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Title: Texas Rocks and Minerals - An Amateur's Guide
Author: Girard, Roselle M.
Language: English
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*** Start of this LibraryBlog Digital Book "Texas Rocks and Minerals - An Amateur's Guide" ***
BUREAU OF ECONOMIC GEOLOGY
The University of Texas
Austin, Texas
Peter T. Flawn, Director
Guidebook 6
TEXAS ROCKS AND MINERALS
_An Amateur’s Guide_
_By_
ROSELLE M. GIRARD
_Sketches by Bill M. Harris_
February 1964
Second Printing, April 1972
Third Printing, April 1976
Fourth Printing, May 1979
Contents
Page
Preface vii
Introduction 1
Earth’s outer crust 2
Geologists 2
Time and rock units 2
Geologic map 6
What are rocks and minerals? 7
Chemical elements 7
Minerals 7
Rocks 8
Igneous rocks 9
Extrusive or volcanic igneous rocks 9
Intrusive igneous rocks 9
Sedimentary rocks 10
Soils 10
Sedimentary rock materials in broken fragments 11
Sedimentary rock materials in solution 12
Cementing materials and chemical sediments 12
Sedimentary rocks formed by plants and animals 12
Metamorphic rocks 12
Static metamorphism 13
Contact metamorphism 13
Dynamic metamorphism 14
Occurrence and properties of minerals 14
How minerals occur 14
Crystalline minerals 14
Crystals 14
Imperfect crystals 14
Amorphous minerals 15
Some distinguishing properties of minerals 15
Color 16
Luster 16
Transmission of light 16
Hardness 16
Streak or powder 17
Cleavage 17
Parting 17
Fracture 17
Specific gravity 18
Effervescence in acid 18
Some special occurrences of minerals 18
Cave deposits 18
Concretions 19
Geodes 19
Petrified wood 20
Collecting rocks and minerals 22
Rock and mineral identification charts 24
How to use the mineral identification charts 24
Key to mineral identification charts 25
Mineral identification charts 26
How to use the rock identification charts 39
Rock identification charts 40
Descriptions of some Texas rocks and minerals 43
Anhydrite 43
Asbestos 43
Barite 44
Basalt 45
Calcite 46
Cassiterite 47
Celestite 48
Cinnabar 49
Clay 51
Copper minerals (chalcocite, chalcopyrite, malachite, azurite)
52
Dolomite 54
Feldspar 55
Fluorite 56
Galena 57
Garnet 58
Gneiss 59
Gold 59
Granite 61
Graphite 62
Gypsum 63
Halite 65
Hematite 66
Limestone 68
Limonite 70
Llanite 71
Magnetite 72
Manganese minerals (braunite, hollandite, pyrolusite) 73
Marble 75
Mica 76
Obsidian and vitrophyre 77
Opal 78
Pegmatite 79
Pyrite 80
Quartz 81
Quartzite 84
Rhyolite 85
Sand and sandstone 85
Schist 87
Serpentine 87
Shale 88
Silver minerals (argentite, cerargyrite, native silver) 89
Sulfur 90
Talc and soapstone 93
Topaz 94
Tourmaline 94
Uranium minerals (carnotite, uranophane, pitchblende) 95
Volcanic ash (pumicite) 97
Composition, hardness, and specific gravity of some Texas minerals
99
Books about rocks and minerals 100
Nontechnical books for beginners 100
Textbooks and other reference books 100
Selected references on Texas rocks and minerals 100
Glossary 102
Index 104
Illustrations
Page
Guadalupe Peak and El Capitan in the Guadalupe Mountains,
Culberson County, Texas 1
Earth’s outer crust 2
Geologic time scale 3
Generalized geologic map of Texas 4-5
A mineral is made up of chemical elements 7
A rock is made up of minerals 8
Extrusive igneous rocks form at the earth’s surface 9
Intrusive igneous rocks form beneath the earth’s surface 10
Soils develop from weathered rock and associated organic material
11
Conglomerate from Webb County, Texas 11
Precipitated sediments lining a teakettle 12
Contact metamorphism 13
A scalenohedron 14
Barite specimen showing radial form 15
Chalcedony showing botryoidal form 16
Transparent mineral 16
Streak plate 17
Conchoidal fracture 18
Stalactites and stalagmites in the Caverns of Sonora, Sutton
County, Texas 19
Calcite geode from Travis County, Texas 20
Petrified wood from Texas Gulf Coastal Plain 20
Prospector’s hammer 22
Hand lens 22
Physiographic outline map of Texas 42
Massive anhydrite 43
Amphibole asbestos from Gillespie County, Texas 44
Barite cleavage fragment from west Texas 44
Basalt from Brewster County, Texas 45
Calcite has perfect rhombohedral cleavage 46
Calcite crystals (dog-tooth spar) from the Terlingua area of
Brewster County, Texas 47
Celestite cleavage fragment from Lampasas County, Texas 48
Cinnabar and calcite crystals from the Terlingua area of Brewster
County, Texas 50
Bentonite is used as a drilling-fluid additive 51
Hazel copper-silver mine, Culberson County, Texas 53
Dolomite rock from Burnet County, Texas 54
Feldspar cleavage fragment from Llano County, Texas 55
Microcline feldspar crystals from Llano County, Texas 56
Fluorite has octahedral cleavage 57
Galena has perfect cubic cleavage 57
Garnet crystal forms 58
Gneiss from Blanco County, Texas 59
Placer gold in stream gravels 60
Granite from Gillespie County, Texas 61
Texas State Capitol building at Austin is made of Burnet County
granite 62
Graphite is used in pencil lead, generator brushes, and lubricants
63
Selenite gypsum crystal from Bastrop County, Texas 64
Selenite gypsum rosettes from Nolan County, Texas 64
Fibrous gypsum from Terlingua area, Brewster County, Texas 65
Salt domes occur on the Gulf Coastal Plain 66
Specular hematite from Carrizo Mountains, Hudspeth County, Texas
67
Limestone from Travis County, Texas 68
Limestone quarry at Georgetown, Williamson County, Texas 69
Limonite ore is changed to metallic iron in a blast furnace 71
Metallic iron is made into steel in an open-hearth furnace 72
Magnetite, Llano County, Texas 73
Hollandite from Jeff Davis County, Texas 74
Precambrian metamorphic marble from Llano County, Texas 75
Mica minerals have perfect cleavage in one direction 76
Obsidian arrowheads 77
Opalized wood from Washington County, Texas 78
Quartz-feldspar pegmatite from Burnet County, Texas 79
Pyrite veins in white marble from Llano County, Texas 80
Cubic crystals of pyrite 80
Quartz crystal from Burnet County, Texas 81
Amethyst geode from the Alpine area of Brewster County, Texas 82
Milky quartz from Burnet County, Texas 82
Smoky-quartz crystals from Burnet County, Texas 83
Polished agate from Rio Grande gravels of Zapata County, Texas 83
Jasper from Uvalde County, Texas 84
Sandstone from Zavala County, Texas 86
Prospector 89
Sulfur is obtained by the Frasch process 92
Talc schist from the Allamoore area of Hudspeth County, Texas 93
Topaz crystal from Mason County, Texas 94
Black tourmaline crystals with milky quartz from Llano County,
Texas 95
A Geiger counter is used to detect radioactivity 96
PREFACE
This booklet has been designed to serve as a brief, simple guide that
will be of help to school children, amateur collectors, and others who
are just beginning to develop an interest in the rocks and minerals of
Texas. It is a companion volume to _Texas Fossils_ by William H.
Matthews III published as Guidebook No. 2 by the Bureau of Economic
Geology.
Numerous present and former staff members of The University of Texas
contributed time and talents to the preparation of this book, and their
help is gratefully acknowledged: Peter T. Flawn, Director of the Bureau
of Economic Geology, Thomas E. Brown, John W. Dietrich, Alan Humphreys,
Elbert A. King, Jr., Peter U. Rodda, and others, including the late John
T. Lonsdale, made many helpful suggestions; John S. Harris and Miss
Josephine Casey edited the manuscript; Cader A. Shelby prepared a number
of the photographs; Bill M. Harris made the illustrative sketches under
the direction of James W. Macon; and Cyril Satorsky designed the cover.
Texas Rocks and Minerals
An Amateur’s Guide
Roselle M. Girard
INTRODUCTION
Texas has a great variety of rocks and minerals—some are common and
others are not. This book is designed to acquaint you with some of them
and to tell you in a nontechnical way what they are like, some of the
places where they are found, and how they are used. Although we do not
know exactly how all of the rocks and minerals formed, some of the ideas
about their origin are mentioned.
If you would like to learn more about rocks and minerals in general, the
names of several reference books are listed on page 100. In addition,
scientific reports that describe in detail many of the rocks and
minerals of Texas have been published by the Bureau of Economic Geology
of The University of Texas, the United States Geological Survey, and
other organizations. A selected list of these reports is given on pages
100-101.
Rocks and minerals are familiar objects to all of us. We pick up
attractive or unusual pebbles for our collections, we admire rocky
mountain peaks, we speak of the mineral resources of our State and
Nation. Rocks and minerals enter, either directly or indirectly, into
our daily living. From them come the soils in which grow the grains, the
fruits, and the vegetables for our food, the trees for our lumber, and
the flowers for our pleasure. The iron, copper, lead, gold, silver, and
manganese, the sulfur and salt, the clays and building stones, and the
other metals and nonmetals that we require for our way of living were
once a part of the earth’s crust.
[Illustration: Texas’ highest mountain is Guadalupe Peak, right, with
an elevation of 8,751 feet. El Capitan, left, has an elevation of 8,078
feet. These peaks in the Guadalupe Mountains in Culberson County
consist largely of Capitan reef limestone, which formed during the
Permian Period.]
Earth’s Outer Crust
Rocks and minerals make up most of the outer layer or crust of our
earth—the actual ground beneath our feet. The crust is approximately 18
to 30 miles thick beneath the continents. In general, the outermost part
consists of many layers of stratified rocks, one above another. The
older rocks normally make up the bottom or the deeper layers, and the
younger rocks form the upper layers. Not all the layers are perfectly
flat and parallel—some are lenticular (lens-shaped), some are tilted,
some are partly eroded away, and some are present in one place and
absent in another. Beneath the continents, the layers of rock rest on
ancient metamorphic rocks and on great masses of igneous rock such as
granite. These lower rocks are known as the _basement_.
[Illustration: Earth’s outer crust (thickness not drawn to scale).]
Over much of the land surface of the earth, the outermost layer is
made up of layers of rock
On the continents, the layers of rock rest on metamorphic rocks and on
igneous rocks such as granite
Geologists
Those who study the earth’s crust—its origin, history, rocks, minerals,
fossils, and structure—are known as _geologists_. The geologists who are
especially interested in a particular phase of _geology_, as this
science is called, are given special names: those who study fossils are
called _paleontologists_; those who study minerals are called
_mineralogists_; those who study rocks are called _petrologists_.
Time and Rock Units
The earth’s crust is believed to be at least 3¼ billion years old. In
order to deal with this vast stretch of time, geologists have divided
the billions of years into various time units and have given each unit a
name. The great divisions of geologic time, called _eras_, are Early
Precambrian, Late Precambrian, Paleozoic, Mesozoic, and Cenozoic. These
eras are divided into smaller units of time called _periods_, and the
periods are divided into _epochs_. The _[xx time scale]_ shows the
geologic time divisions. Earliest geologic time is shown at the bottom
of the scale; most recent is shown at the top.
By examining and studying the different rocks and rock layers,
geologists try to discover in which unit of geologic time these rocks
formed. Those rocks that formed during a _period_ of geologic time are
called a _system_ of rocks; those that formed during an _epoch_ are
called a _series_. For example, the Cambrian System of rocks formed
during the Cambrian Period; the Cretaceous System of rocks formed during
the Cretaceous Period; the Tertiary System of rocks formed during the
Tertiary Period. We are now in the younger epoch (called Recent) of the
Quaternary Period of the Cenozoic Era. The rocks that are forming now
are the Recent Series of rocks.
[Illustration: Geologic time scale]
ERA
PERIOD
EPOCH
CENOZOIC
QUATERNARY (lasted 0-1 million years)
Recent
Pleistocene
TERTIARY (lasted 62 million years)
Pliocene
Miocene
Oligocene
Eocene
Paleocene
—63 million years ago—
MESOZOIC
CRETACEOUS (lasted 72 million years)
JURASSIC (lasted 46 million years)
TRIASSIC (lasted 49 million years)
—230 million years ago—
PALEOZOIC
PERMIAN (lasted 50 million years)
PENNSYLVANIAN (lasted 30 million years)
MISSISSIPPIAN (lasted 35 million years)
DEVONIAN (lasted 60 million years)
SILURIAN (lasted 20 million years)
ORDOVICIAN (lasted 75 million years)
CAMBRIAN (lasted 100? million years)
—600? million years ago—
LATE PRECAMBRIAN
EARLY PRECAMBRIAN
These time estimates are from the paper, Geologic Time Scale, by J.
Lawrence Kulp, published in Science, Vol. 133, No. 3459, April 14, 1961.
(The time divisions are not drawn to scale)
[Illustration: Plate 10. GENERALIZED GEOLOGIC MAP OF TEXAS
Modified from Geologic Map of Texas, 1933]
EXPLANATION
CENOZOIC
1 Quaternary
2 Tertiary (Oligocene, Miocene, and Pliocene)
3 Tertiary (Eocene)
4 Volcanic (extrusive) igneous rocks
MESOZOIC
5 Upper Cretaceous (Gulf series)
6 Lower Cretaceous (Comanche series)
7 Jurassic
8 Triassic
PALEOZOIC
9 Permian
10 Mississippian and Pennsylvanian
11 Cambrian, Ordovician, Silurian, Devonian and undivided
Paleozoic
12 Rocks (Precambrian) older than Paleozoic
13 Intrusive igneous rocks (Precambrian, Mesozoic or Cenozoic)
These rocks are found either at the surface or directly beneath the
soils and subsoils which cover most of Texas.
Geologists also subdivide rocks into lesser units. One of these, called
a _group_, is made up of two or more _formations_. A _formation_
comprises rocks or strata (layers of rock) that are recognized and
mapped as a unit. Some formations consist of layers of one particular
type of rock, such as limestone or shale. Formations are named after a
nearby geographic locality, and in some formation names, the type of
rock is included. For example, three of the Texas geologic formations
are called Buda Limestone, Del Rio Clay, and Eagle Ford Shale.
Geologic Map
The _geologic map_ (pp. 4-5) shows the rocks that are found at the
surface in Texas. Some of these are extremely old. Some, geologically
speaking, are very young.
WHAT ARE ROCKS AND MINERALS?
Although _rocks_ and _minerals_ are often mentioned together, and to
some people they have similar meanings, geologists make a distinction
between the two words. In general, rocks are made up of minerals, and
minerals are made up of chemical elements.
Chemical Elements
The _chemical elements_ include oxygen, silicon, calcium, sulfur,
carbon, gold, silver, and many others. There are 90 naturally occurring
elements. Each is made up of molecules that consist of only one kind of
atom. Chemical elements may either be combined with each other or occur
alone. They are the building blocks of our world for they make up all
the gases, all the liquids, all the minerals, all the plant and animal
life, and all the other physical matter. Some of the chemical elements
that occur in the rocks and minerals mentioned in this book are listed
below.
Aluminum Al
Barium Ba
Beryllium Be
Boron B
Calcium Ca
Carbon C
Cerium Ce
Chlorine Cl
Copper Cu
Fluorine F
Gold Au
Hydrogen H
Iron Fe
Lead Pb
Magnesium Mg
Manganese Mn
Mercury Hg
Molybdenum Mo
Oxygen O
Potassium K
Silicon Si
Silver Ag
Sodium Na
Strontium Sr
Sulfur S
Thorium Th
Tin Sn
Uranium U
Vanadium V
Yttrium Y
Zinc Zn
Zirconium Zr
We can compare the chemical elements to the letters of our alphabet. The
letters, like the chemical elements, are fundamental building blocks,
and they can be brought together in various combinations to form words.
Minerals
A _mineral_ can be compared to a word of our language. We combine
letters to form a word, and nature combines certain chemical elements to
form each particular mineral. For example, calcite, a mineral that is
abundant in Texas, is always made up of the same proportions of the same
three elements: calcium, carbon, and oxygen.
[Illustration: A mineral is made up of chemical elements. The mineral
_calcite_, for example, always consists of the same proportions of
calcium, carbon, and oxygen.]
Each mineral has its own characteristic internal structure and other
properties. At ordinary temperatures, nearly all the minerals are solids
rather than gases or liquids. (Water and mercury are the principal
exceptions.) In addition, minerals are inorganic rather than being
composed of plant or animal matter.
When a single chemical element is found alone in nature as a _solid_, it
is considered to be a mineral, too. Gold, silver, copper, lead, and
sulfur are some of the chemical elements that can occur alone as solid
minerals. When they occur this way, we refer to them as _native_ silver,
_native_ copper, or _native_ sulfur. Although the element mercury is a
liquid rather than a solid at ordinary temperatures, it too is a mineral
when it occurs alone in nature. It is then called _native_ mercury.
Rocks
We have already compared the chemical elements to the alphabet and the
minerals to words. We can now go a step further and compare rocks to
sentences. We put words together to make sentences; nature puts minerals
together to make rocks. A sentence does not have to be made up of a
definite number of words, nor does a rock have to be made up of a
definite number of minerals. Some rocks, such as granite, may be
composed of several minerals. Others, such as dolomite and rock gypsum,
consist of only one mineral.
Minerals do not lose their identities when they make up a rock. Instead,
they are merely associated together in varying proportions. Some rocks,
as we will find later, instead of being composed of the minerals
themselves, are made up of fragments of earlier-formed rocks.
Ordinarily, we think of rocks as hard and solid substances, such as
limestone and granite, but some geologists consider loose and uncemented
materials, such as sand, gravel, or volcanic ash, to be rocks also. The
words _sediments_ or _deposits_ are often used to describe this
uncemented or loose material.
Rocks are commonly grouped, according to how they formed, into three
great classes known as _igneous_, _metamorphic_, and _sedimentary_.
[Illustration: A rock is made up of minerals. The igneous rock
_granite_, for example, consists chiefly of quartz and feldspar; other
minerals such as mica and hornblende are commonly present.]
IGNEOUS ROCKS
Igneous rocks result from the cooling of hot, molten rock material or
_magma_. Magma that reaches the surface through volcanoes is called
_lava_. Magma comes from deep within the earth and is made up of a
mixture of molten mineral materials. Igneous rocks have been forming
throughout the geologic past and are still forming today. We can
understand how they form when we look at pictures of hot, molten lava
flowing from volcanoes, such as Mauna Loa in Hawaii. As lava cools, it
hardens into rock.
Extrusive or Volcanic Igneous Rocks
The igneous rocks that form on the earth’s surface are called
_extrusive_ or _volcanic_ igneous rocks. When magma flows to the
surface, it cools and hardens quickly. The mineral grains that form
during this fast cooling may be too small to be distinguished from each
other. Some lava cools too quickly for minerals to crystallize—then the
rock is volcanic glass.
[Illustration: Extrusive igneous rocks form at the earth’s surface from
lava that cools and hardens relatively quickly.]
No volcanic igneous rocks are forming in Texas now. However, during
Tertiary time, in the Big Bend area and in other parts of the
Trans-Pecos country of west Texas, lava came to the surface and
hardened. (The physiographic outline map, p. 42, shows where these areas
are located.)
Intrusive Igneous Rocks
The cooling and hardening of hot, molten magma also takes place below
the earth’s surface. Here, the magma cools slowly to form rocks made up
of mineral grains that are large enough to be readily visible. These
rocks are known as _intrusive_ igneous rocks. We know that they are
present below the surface in Texas because of wells drilled in many
areas of the State. In Pecos County, a well reached granite, an
intrusive igneous rock, at a depth of 16,510 feet. Other wells in Texas
have reached the granite basement rocks at much shallower depths. But
not all intrusive igneous rocks in Texas are found underground. In the
Trans-Pecos country of west Texas, in the Balcones fault zone, and in
the Llano uplift of central Texas, some are now seen at the surface.
They, like all intrusive rocks, were formed below the ground, but
earth’s processes of uplift and erosion have gradually uncovered them.
[Illustration: Intrusive igneous rocks form from molten rock material
(magma) that cools and hardens beneath the earth’s surface.]
SEDIMENTARY ROCKS
Sedimentary rocks are made up of sediments, which are rock and mineral
grains that have come from weathered rocks of all kinds. Rocks are
weathered when water, ice, snow, wind, and other agents cause them
either to dissolve, as table salt does when put in water, or to break
apart, as old pavement commonly does.
Soils
Some of the broken-down rocks, along with associated plant and animal
matter, develop into soils. When you examine soil with a magnifying
glass, you may be able to see some of the small rock and mineral grains
that still remain in it. Some soils have formed on top of the rocks from
which they came, and some have been moved in from another place.
[Illustration: Soils develop from weathered rock and associated organic
material.]
SOIL
SUBSOIL
WEATHERED ROCK
BEDROCK
Sedimentary Rock Materials in Broken Fragments
Water and wind not only weather the rocks and soils but also move the
weathered materials (the sediments) and deposit them in other places.
Whenever you see a dust or sand storm, or a muddy creek or river, you
are observing the movement of sediments by wind and water to other land
areas or to the sea. The combination of weathering and movement is
called _erosion_.
[Illustration: Conglomerate from Webb County, Texas, is composed of
rounded gravel that has been cemented together.]
Some of the rock fragments carried by water are still fairly large when
they reach their destinations. On the basis of size, they are called
_boulders_, _cobbles_, _pebbles_, and _granules_. Loose deposits of
these larger-size sediments make up what is known as _gravel_. Nature
cements gravels together to form rocks such as _conglomerates_ (made up
of rounded gravel) and _breccias_ (made up of sharp-cornered gravel).
The finer sediments are called _sand_, _silt_, _mud_, and _clay_. When
cemented, the sand grains become _sandstones_, the silt particles become
_siltstones_, and the mud and clay particles become _shale_. The
sedimentary rocks that are made up of these rock fragments are called
_clastic_ or _fragmental_ rocks.
Sedimentary Rock Materials in Solution
As they are weathered, some rocks dissolve and go into solution. For
example, a number of the Texas creeks and rivers carry calcium carbonate
in solution because they flow through areas where limestone rocks, which
consist mostly of calcium carbonate, are being weathered. (Water that
contains a large amount of dissolved rock material is called _hard_
water.)
_Cementing materials and chemical sediments._—
Some of the waters containing dissolved rock material seep through loose
sediments where the dissolved material may come out of solution and form
a _cement_, which binds the sediments together. For example, when loose
sand sediments are cemented, they form sandstone. Three of the most
common cements are iron oxide, calcium carbonate, and silicon dioxide,
although a number of other materials also serve as cements.
Dissolved rock materials come out of solution not only to serve as
cementing agents but to form the chief mineral of some sedimentary rocks
as well. Sedimentary rocks of this kind form mostly in lakes and seas
into which much dissolved material is carried by rivers. When the
dissolved material comes out of solution, it is said to be
_precipitated_ and the mineral sediments it forms are the _chemical_
sediments. Some limestones originate this way. You can see examples of
precipitated materials by noting the crust-like deposits that form
inside some water pipes and teakettles, as dissolved material in the
water comes out of solution.
[Illustration: Precipitated sediments are commonly observed lining a
teakettle.]
_Sedimentary rocks formed by plants and animals._—
The dissolved rock material can come out of solution in another way.
Some plants and animals are able to take dissolved calcium carbonate out
of the sea water and use it to build their shells and other structures.
Some of these organisms, such as corals and algae, can grow upward from
the sea floor in large groups to form reefs that later become reef
limestones. Other limestones are made up of the remains of plants and
animals that collect on the sea floor and become cemented together.
METAMORPHIC ROCKS
Metamorphic rocks come from earlier-formed rocks that have undergone a
change or a _metamorphosis_. All igneous and sedimentary rocks, and
earlier-formed metamorphic rocks too, can be changed, without being
moved to some other place, into new and different rocks. As they are
changed, they may become harder, new minerals may form, and they may
look entirely different. For example, granite, an igneous rock, can be
changed into the metamorphic rock known as _gneiss_; limestone, a
sedimentary rock, can be changed into _marble_; shale, a sedimentary
rock, can be changed into _slate_. These changes occur because the earth
is a big and complex chemical system. The agents that bring about these
changes, which always occur below the surface of the earth, are heat,
pressure, and fluids—both liquids and gases. Several different kinds of
change or metamorphism can take place.
Static Metamorphism
Some of the changes occur because the rocks are at great depths. As more
and more younger rocks are deposited on top of them, the older rocks
become deeply buried. The great thicknesses of younger rocks are heavy,
and they squeeze and press down on the rocks beneath them. The deeply
buried rocks are also hotter than surface rocks. In general, the
temperature increases about 1° Fahrenheit for each 50 feet of depth
below the surface. The change of deeply buried rocks into new rocks by
pressure and heat is known as _static metamorphism_.
Contact Metamorphism
Another method of change or metamorphism involves molten igneous rock
material. When hot magma moves up through rocks, it not only heats and
pushes them, but it also may soak them with liquids and gases, causing
the nearby rocks to change into new rocks, by a process called _contact
metamorphism_.
[Illustration: Some rocks are altered by heat and fluids when they are
invaded by hot magma in a process called contact metamorphism.]
UNALTERED ROCK
METAMORPHIC ROCK
MAGMA
Dynamic Metamorphism
Still another rock-changing process is one that is associated with
mountain building. When mountains are formed, heat and great pressures
develop deep within the earth’s crust. The flat layers of rock are then
slowly pushed and squeezed so that they bend up into arches, fracture,
or slide over each other. These forces cause great changes in the rocks
in widespread areas. This process of change is known as _dynamic
metamorphism_.
Occurrence and Properties of Minerals
HOW MINERALS OCCUR
Rocks are made up of minerals. In addition, minerals are associated with
rocks in other ways. For example, minerals fill or coat cracks and
cavities that have developed in some of the rocks. Minerals are either
crystalline or amorphous.
Crystalline Minerals
Most minerals are crystalline. In crystalline minerals, combinations of
atoms are arranged in ordered patterns, which are repeated over and
over. This orderly internal structure of atoms is a characteristic of
each crystalline mineral, as mineralogists are able to determine by
using X-rays and special microscopes.
_Crystals._—
When a mineral occurs as a well-formed individual crystal, it has a
definite, precise shape. The kind of crystal shape it has depends on its
own type of crystalline internal structure. A well-formed crystal has
smooth, flat, outer surfaces called _crystal faces_, which are arranged
together to form prisms, cubes, pyramids, and many other geometric
shapes. For example, quartz, a common Texas mineral, is commonly found
as a six-sided, prism-shaped crystal that is topped by pyramid-like
forms. Pyrite, another common mineral, occurs as cube-shaped crystals.
We can identify some minerals more readily by learning to recognize
their crystal shapes.
[Illustration: A scalenohedron, one of the many crystal forms of
calcite.]
_Imperfect crystals._—
A crystalline mineral commonly forms under conditions that do not permit
it to become a well-shaped crystal. Although the mineral may show a few
crystal faces, it does not have a complete crystal shape and so is
described as _massive_, or is said to occur in _masses_. Some of the
minerals that make up rocks occur as crystalline masses. For example,
_calcite_ is a crystalline mineral that occurs in the metamorphic rock
_marble_ without its normal crystal shape.
Many crystalline minerals occur as incomplete and imperfect crystals
that are grouped together in various arrangements. If these incomplete
crystals are arranged around a common center like the spokes of a wheel,
they are said to be _radial_ or _radiated_. If the groups of incomplete
crystals look like bundles of strings or fibers, they are described as
_fibrous_. If they are in rounded masses that resemble bunches of
grapes, they are called _botryoidal_. If they look like fish scales,
they are described as _scaly_. Some crystalline minerals are made up of
tiny grains that are grouped together like the grains in a lump of
sugar. A mineral occurring in this way is described as _granular_. More
descriptions of crystalline minerals are found in the section on Texas
rocks and minerals (pp. 43-98).
[Illustration: Barite specimen showing radial form.]
Amorphous Minerals
An amorphous mineral, unlike a crystalline mineral, does not have a
definite, orderly arrangement of its atoms. Because of this lack of
internal structure, the mineral occurs in masses that have no regular
geometric shapes, and it has no crystal form of its own. Only a few
minerals are amorphous.
SOME DISTINGUISHING PROPERTIES OF MINERALS
We use our senses of sight, hearing, smell, touch, and taste to become
aware of the world around us. For example, we recognize a flower by
noting its color, its fragrance, and the texture, shape, and arrangement
of its petals. These are some of its characteristic properties. A
mineral also has distinguishing properties, among them color, luster,
and hardness, which help us identify it. Some minerals have a single
outstanding property, such as the magnetism of magnetite, that makes
them easier to recognize. But to identify most minerals, we need to
determine not just one, but several properties.
[Illustration: Chalcedony showing botryoidal form.]
Color
Color is one of the properties we notice first. The color of some
minerals is always the same, and it helps us to identify them. But it is
not a dependable property to use in identifying all minerals, because
some contain impurities that change or hide the real color.
Luster
The luster is the way the surface of a mineral reflects light. The
luster of a mineral may be _nonmetallic_, _submetallic_, or _metallic_.
Mineral metals such as gold, silver, galena, and pyrite have a
_metallic_ luster. A few minerals have a luster that is almost, but not
quite metallic—their luster is _submetallic_. A mineral with a
nonmetallic luster may look _vitreous_ (glassy), _silky_, _resinous_
(like resin), _greasy_, _earthy_ (dull), _pearly_, or _adamantine_
(brilliant).
Transmission of Light
Some minerals allow light to pass through them; others do not. A mineral
is _transparent_ if you can see both light and objects through it, as
through clear glass. If you can see only light, but no objects, as
through frosted glass, the mineral is _translucent_. When you hold an
_opaque_ mineral up to the light, it looks dark. No light at all comes
through it, even through the thin edges.
[Illustration: Transparent mineral.]
Hardness
Some minerals are soft and can be scratched easily. Others, which are
harder, are resistant to scratching. To measure a mineral’s hardness, we
try to find out which substances will scratch it and which substances
will not scratch it. To do this in a general way, several ordinary
objects—such as a fingernail, a copper penny, a pocket knife, a piece of
window glass, and a steel file—can be used. For a more exact way of
testing hardness, we can use ten minerals that make up what is known as
_Mohs scale_. Each mineral in this scale has a different hardness, and
each one has been given a number that represents its hardness. For
example, talc, the softest mineral in this scale, is given a hardness of
_1_. Gypsum, the next softest mineral in the scale, has a hardness of
_2_. Diamond, the hardest mineral known, is given the top hardness of
_10_ in this scale. These ten minerals are listed below. Alongside them
are five common objects with their hardnesses.
1—Talc
2—Gypsum Fingernail—slightly over 2
3—Calcite Copper penny—about 3
4—Fluorite
5—Apatite Pocket knife—slightly over 5
6—Orthoclase Window glass—5½
7—Quartz Steel file—about 6½
8—Topaz
9—Corundum
10—Diamond
Suppose, for example, that a mineral can be scratched by fluorite, which
has a hardness of _4_ on Mohs scale, but cannot be scratched by calcite,
which has a hardness of _3_. We then know that this mineral is softer
than fluorite, but harder than calcite; therefore, it has a hardness of
about _3½_. In the same way, if a mineral can be scratched by a pocket
knife, which is slightly more than _5_ in hardness, but not by a copper
penny, which has a hardness of about _3_, we know then that its hardness
is between _3_ and _5_.
Streak or Powder
The streak is the mark, made of fine powder, that a mineral leaves as
you rub it across a streak plate. A streak plate is a flat piece of
white tile or porcelain that has a dull, unglazed surface. The streak
plate is about as hard as quartz, which is _7_ on Mohs scale, and you
will not be able to use it for minerals that have a greater hardness.
For these, you can obtain the powder by scratching the mineral or by
crushing a small piece of it.
[Illustration: A streak plate is used to determine the color of the
streak or powder of a mineral.]
The color of the streak or powder is extremely helpful in identifying
some minerals. For example, hematite is a mineral that may be any one of
several different colors, but its streak or powder is always reddish
brown.
Cleavage
As they break, some crystalline minerals always split along a smooth,
flat surface. This property is known as cleavage. Some cleavages are
smooth and perfect; others are not so perfect. The cleavage surfaces,
because of the mineral’s crystalline internal structure, are parallel to
possible crystal faces, even though the mineral itself may occur as a
crystalline mass without a perfect crystal shape.
Some minerals will cleave in only one direction; some, in several
directions. For example, galena, a mineral found in Texas, has perfect
_cubic_ cleavage. It cleaves in three directions that are at right
angles to each other. These cleavage directions are parallel to possible
cubic crystal faces, and some of the cleavage fragments are cubes.
Parting
A few minerals sometimes show a kind of false cleavage known as
_parting_. Parting, unlike cleavage, is not constant and does not occur
in every specimen of a particular mineral. For this reason, it is not a
very dependable means of identification.
Fracture
Minerals also break in another way. When the break is in a different
direction from that of the cleavage or parting, it is known as the
fracture. A fracture is called _conchoidal_ if the mineral’s broken
surface is curved like the inside of a spoon or shell. Thick pieces of
glass break with this conchoidal fracture. A fracture is described as
_hackly_ if the broken surface has sharp, jagged edges; as _even_, if
the surface is generally flat; and as _uneven_, if it is rough and not
flat. If the mineral breaks into splinters, its fracture is called
_splintery_.
[Illustration: Conchoidal fracture.]
Specific Gravity
The specific gravity is a measure of whether a mineral is heavy or
light. It is a comparison of the weight of a piece of the mineral with
the weight of an equal volume of water. The mineral quartz, for example,
has a specific gravity of 2.65. This means that a piece of quartz is a
little more than 2½ times as heavy as an equal volume of water. Accurate
measurements of specific gravity can be made in a laboratory. You can,
however, learn to estimate specific gravities just by lifting various
minerals and judging whether they are heavy or light.
Effervescence in Acid
This is a property that depends on the chemical composition of the
mineral. Carbonate minerals, which contain (in addition to at least one
other element) three parts of oxygen and one part of carbon, can be
tested with dilute hydrochloric acid. When a drop or two of this acid is
put on a carbonate mineral such as calcite (calcium carbonate, CaCO₃),
the acid begins to bubble and fizz. The fizzing or effervescence is
caused by the carbon dioxide gas that is formed when the acid and
mineral come in contact with each other. This test is also helpful in
identifying rocks, such as limestone and marble, that contain carbonate
minerals.
SOME SPECIAL OCCURRENCES OF MINERALS
Cave Deposits
Beautiful mineral deposits occur in some natural caves. Deposits that
look like icicles, called _stalactites_, are found hanging from the
ceiling of a cave. Other deposits, _stalagmites_, are like the
stalactites except that they jut upward from the floor. _Columns_ are
formed from stalactites and stalagmites that have joined together. In
addition, some caves contain sheet-like deposits that are spread along
the ceiling, floor, and walls. These deposits are called _flowstone_.
Calcite is one of the minerals that commonly form cave deposits.
Just a few of the caves in Texas contain these deposits. They occur
mostly in the limestone rocks that are south and southwest of the Llano
uplift area of central Texas. Some of the commercial caves that contain
good examples of calcite deposits are located near Boerne in Kendall
County and near Sonora in Sutton County. Calcite deposits also occur in
Longhorn Cavern, a large cave located in the Longhorn Cavern State Park
of Burnet County. These caves were formed by underground waters that
moved through cracks and pores in the limestone rocks and dissolved
passageways in them. After the cave passages were made, water containing
dissolved calcium carbonate dripped into the cave. As it evaporated,
this water left behind a deposit of calcium carbonate—the mineral
calcite.
You can better understand how the cave deposits are formed by watching
icicles grow in wet, freezing weather. First, small hanging drops of
water freeze, and a small icicle forms. Then, as more water drips over
it and freezes, the icicle grows longer and wider. Some of the water
drips completely over the icicle and falls to the ground. There, it
either freezes into a sheet of ice, or it begins to build upward to form
an upside-down icicle. The water dripping down in the caves evaporates
instead of freezing, and in doing so it leaves behind a deposit of
calcite.
[Illustration: Calcite stalactites and stalagmites in the Caverns of
Sonora, Sutton County, Texas. Photograph courtesy of the Travel and
Information Division of the Texas Highway Department.]
Concretions
Limestone, shale, and other sedimentary rocks commonly have scattered
throughout them masses of other rocks and minerals, such as limonite,
chert, and pyrite. These masses are called _concretions_. Concretions
may be round or oval, or they may have odd, irregular shapes. They—such
as some of the limonite concretions of east Texas—even may look like
gourds or sweet potatoes. Concretions generally are harder than the
surrounding rocks. Some are smaller than peas, but others are several
feet wide. (The word _nodule_ is used to describe small, rounded
concretions as well as other small, rounded mineral occurrences.)
It is believed that some concretions form at the same time as the rocks
in which they occur. Other concretions develop after the rocks
themselves have formed. These are deposited by underground water that
contains dissolved mineral matter. The water seeps through the rocks and
deposits mineral matter around an object in the rock, such as a fossil
or a grain of sand, to form a concretion.
Geodes
Geodes are rounded, generally hollow masses that occur mostly in
limestones. They are scattered through the rocks and can be lifted or
dug out. Some geodes are as small as walnuts, and some are as large as
basketballs. Most of them have a rough, dull-looking outer surface. If
you break geodes open, you will find that many are lined with beautiful
crystals of calcite, celestite, or quartz that point inward toward the
hollow center.
[Illustration: Calcite geode found in Lower Cretaceous strata of western
Travis County, Texas.]
It is thought that a geode forms when water, carrying dissolved mineral
material, seeps into a cavity in the rock, then deposits the mineral
material as a lining in the cavity. This lining becomes the outer part
of the geode. Thus a geode—unlike a concretion, which grows from the
center outward—forms from outside to inside.
Some of the Lower Cretaceous limestone rocks of Travis, Williamson, and
Lampasas counties contain calcite and celestite geodes. Celestite geodes
have also been found in Permian rocks in parts of Coke, Fisher, and
Nolan counties.
Petrified Wood
[Illustration: Petrified wood from Texas Gulf Coastal Plain.]
We often find some minerals occurring as petrified wood. (Petrified wood
includes silicified wood, opalized wood, agatized wood, and carbonized
wood.) Petrified wood forms when plant material, such as a tree or a
bush, is replaced by a mineral. It is formed by underground water
carrying dissolved mineral matter. As this water seeps through sediments
in which the plants are buried, it gradually deposits agate, chalcedony,
calcite, opal, chalcocite, or some other mineral in the place of each
fiber of the wood. By this slow change from plant to mineral matter, the
original shape and structure of the wood remain unchanged.
Petrified wood is commonly found in some of the Tertiary, Permian, and
Lower Cretaceous rocks of Texas. (_See_ Opal, Quartz, Copper Minerals,
pp. 78, 84, 52).
COLLECTING ROCKS AND MINERALS
Perhaps you would like to start your own collection of rocks and
minerals. For this purpose you will need a _hammer_ (a prospector’s
hammer with a pick on one end of it is a good tool), some _newspapers_
to wrap around the specimens to keep them from breaking, and a _cloth
bag_ in which to carry the specimens.
[Illustration: Prospector’s hammer.]
Before you start to collect, be sure to ask the owner’s permission to go
on his property. If he agrees to let you come on his land, be careful
about closing gates, and do not leave holes into which his livestock
might step and be injured. Look out for snakes. Plenty of rattlers,
copperheads, and moccasins are still left in Texas. And, incidentally,
collecting is not allowed in State or National parks.
To identify the rocks and minerals that you collect, you probably will
need several articles with which to make simple tests. The following can
be easily obtained:
1. A _pocket knife_, a _copper penny_, a piece of _window glass_, a
_steel file_, and a piece of _quartz_ to test the hardness. If you
prefer to use a group of minerals of known hardness, such as those of
Mohs scale described on pages 16-17, you can either collect your own
or buy a prepared set from a mineral supply house.
2. A _streak plate_ to test the color of the mineral’s streak. Mineral
streak plates can be purchased, or a piece of unglazed tile can be
used.
3. A _magnifying glass_ to examine small cleavage surfaces, crystals,
and rock grains. A number of different kinds can be bought, from the
simple reading glass to the precisely made hand lens. A lens with
ten-power magnification is good for general use.
4. A small _magnet_ to test whether or not a mineral is magnetic.
5. _Dilute_ (10%) _hydrochloric acid_ (HCl), also known as _muriatic
acid_, to test carbonate rocks and minerals. You can buy a small
bottle at a drug store. Be extremely careful in handling this acid,
and keep it away from small children—it is a _POISON_. If you spill
any on yourself, it will burn your skin and eat holes in your clothes.
[Illustration: Hand lens.]
The rock and mineral identification charts on pages 24-41 will help you
to make the simple identification tests in a methodical way.
It is a good idea to have some system of labeling your rock and mineral
specimens. Some collectors carry note paper with them on field trips.
Then they can write down the location and, if possible, the name of the
rock or mineral. This information is either wrapped with the specimen or
stuck to it with tape. One way to label large collections is to put a
small spot of paint or fingernail polish on each of the rock and mineral
specimens. When the paint has dried, a number can be written on it in
black India ink. Then, on a file card, the name and the number of the
specimen can be written, together with the place where it was found, the
date of collection, and the name of the collector.
ROCK AND MINERAL IDENTIFICATION CHARTS
To help you identify them, various Texas rocks and minerals are listed
together in the following charts according to properties that they have
in common. Although useful, the identification charts may not always
give you perfect results. For example, hardness, which is used as a
guide, is not to be completely relied upon in the identification of
rocks.
The charts on the following pages pertain only to the rocks and minerals
that are described in this book. It is quite possible that you will find
rocks and minerals in Texas that are not included in these charts.
If you find a rock or a mineral that you are unable to identify, you can
check your local library for reference books that may aid you (several
such references are noted on pages 100-101). If you need further help,
possibly the science teacher at a nearby public school will be able to
identify the specimen for you. Or if a college or university is located
in your area (especially one that has a department of geology), you can
obtain help there. In Texas, the Bureau of Economic Geology is a mineral
information center. Most other states have similar geological research
and public-service organizations. Other sources of information might be
the gem and mineral societies that are found in a number of communities.
Many of the members of these organizations are experts in the
identification of rocks and minerals.
How To Use the Mineral Identification Charts
In the mineral identification charts (pp. 26-38), the minerals have been
grouped, first of all, on the basis of _luster_: the first group
includes the minerals that appear _metallic_ and _almost metallic_
(_submetallic_); the second group includes those that appear
_nonmetallic_. Next, the minerals have been arranged within the two
groups according to _color_.
After you have determined the luster and the color of an unknown
mineral, turn to the _Key to Mineral Identification Charts_ on page 25.
It will direct you to the proper mineral chart.
Mineral Charts 1 through 5, which include the minerals of various colors
with _metallic_ and _submetallic_ lusters, are subdivided according to
the _hardness_ of the minerals. To determine the hardness of a mineral
that has one of these lusters, you can make the following tests:
1. Will the mineral readily leave a mark on paper?
2. If it will not readily leave a mark on paper, will an ordinary
pocket knife scratch it?
3. Is it too hard to be scratched by an ordinary pocket knife?
Mineral Charts 6 through 15 are for the _nonmetallic_ minerals of
various colors. They, too, are subdivided according to the _hardness_ of
the minerals, as follows:
1. Can the mineral be scratched by a fingernail?
2. If it cannot be scratched by a fingernail, can it be scratched by a
copper penny?
3. If it cannot be scratched by a copper penny, can it be scratched by
an ordinary pocket knife?
4. If it cannot be scratched by an ordinary pocket knife, can it be
scratched by a piece of quartz?
5. Is it too hard to be scratched by quartz?
When the luster, color, and hardness of a mineral have been determined,
you may find that several minerals on the charts fit the description. To
narrow your choice, you can then test other properties of the mineral.
Notice the “remarks” column on the charts. In it, is mentioned anything
that is distinctive about the mineral.
For more complete mineral identification lists and tables, you can use
textbooks, such as _Dana’s Manual of Mineralogy_, revised by C. S.
Hurlbut, Jr., or _Mineralogy_, by E. H. Kraus, W. F. Hunt, and L. S.
Ramsdell.
Key to Mineral Identification Charts
If the mineral has a _metallic_ or _submetallic_ luster,
and is: Consult Mineral Chart
white 1
gray 2
yellow 3
brown 4
black 5
If the mineral has a _nonmetallic_ luster,
and is: Consult Mineral Chart
white 6
gray 7
yellow 8
brown 9
black 10
green 11
blue 12
red or pink 13
purple or 14
violet
colorless 15
Mineral Identification Charts
_Chart _Mineral_ _Streak_ _Remarks_ _Hardness_
No._
1. METALLIC luster, WHITE color
A. Does not readily leave mark on paper but can be scratched by
ordinary pocket knife
Native silver Shiny silver Silver-white 2½-3
white, unless color that
tarnished tarnishes to
gray, black, or
yellowish brown;
heavy; can be
flattened when
hit with hammer
2. METALLIC or SUBMETALLIC luster, GRAY color
A. Will leave mark on paper
Argentite Shiny, blackish Lead-gray color 2-2½
to lead gray that tarnishes to
dull black; knife
cuts it smoothly;
heavy; may occur
as masses and
coatings
Galena Grayish black Shiny lead-gray 2½
color; heavy;
cube-shaped
fragments and
crystals
Graphite Black Steel-gray color; 1-2
greasy feel; very
soft; splits into
thin flakes
B. Does not readily leave mark on paper but can be scratched by
ordinary pocket knife
Chalcocite Grayish black Shiny lead-gray 2½-3
color that
tarnishes to dull
black; knife cuts
it smoothly; may
have black sooty
coating; commonly
occurs as compact
or granular masses
Hollandite Black Silvery-gray 4-6
color; may occur
as rounded masses
C. Cannot be scratched by ordinary pocket knife
Braunite Steel gray or Dark steel-gray 6-6½
black color and
submetallic luster
Hematite Dark reddish brown Steel-gray color; 5½-6½
commonly occurs (may be
as granular or softer)
compact masses;
shiny, scaly
variety is
_specular_
hematite; notice
streak
Hollandite Black Silvery-gray 4-6
color; may occur
as rounded masses
3. METALLIC luster, YELLOW color
A. Does not readily leave mark on paper but can be scratched by
ordinary pocket knife
Chalcopyrite Greenish black Brass-yellow or 3½-4
golden-yellow
color that may
tarnish and show
rainbow-like
colors; commonly
massive; notice
streak
Gold Shiny golden Shiny yellow 2½-3
yellow color; extremely
heavy; flattens
when hit with
hammer; notice
streak
B. Cannot be scratched by ordinary pocket knife
Pyrite Black, greenish Shiny, pale 6-6½
black, or golden-yellow or
brownish black brass-yellow
color that may
tarnish; occurs
as grains, as
masses, or as
cubes or other
crystal shapes;
notice hardness
and streak
4. METALLIC or SUBMETALLIC luster, BROWN color
A. Does not readily leave mark on paper but can be scratched by
ordinary pocket knife
Limonite Rusty yellowish Dark-brown color; 5-5½
brown some specimens
have a shiny
black surface;
notice streak
B. Cannot be scratched by an ordinary pocket knife
Cassiterite Pale brown, pale Brown; 6-7
yellow or white submetallic;
heavy; notice
streak
Hematite Dark reddish brown Dark brown color; 5½-6½
commonly occurs (may be
as granular or softer)
compact masses;
notice streak
Limonite Rusty, yellowish Dark brown color; 5-5½
brown some specimens
have a shiny
black surface;
notice streak
5. METALLIC or SUBMETALLIC luster, BLACK color
A. Will leave mark on paper
Argentite Shiny, blackish Lead-gray color 2-2½
to lead grey that tarnishes to
dull black; knife
cuts it smoothly;
heavy; may occur
as masses and
coatings
Graphite Black Greasy feel; very 1-2
soft; splits into
thin flakes
Pyrolusite Black Very soft; will 1-2
soil fingers; may
be powdery
B. Does not readily leave mark on paper but can be scratched by an
ordinary pocket knife
Chalcocite Grayish black Shiny lead-gray 2½-3
color that
tarnishes to dull
black; knife cuts
it smoothly; may
have a black
sooty coating;
commonly occurs
as compact or
granular masses
Hollandite Black May occur as 4-6
rounded masses
Limonite Rusty, yellowish Some specimens 5-5½
brown have shiny black
surface; notice
streak
C. Cannot be scratched by an ordinary pocket knife
Braunite Steel gray or Luster is 6-6½
black submetallic
Cassiterite Pale brown, pale Submetallic 6-7
yellow, or white luster; heavy;
notice streak
Hematite Dark reddish brown Notice streak; 5½-6½
commonly occurs (may be
as granular or softer)
compact masses
Hollandite Black May occur as 4-6
rounded masses
Limonite Rusty yellowish Some specimens 5-5½
brown have shiny black
surface; notice
streak
Magnetite Black Fragments cling 6
to a magnet
Pitchblende Brownish black Brownish black, 5½
greenish black,
or black;
radioactive;
heavy; may appear
dull or greasy
6. NONMETALLIC luster, WHITE color
A. Can be scratched by a fingernail
Cerargyrite Shiny white or Appears waxy; 1-1½
gray knife cuts it
smoothly; turns
violet brown to
black when
exposed to light
Gypsum White Soft; occurs as 2
crystals or as
fibrous,
granular,
compact, or
earthy masses
Talc White Knife cuts it 1
smoothly; feels
soapy or greasy;
splits into thin
flakes
B. Cannot be scratched by a fingernail but can be scratched by a copper
penny
Anhydrite White Commonly occurs 3-3½
as sugary-looking
masses
Barite White Rather heavy; 3-3½
cleavage
fragments may be
flat and slab-like
Calcite White Dilute 3
hydrochloric acid
fizzes on
calcite; perfect
cleavage in 3
directions gives
rhomb-shaped
fragments
Celestite White Not quite as 3-3½
heavy as barite;
crystals commonly
prism-shaped or
flat-looking;
some cleavage
fragments are
flat and slab-like
Halite White Salty taste; 2½
dissolves in
water;
cube-shaped
cleavage fragments
C. Cannot be scratched by a copper penny but can be scratched by an
ordinary pocket knife
Anhydrite White Commonly occurs 3-3½
as sugary-looking
masses
Barite White Rather heavy; 3-3½
cleavage
fragments may be
flat and slab-like
Celestite White Not quite as 3-3½
heavy as barite;
crystals commonly
prism-shaped or
flat-looking;
some cleavage
fragments are
flat and slab-like
Dolomite White Commonly occurs 3½-4
as granular
masses and as
rhomb-shaped
crystals; dilute
hydrochloric acid
may fizz slightly
on dolomite
Fluorite White Cleavage in 4 4
directions can
give fragments
that are shaped
like octahedrons;
crystals commonly
cubes
Opal White Curved, 5-6
conchoidal
fracture; may
appear glassy,
greasy, resinous,
or dull; milky
white and
bluish-white
precious opal
shows plays of
colors
D. Cannot be scratched by an ordinary pocket knife but can be scratched
by quartz
Feldspar White Glassy or pearly 6
luster; good
cleavage in 2
directions that
meet at an angle
of 90° or near
90°; common in
granite and
pegmatite rocks
Opal White Curved, 5-6
conchoidal
fracture; may
appear glassy,
greasy, resinous,
or dull; milky
white and
bluish-white
precious opal
shows plays of
colors
Quartz White Curved conchoidal 7
fracture; occurs
as milky quartz,
chert, and
chalcedony;
crystals commonly
6-sided prisms
with pyramid-like
ends
7. NONMETALLIC luster, GRAY color
A. Can be scratched by a fingernail
Amphibole asbestos White Made up of 1-2½
slender, flexible
fibers that can
be pulled apart
Cerargyrite Shiny white or Appears waxy; 1-1½
gray knife cuts it
smoothly; turns
violet brown to
black when
exposed to light
Gypsum White Soft; occurs as 2
crystals or as
fibrous,
granular,
compact, or
earthy masses
Sulfur White or pale Will burn with a 1½-2½
yellow blue flame;
commonly found as
crystals, crusts,
or grains
Talc White Knife cuts it 1
smoothly; feels
soapy or greasy;
splits into thin
flakes
B. Cannot be scratched by a fingernail but can be scratched by a copper
penny
Amphibole asbestos White Made up of 1-2½
slender, flexible
fibers that can
be pulled apart
Anhydrite White Commonly occurs 3-3½
as sugary-looking
masses
Calcite White Dilute 3
hydrochloric acid
fizzes on
calcite; perfect
cleavage in 3
directions gives
rhomb-shaped
fragments
Celestite White Crystals commonly 3-3½
prism-shaped or
flat-looking;
some cleavage
fragments are
flat and slab-like
Halite White Salty taste; 2½
dissolves in
water;
cube-shaped
cleavage fragments
Sulfur White or pale Will burn with a 1½-2½
yellow blue flame;
commonly found as
crystals, crusts,
or grains
C. Cannot be scratched by a copper penny but can be scratched by an
ordinary pocket knife
Anhydrite White Commonly occurs 3-3½
as sugary-looking
masses
Celestite White Crystals commonly 3-3½
prism-shaped or
flat-looking;
some cleavage
fragments are
flat and slab-like
Dolomite White Commonly occurs 3½-4
as granular
masses and as
rhomb-shaped
crystals; dilute
hydrochloric acid
may fizz slightly
on dolomite
Opal White Curved, 5-6
conchoidal
fracture; may
appear glassy,
greasy, resinous,
or dull
D. Cannot be scratched by an ordinary pocket knife but can be scratched
by quartz
Feldspar White Glassy or pearly 6
luster; good
cleavage in 2
directions that
meet at an angle
of 90° or near
90°; common in
granite and
pegmatite rocks
Opal White Curved, 5-6
conchoidal
fracture; may
appear glassy,
greasy, resinous,
or dull
Quartz White Curved conchoidal 7
fracture; occurs
as chert and
chalcedony
8. NONMETALLIC luster, YELLOW color
A. Can be scratched by a fingernail
Carnotite Yellow Bright canary 2
yellow or lemon
yellow;
radioactive;
occurs as crusts
and powdery masses
Gypsum White Yellowish; soft; 2
occurs as
crystals or as
fibrous,
granular, compact
or earthy masses
Limonite Rusty yellowish Brownish-yellow 1+
brown color; may be
soft and earthy
Muscovite (white White Light colored; 2-2½
mica) splits into thin,
flat, transparent
sheets that will
bend without
breaking
Sulfur White or pale Will burn with a 1½-2½
yellow blue flame;
commonly found as
crystals, crusts,
or grains
Uranophane Light yellow to Yellow to 2-3
light yellow yellow-orange
orange color; radioactive
B. Cannot be scratched by a fingernail but can be scratched by a copper
penny
Barite White Rather heavy; 3-3½
cleavage
fragments may be
flat and slab-like
Calcite White Yellowish; dilute 3
hydrochloric acid
fizzes on
calcite; perfect
cleavage in 3
directions gives
rhomb-shaped
fragments
Muscovite (white White Light colored; 2-2½
mica) splits into thin,
flat, transparent
sheets that will
bend without
breaking
Sulfur White or pale Will burn with a 1½-2½
yellow blue flame;
commonly found as
crystals, crusts,
or grains
Uranophane Light yellow to Yellow to 2-3
light yellow yellow-orange
orange color; radioactive
C. Cannot be scratched by a copper penny but can be scratched by an
ordinary pocket knife
Barite White Rather heavy; 3-3½
cleavage
fragments may be
flat and slab-like
Opal White Curved, 5-6
conchoidal
fracture; may
appear glassy,
greasy, resinous,
or dull
D. Cannot be scratched by an ordinary pocket knife but can be scratched
by quartz
Feldspar White Glassy or pearly 6
luster; good
cleavage in 2
directions that
meet at an angle
of 90° or near 90°
Garnet White Commonly occurs 6½-7
as crystals
Opal White Curved, 5-6
conchoidal
fracture; may
appear glassy,
greasy, resinous,
or dull
Quartz White Curved conchoidal 7
fracture;
brownish-yellow
_smoky_ quartz
crystals commonly
6-sided prisms
with pyramid-like
ends;
_chalcedony_ and
_jasper_ may be a
shade of yellow,
too
9. NONMETALLIC luster, BROWN color
A. Can be scratched by a fingernail
Gypsum White Brownish; soft; 2
occurs as
crystals or as
fibrous,
granular, compact
or earthy masses
Limonite Rusty yellowish May be soft and 1+
brown earthy
Muscovite (white White Light colored; 2-2½
mica) splits into thin,
flat, transparent
sheets that will
bend without
breaking
Sulfur White or pale Will burn with a 1½-2½
yellow blue flame;
commonly found as
crystals, crusts,
or grains
B. Cannot be scratched by a fingernail but can be scratched by a copper
penny
Barite White Rather heavy; 3-3½
cleavage
fragments may be
flat and slab-like
Biotite (black mica) White Dark brown; 2½-3
splits into thin,
flat sheets that
will bend without
breaking
Calcite White Dilute 3
hydrochloric acid
fizzes on
calcite; perfect
cleavage in 3
directions gives
rhomb-shaped
fragments
Halite White Salty taste; 2½
dissolves in
water;
cube-shaped
cleavage fragments
Muscovite (white White Light colored; 2-2½
mica) splits into thin,
flat, transparent
sheets that will
bend without
breaking
Sulfur White or pale Will burn with a 1½-2½
yellow blue flame;
commonly found as
crystals, crusts,
or grains
C. Cannot be scratched by a copper penny but can be scratched by an
ordinary pocket knife
Barite White Rather heavy; 3-3½
cleavage
fragments may be
flat and slab-like
Dolomite White Commonly occurs 3½-4
as granular
masses and as
rhomb-shaped
crystals; dilute
hydrochloric acid
may fizz slightly
on dolomite
Fluorite White Cleavage in 4 4
directions can
give fragments
that are shaped
like octahedrons;
crystals commonly
cube-shaped
D. Cannot be scratched by an ordinary pocket knife but can be scratched
by quartz
Cassiterite Pale brown, pale Brown, reddish 6-7
yellow, or white brown, or
yellowish brown;
heavy; dull to
brilliant luster
Feldspar White Glassy or pearly 6
luster; good
cleavage in 2
directions that
meet at an angle
of 90° or near
90°; common in
granite and
pegmatite rocks
Garnet White Commonly occurs 6½-7
as crystals
Quartz White Curved conchoidal 7
fracture; brown
_smoky_ quartz
crystals commonly
6-sided prisms
with pyramid-like
ends;
_chalcedony_,
_chert_, and
_jasper_ may be a
shade brown, too
Tourmaline White Dark brown 7-7½
variety is
_dravite_; notice
hardness,
striations on
crystals, and
triangular cross
section of some
crystals
E. Cannot be scratched by quartz
Tourmaline White Dark brown 7-7½
variety is
_dravite_; notice
hardness,
striations on
crystals, and
triangular cross
section of some
crystals
10. NONMETALLIC luster, BLACK color
A. Cannot be scratched by a fingernail but can be scratched by a copper
penny
Biotite (black mica) White Splits into thin, 2½-3
flat sheets that
will bend without
breaking
B. Cannot be scratched by an ordinary pocket knife but can be scratched
by quartz
Garnet White Commonly occurs 6½-7
as crystals
Quartz White Curved conchoidal 7
fracture;
brownish-black
smoky quartz
crystals commonly
6-sided prisms
with pyramid-like
ends;
_chalcedony_ and
_chert_ may be
black, too
Tourmaline White Black variety is 7-7½
_schorl_; notice
hardness,
striations on
crystals, and
triangular cross
section of some
crystals
C. Cannot be scratched by quartz
Tourmaline White Black variety is 7-7½
_schorl_; notice
hardness,
striations on
crystals, and
triangular cross
section of some
crystals
11. NONMETALLIC luster, GREEN color
A. Can be scratched by a fingernail
Amphibole asbestos White Made up of 1-2½
slender, flexible
fibers that can
be pulled apart
Cerargyrite Shiny white or Light greenish 1-1½
gray color; appears
waxy; knife cuts
it smoothly;
turns violet
brown to black
when exposed to
light
Muscovite (white White Light colored; 2-2½
mica) splits into thin,
flat, transparent
sheets that will
bend without
breaking
Sulfur White or pale Greenish; will 1½-2½
yellow burn with a blue
flame; commonly
found as
crystals, crusts,
or grains
Talc White Light greenish 1
color; knife cuts
it smoothly;
feels soapy or
greasy; splits
into thin flakes
B. Cannot be scratched by a fingernail but can be scratched by a copper
penny
Amphibole asbestos White Made up of 1-2½
slender, flexible
fibers that can
be pulled apart
Biotite (black mica) White Dark green; 2½-3
splits into thin,
flat, translucent
sheets that will
bend without
breaking
Calcite White Dilute 3
hydrochloric acid
fizzes on
calcite; perfect
cleavage in 3
directions gives
rhomb-shaped
fragments
Halite White Greenish tint; 2½
salty taste;
dissolves in
water;
cube-shaped
cleavage fragments
Muscovite (white White Light colored; 2-2½
mica) splits into thin,
flat, transparent
sheets that will
bend without
breaking
Serpentine White Two kinds: silky 2½-4
and fibrous, waxy
and platy
Sulfur White or pale Greenish; will 1½-2½
yellow burn with a blue
flame; commonly
found as
crystals, crusts,
or grains
C. Cannot be scratched by a copper penny but can be scratched by an
ordinary pocket knife
Fluorite White Cleavage in 4 4
directions can
give fragments
shaped like
octahedrons;
crystals commonly
cubes
Malachite Green Bright green 3½-4
color; dilute
hydrochloric acid
will fizz on
malachite
Serpentine White Two kinds: silky 2½-4
and fibrous, waxy
and platy
D. Cannot be scratched by an ordinary pocket knife but can be scratched
by quartz
Feldspar White Glassy or pearly 6
luster; good
cleavage in 2
directions that
meet at an angle
of 90° or near 90°
Garnet White Commonly occurs 6½-7
as crystals
12. NONMETALLIC luster, BLUE color
A. Cannot be scratched by a fingernail but can be scratched by a copper
penny
Anhydrite White Commonly occurs 3-3½
as sugary-looking
masses
Barite White Rather heavy; 3-3½
cleavage
fragments may be
flat and slab-like
Calcite White Dilute 3
hydrochloric acid
fizzes on
calcite; perfect
cleavage in 3
directions gives
rhomb-shaped
fragments
Celestite White Not quite as 3-3½
heavy as barite;
crystals commonly
prism-shaped or
flat-looking;
some cleavage
fragments are
flat and slab-like
Halite White Salty taste; 2½
dissolves in
water;
cube-shaped
cleavage fragments
B. Cannot be scratched by a copper penny but can be scratched by an
ordinary pocket knife
Anhydrite White Commonly occurs 3-3½
as sugary-looking
masses
Azurite Blue Bright, intense 3½-4
blue color;
dilute
hydrochloric acid
will fizz on
azurite
Barite White Rather heavy; 3-3½
cleavage
fragments may be
flat and slab-like
Celestite White Not quite as 3-3½
heavy as barite;
crystals commonly
prism-shaped or
flat-looking;
some cleavage
fragments are
flat and slab-like
Fluorite White Cleavage in 4 4
directions can
give fragments
that are shaped
like octahedrons;
crystals commonly
cube-shaped
Opal White Curved, 5-6
conchoidal
fracture; may
appear glassy,
greasy, resinous,
or dull; milky
white and
bluish-white
precious opal
shows plays of
colors
C. Cannot be scratched by an ordinary pocket knife but can be scratched
by quartz
Feldspar White Glassy or pearly 6
luster; good
cleavage in 2
directions that
meet at an angle
of 90° or near 90°
Opal White Curved, 5-6
conchoidal
fracture; may
appear glassy,
greasy, resinous,
or dull; milky
white and
bluish-white
precious opal
shows plays of
colors
Quartz White Curved conchoidal 7
fracture; occurs
as crystalline
quartz and as
bluish
_chalcedony_
D. Cannot be scratched by quartz
Topaz White Perfect basal 8
cleavage gives
flat, plate-like
fragments; notice
hardness
13. NONMETALLIC luster, RED or PINK color
A. Can be scratched by a fingernail
Gypsum White Reddish; soft; 2
occurs as
crystals or as
fibrous,
granular,
compact, or
earthy masses
Hematite Dark reddish brown Brownish-red 1+
color; soft and
earthy
Sulfur White or pale Reddish; will 1½-2½
yellow burn with blue
flame; commonly
found as
crystals, crusts,
or grains
B. Cannot be scratched by a fingernail but can be scratched by a copper
penny
Anhydrite White Pinkish tint; 3-3½
commonly occurs
as sugary-looking
masses
Barite White Pinkish tint; 3-3½
rather heavy;
cleavage
fragments may be
flat and slab-like
Calcite White Pink color; 3
dilute
hydrochloric acid
fizzes on
calcite; perfect
cleavage in 3
directions gives
rhomb-shaped
fragments
Cinnabar Dark red Dark red or 2½
bright
yellowish-red
color; shiny,
brilliant luster
when pure; dull
and earthy when
impure; heavy
Halite White Reddish tint; 2½
salty taste;
dissolves in
water;
cube-shaped
cleavage fragments
Sulfur White or pale Reddish; will 1½-2½
yellow burn with blue
flame; commonly
found as
crystals, crusts,
or grains
C. Cannot be scratched by a copper penny but can be scratched by an
ordinary pocket knife
Anhydrite White Pinkish tint; 3-3½
commonly occurs
as sugary-looking
masses
Barite White Pinkish tint; 3-3½
rather heavy;
cleavage
fragments may
look flat and
slab-like
Dolomite White Pink color; 3½-4
commonly occurs
as granular
masses and as
rhomb-shaped
crystals; dilute
hydrochloric acid
may fizz slightly
on dolomite
Fluorite White Pink color; 4
cleavage in 4
directions can
give fragments
that are shaped
like octahedrons;
crystals commonly
cubes
Opal White Reddish color; 5-6
curved,
conchoidal
fracture; may
appear glassy,
greasy, resinous,
or dull
D. Cannot be scratched by an ordinary pocket knife but can be scratched
by quartz
Feldspar White Glassy or pearly 6
luster; good
cleavage in 2
directions that
meet at an angle
of 90° or near 90°
Garnet White Commonly occurs 6½-7
as crystals
Opal White Reddish color; 5-6
curved,
conchoidal
fracture; may
appear glassy,
greasy, resinous,
or dull
Quartz White Curved, 7
conchoidal
fracture; occurs
as _rose quartz_,
as pink _chert_,
and as _agate_
and _jasper_
14. NONMETALLIC luster, PURPLE or VIOLET color
A. Cannot be scratched by a copper penny but can be scratched by an
ordinary pocket knife
Fluorite White Cleavage in 4 4
directions can
give fragments
that are shaped
like octahedrons;
crystals commonly
cubes
B. Cannot be scratched by an ordinary pocket knife but can be scratched
by quartz
Quartz, variety: White Curved, 7
amethyst conchoidal
fracture;
_amethyst_
crystals commonly
6-sided prisms
with pyramid-like
ends
15. NONMETALLIC luster, COLORLESS
A. Can be scratched by a fingernail
Cerargyrite Shiny white or Appears waxy; 1-1½
gray knife cuts it
smoothly; turns
violet brown to
black when
exposed to light
Gypsum White Transparent 2
selenite variety
commonly occurs
as flat,
diamond-shaped
crystals; splits
into thin, flat
sheets that will
not bend without
breaking
Muscovite (white White Splits into thin, 2-2½
mica) flat, transparent
sheets that will
bend without
breaking
B. Cannot be scratched by a fingernail but can be scratched by a copper
penny
Barite White Rather heavy; 3-3½
cleavage
fragments may be
flat and slab-like
Calcite White Dilute 3
hydrochloric acid
fizzes on
calcite; perfect
cleavage in 3
directions gives
rhomb-shaped
fragments
Celestite White Not quite as 3-3½
heavy as barite;
crystals commonly
prism-shaped or
flat-looking;
some cleavage
fragments are
flat and slab-like
Halite White Salty taste; 2½
dissolves in
water;
cube-shaped
cleavage fragments
Muscovite (white White Splits into thin, 2-2½
mica) flat, transparent
sheets that will
bend without
breaking
C. Cannot be scratched by a copper penny but can be scratched by an
ordinary pocket knife
Barite White Rather heavy; 3-3½
cleavage
fragments may be
flat and slab-like
Celestite White Not quite as 3-3½
heavy as barite;
crystals commonly
prism-shaped or
flat-looking;
some cleavage
fragments are
flat and slab-like
Dolomite White Commonly occurs 3½-4
as granular
masses and as
rhomb-shaped
crystals; dilute
hydrochloric acid
may fizz slightly
on dolomite
Fluorite White Cleavage in 4 4
directions can
give fragments
that are shaped
like octahedrons;
crystals commonly
cubes
Opal White Curved, 5-6
conchoidal
fracture;
transparent
_hyalite_ variety
resembles ice
D. Cannot be scratched by an ordinary pocket knife but can be scratched
by quartz
Opal White Curved, 5-6
conchoidal
fracture;
transparent
_hyalite_ variety
resembles ice
Quartz White Curved, 7
conchoidal
fracture; _rock
crystal_ quartz
commonly 6-sided
prism with
pyramid-like ends
E. Cannot be scratched by quartz
Topaz White Perfect basal 8
cleavage gives
flat, plate-like
fragments; notice
hardness
How To Use the Rock Identification Charts
In the rock identification charts on pages 40-41, the Texas rocks
described in this book are arranged in four major groups according to
their texture.
1. _Glassy_ (the rocks are smooth, dark, and shiny)
2. _Compact, dull, or stony_ (the rocks are smooth and dull, but the
individual grains are too small to be recognized)
3. _Granular_ (at least some of the individual grains of the rocks are
large enough to be seen without a magnifying glass)
4. _Fragmental_ (the rocks are made up of fragments that are either
loose or cemented together)
Consult Rock Chart 1, if the rock is glassy; Chart 2, if it is compact,
dull, or stony; Chart 3, if it is granular; and Chart 4, if it is
fragmental.
Two of the rock charts are subdivided. In Rock Chart 2, the compact,
dull, or stony rocks are arranged according to hardness as follows:
A. Rocks that can be scratched by a fingernail
B. Rocks that cannot be scratched by a fingernail but can be scratched
by an ordinary pocket knife
C. Rocks that cannot be scratched by an ordinary pocket knife
In Rock Chart 3, the granular rocks also are arranged according to
_hardness_ into:
A. Rocks that can be scratched by an ordinary pocket knife
B. Rocks that cannot be scratched by an ordinary pocket knife
These harder rocks are subdivided into three groups:
1. Those that have grains of about equal size
2. Those with large easily seen grains that are scattered through a
mass of finer grains
3. Those rocks whose grains are arranged in layers
In the “remarks” column of the rock identification charts are included
further tests that will aid you in identifying the rock.
For a more complete rock determination chart, you can consult a
textbook, such as _Rocks and Rock Minerals_, by L. V. Pirsson and A.
Knopf.
Rock Identification Charts
_Chart _Rock_ _Remarks_
No._
1. GLASSY appearance (rock is dark, smooth, and shiny)
Obsidian Entire rock is glassy
Vitrophyre Crystalline grains are scattered
through the dark glassy mass
2. COMPACT, DULL, OR STONY appearance (individual grains too small to be
recognized)
A. Can be scratched by a fingernail
Chalk Dilute hydrochloric acid fizzes on it
Clay Earthy odor when breathed on
Diatomite Crumbly
Rock gypsum Made up of the mineral gypsum
Soapstone Soapy or greasy feel
B. Cannot be scratched by a fingernail but can be scratched by
ordinary pocket knife
Dolomite Dilute hydrochloric acid may fizz
slightly on it; will fizz if rock is
powdered
Limestone Dilute hydrochloric acid fizzes on it
Serpentine rock Commonly some shade of green
Shale Breaks in flat, thin flakes; earthy odor
C. Cannot be scratched by an ordinary pocket knife
Basalt Dark colored and heavy
Chert Hard, smooth, and porcelain-like
Rhyolite Light to dark colored; may show streaks
or flow structure
3. GRANULAR appearance (at least some of the individual grains are large
enough to be seen without a magnifying glass)
A. Can be scratched by an ordinary pocket knife
Limestone Dilute hydrochloric acid will fizz on it
Marble Dilute hydrochloric acid will fizz on
calcite marble, and it may fizz
slightly on dolomite marble
Rock gypsum Made up of the mineral gypsum
Rock salt Has a salty taste; made up of the
mineral _halite_
B. Generally cannot be scratched by an ordinary pocket knife (some
schist is softer)
Grains are of about equal size (equigranular)
1.
Granite Quartz and feldspar grains interlock
Pegmatite Large interlocking grains; commonly
feldspar, quartz, mica
Quartzite Rock breaks across the quartz grains
Sandstone Rock breaks through the cement but
around the sand grains
Easily seen grains are scattered through a mass of finer grains
2.
Basalt Dark colored, heavy rock
Llanite Rusty pink feldspar and blue quartz
grains embedded in a brownish rock mass
Rhyolite porphyry Light to dark colored rock; may show
streaks or flow structure
Grains are arranged in layers
3.
Gneiss Interlocking grains are in straight or
wavy bands
Schist Splits in thin layers; some schist is
soft enough to be scratched by a knife
4. FRAGMENTAL appearance (rocks are made up of fragments that are either
loose or cemented together)
Breccia Angular, gravel-size fragments that are
cemented together
Conglomerate Rounded, gravel-size fragments that are
cemented together
Coquina Shells and shell fragments that are
cemented together
Gravel Loose fragments
Pulverulent Loose, powdery fragments; dilute
limestone hydrochloric acid fizzes on them
Sand Loose fragments no larger than a pinhead
Sandstone Sand fragments that are cemented
together
Volcanic ash Loose, fine, gritty particles
[Illustration: Physiographic outline map of Texas.]
DESCRIPTIONS OF SOME TEXAS ROCKS AND MINERALS
The pages that follow contain descriptions of Texas rocks and minerals.
The descriptions are given in alphabetical order, except that related
varieties are described together. For example, agate, amethyst, chert,
flint, jasper, onyx, and chalcedony are discussed under quartz, because
they are varieties of quartz. The descriptions include the properties of
the rock or mineral that will help you identify it and also include
information on where the rock or mineral can be found in Texas, some of
its uses, and how it may have formed. The chart on page 99 lists
chemical composition, specific gravity, and hardness of various Texas
minerals.
Agate. _See_ Quartz.
Agatized Wood. _See_ Quartz.
Alabaster. _See_ Gypsum.
Albite. _See_ Feldspar.
Almandite. _See_ Garnet.
Amethyst. _See_ Quartz.
Amphibole Asbestos. _See_ Asbestos.
Anhydrite
Anhydrite, calcium sulfate, is a rather soft mineral that you can
scratch with a pocket knife, although not with a fingernail. It has a
glassy or a pearly luster and is transparent or translucent. Most
anhydrite is white, but impurities cause it to be grayish, bluish, or
reddish. When rubbed across a streak plate, anhydrite gives a white
streak. This mineral has an uneven fracture, and it cleaves in three
directions that are at right angles to each other. It commonly occurs as
rectangular cleavage fragments or as sugary crystalline masses.
Anhydrite resembles dolomite, limestone, or gypsum. You can use a
hardness test to distinguish it from gypsum (anhydrite is harder) and an
acid test to distinguish it from limestone and dolomite. A drop of
dilute hydrochloric acid will fizz when you put it on limestone or
powdered dolomite. On anhydrite, the acid does not fizz.
[Illustration: Massive anhydrite.]
Anhydrite occurs at several places in Texas. It is, for example, seen in
bluffs along the Double Mountain Fork and the Salt Fork of the Brazos
River in north-central Texas. Most of the Texas anhydrite, however,
occurs underground. In the Gulf Coastal Plain, the anhydrite is found
below the surface in salt domes. (Salt domes are described with halite
on p. 66 and with sulfur on p. 91.)
Another anhydrite locality is in the subsurface Permian basin of west
Texas. Oil wells drilled in this basin penetrate great, thick deposits
of massive anhydrite. The anhydrite was deposited during the Permian
Period from a sea that covered this area. As the sea gradually
evaporated, the mineral matter that was dissolved in it came out of
solution to form anhydrite, halite, and several other minerals.
Antigorite. _See_ Serpentine.
Argentite. _See_ Silver Minerals.
Asbestos
Asbestos is not really any one particular mineral. It is the name given
to several minerals that occur in masses of slender, delicate fibers. In
the more typical kinds of asbestos, these fibers—when pulled
apart—resemble soft, fluffy, silk strings.
Several small deposits of _amphibole asbestos_ have been found in the
Llano uplift area of central Texas. This asbestos is a variety of the
mineral _tremolite_, a calcium-magnesium silicate. It has fibers that
break rather easily, and it has a silky luster. It is a shade of green
or gray and gives a white streak when rubbed across a streak plate. When
you pull its fibers apart, you actually are breaking the mineral along
its two directions of perfect cleavage. This amphibole asbestos is
softer than other varieties of the mineral tremolite—a copper penny
scratches it easily.
[Illustration: Greenish, silky amphibole asbestos from northeastern
Gillespie County, Texas.]
The asbestos occurs in veins in Precambrian metamorphic rocks in
southern Llano County, northwestern Blanco County, and northeastern
Gillespie County. These deposits are small.
A variety of the mineral _serpentine_ called _chrysotile asbestos_ is
the kind most used by industry. Its fibers are commonly flexible enough
and strong enough to be woven into cloth. This cloth is made into
articles, such as fireproof suits, gloves, and theater curtains. Some
chrysotile has been found in Precambrian metamorphic rocks in
northwestern Blanco County, but it does not break into fibers fine
enough or flexible enough to be called asbestos.
Azurite. _See_ Copper Minerals.
Barite
Barite, barium sulfate, is a fairly common mineral in Texas. It has a
glassy or a pearly luster, and it is transparent to translucent. Barite
is colorless, white, brownish, bluish, yellowish, or reddish. When
rubbed across a streak plate, it gives a white streak. It is not
extremely hard—you can scratch it with a pocket knife, although not with
a fingernail.
Barite is distinctive because of its weight and cleavage. It cleaves in
three directions, and some cleavage fragments are flat or platy. For a
mineral with a nonmetallic luster, barite is heavy—it has a specific
gravity of 4.5.
[Illustration: Barite cleavage fragment from west Texas.]
Barite commonly occurs as prism-shaped and as flat crystals, as granular
masses, as cleavable masses, and as rounded masses called _nodules_. In
Texas, some of it was deposited in sedimentary rocks by underground
waters. As the waters seeped through these rocks, mineral matter came
out of solution to form the barite. Some of the barite in Texas also
formed from solutions that came from hot magmas.
A number of barite deposits have been found in Texas, but many of them
are small. Barite occurs in Precambrian metamorphic rocks in Gillespie
and Llano counties, in Pennsylvanian shale in Brewster County, in
Permian shales in Baylor and Taylor counties, and in Permian limestones
in Culberson County. It is found in Triassic red shales in Howard County
and in Cretaceous sedimentary rocks in Brewster, Brown, Hudspeth, Jeff
Davis, Kinney, and Val Verde counties. In Live Oak County, barite occurs
in Tertiary bentonitic clays. Barite is being mined from a deposit in
the Seven Heart Gap area northeast of Van Horn in Culberson County.
Barite is used in a number of ways. It is a source of barium chemicals,
and it also is powdered and used as an ingredient in paint. The oil
industry uses large amounts of barite. In drilling for oil by the rotary
method, water and muds are pumped down the hole to aid drilling. Barite
is added to these drilling fluids to make them heavy, since
high-pressure gases are not as likely to blow heavy fluids out of the
hole.
Basalt
Basalt is a heavy igneous rock that is black, dark gray, or dark brown.
This rock is made up chiefly of a feldspar mineral, such as
_labradorite_, and a pyroxene mineral, such as _augite_. Other minerals
may be present.
The mineral grains of some basalts are so small that you cannot
distinguish them even with a magnifying glass. Other basalts, however,
are _porphyritic_, which means that they contain larger, easily seen
crystals and grains of feldspar and pyroxene scattered either through a
mass of the small mineral grains or through glassy material.
Some basalts contain many small holes. These holes, called _vesicles_,
were formed when bubbles of gas were trapped in the hardening magma.
Later, solutions moving through the rocks may have deposited another
mineral—such as calcite or chalcedony—in some of the vesicles.
Basalt forms from molten rock material that hardens either on or beneath
the surface—it can be extrusive or intrusive. Much of the basalt now
found in the Trans-Pecos country of west Texas formed from lava that
flowed out onto the surface during the Tertiary Period. A few of the
places where basalt occurs in west Texas are the Chinati Mountains of
Presidio County, the Chisos Mountains of Brewster County, the Davis
Mountains of Jeff Davis County, and the Van Horn Mountains of Culberson
and Hudspeth counties.
[Illustration: Basalt from Brewster County, Texas.]
Several varieties of basalt occur in the Balcones fault region of
Bandera, Comal, Hays, Kinney, Medina, Travis, and Uvalde counties. These
basalts formed from molten magma that forced its way into rocks just
below the earth’s surface.
Some basalt, which is known commercially as _trap rock_, is produced in
Uvalde County. It is crushed and used for railroad ballast, road
building material, and as concrete aggregate.
Bentonite. _See_ Clay.
Biotite. _See_ Mica.
Braunite. _See_ Manganese Minerals.
Calcite
Calcite, calcium carbonate, is one of the most abundant minerals in
Texas. It is the chief mineral in limestone and in some marble. It also
serves as the cementing material in many sandstones. Crystals, grains,
and cleavable masses of calcite, which have been deposited by
underground water, occur in cracks and cavities in many of the igneous,
metamorphic, and sedimentary rocks of Texas. Calcite also occurs as
cave, spring, and stream deposits and as caliche.
Calcite is transparent or translucent, and—depending on the variety—its
luster is glassy to earthy. Most calcite is white or colorless, but it
can be a shade of pink, blue, green, brown, yellow, or gray. It gives a
white streak when you rub it across a streak plate.
Two properties of calcite to notice are the hardness and the cleavage.
This mineral cleaves perfectly in three directions that are not at right
angles to each other, and some of the cleavage fragments are
rhombohedrons. Calcite is rather soft—you can scratch it with a copper
penny but not with a fingernail. A drop or two of dilute hydrochloric
acid also will help you to identify this mineral. The acid will readily
fizz and bubble when it is placed on calcite.
[Illustration: Calcite has perfect rhombohedral cleavage. The three
directions of cleavage are not at right angles to each other.]
Calcite occurs in more different kinds of crystal shapes than any other
mineral. Some of these crystals are flat and tabular; some are
rhombohedrons; some are prisms. Pointed crystals, called _dog-tooth
spar_, and twinned crystals have been found in the Terlingua area of
Brewster County in west Texas. A somewhat unusual occurrence of calcite
crystals is in geodes. Some of these are found in Lower Cretaceous rocks
west of Austin in Travis County.
Transparent crystals and cleavage fragments of calcite show a property
called _double refraction_ (other minerals show it, too). To test this
property, you can mark a single dot on paper. When you look at the dot
through a piece of clear calcite, you will see two dots instead of one.
This happens because a ray of light is bent (refracted) and is split
into two rays as it enters the mineral. These two rays travel through
the calcite in slightly different directions, and each carries an image
of the dot through the mineral. The two images that you see are at the
points where the two rays leave the calcite.
Calcite that is deposited at springs, along river and creek banks, and
in caves is known as _travertine_. Cave forms of travertine, including
stalagmites and stalactites, occur in several caves in Texas. Another
kind of travertine is called _calcareous tufa_ or _calcareous sinter_.
It is a porous and spongy-looking material deposited from water carrying
dissolved limestone and is found around the openings of some springs and
along some creek and river banks.
A dull, earthy calcite deposit, known as _caliche_, occurs in areas of
Texas that have scant rainfall, such as the High Plains, west Texas, and
the southwestern part of the Gulf Coastal Plain. Caliche commonly is
found mixed with other materials, such as clay, sand, or gravel. This
substance may be firm and compact or loose and powdery.
It is thought that caliche forms when ground moisture, containing
dissolved calcium bicarbonate, moves upward. In dry areas of the
country, this moisture evaporates. As it does, it leaves a crust of
calcium carbonate in the form of caliche on or near the surface of the
ground.
[Illustration: Calcite crystals (dog-tooth spar) from the Terlingua
area of Brewster County, Texas.]
Caliche is quarried in many counties in Texas and is used chiefly as
road material and as an aggregate.
Caliche. _See_ Calcite.
Carnotite. _See_ Uranium Minerals.
Cassiterite
Cassiterite, tin dioxide, is the mineral that serves as the chief source
of tin. Tin does not corrode and tarnish, and one of its main uses is in
the making of tin cans. (Actually, our tin cans are made from thin
sheets of steel that have been coated with a protective layer of tin.)
Cassiterite has either a nonmetallic or a submetallic luster. Some
specimens are brilliant and shiny; others are dull. Cassiterite may be
translucent to transparent. It may be black, brown, gray, reddish brown,
or yellowish brown. When rubbed across a streak plate, this mineral
leaves a pale brown, a pale yellow, or a white streak. Cassiterite is
quite heavy—it has a specific gravity of 6.8 to 7.1. It is too hard to
be scratched by an average pocket knife.
Sometimes, prospectors use a chemical test to help them identify
cassiterite. They put small pieces of metallic zinc into a jar or test
tube containing dilute hydrochloric acid. Then they add a few fragments
of the mineral that they suspect is cassiterite. If the fragments are
cassiterite, they become covered with a pale gray coating of metallic
tin.
Cassiterite’s most common crystal shape is a short, 8-sided prism with
pyramids at each end, but perfect crystals are not often found. Most
Texas cassiterite does not show a crystal shape. Instead, it occurs as
crystalline masses in igneous rocks and as loose pebbles that have
weathered out of these rocks.
Cassiterite occurs in a number of places in the United States but not in
large quantities. A small amount of cassiterite has been found in quartz
veins in Precambrian granite in both central Texas and west Texas. In El
Paso County, the cassiterite is found on the east side of the Franklin
Mountains a few miles north of El Paso, where some of it has been mined.
In central Texas, cassiterite occurs in the Streeter area of Mason
County.
When the granite rocks in these areas were formed, probably not all of
the hot magmas cooled and hardened at the same time. The fluids given
off by the remaining magmas contained tin and several other elements. It
is believed that these fluids moved up into cracks in the granite rocks
and formed the cassiterite.
Celestite
Celestite is a strontium sulfate mineral. It is colorless, white,
yellow, or gray. Light blue specimens of this mineral also are found,
and it is because of this sky-like color that celestite gets its name.
The word celestite comes from the Latin word _caelestis_, meaning _of
the sky_.
Celestite has a glassy to a pearly luster, and it is either transparent
or translucent. It gives a white streak when rubbed across a streak
plate. Celestite has a specific gravity of 3.95 to 3.97. It is, however,
lighter than barite, a mineral that it resembles. Celestite is not very
hard—a knife will scratch it, although your fingernail will not. It
cleaves in three directions, and some of the fragments are flat and
slabby.
[Illustration: Celestite cleavage fragment from Lampasas County, Texas.]
Celestite occurs commonly either as prism-shaped or flat crystals and as
cleavable, granular, or fibrous crystalline masses. In Texas, it is
found in geodes, as rounded nodules, or as bedded or layer-like deposits
in limestones and other sedimentary rocks. In Real County, celestite
occurs on the walls of a cave in Cretaceous limestone.
Some celestite may be deposited by sea water, but much of the Texas
celestite is believed to have been deposited by underground water that
seeped through cracks and pores in the limestones and other sedimentary
rocks. This water picked up and dissolved strontium compounds that were
scattered in small amounts through the rocks. Then, it re-deposited the
strontium in the rocks as celestite.
In Texas, beds of celestite occur in Permian rocks in Coke, Fisher, and
Nolan counties and in Lower Cretaceous rocks in Brown, Comanche, and
Mills counties. Celestite geodes and nodules are found in Lower
Cretaceous limestone rocks in Lampasas, Travis, and Williamson counties,
and in Permian rocks in Coke, Fisher, Nolan, and Taylor counties.
Celestite is one of two minerals (the other mineral is _strontianite_,
strontium carbonate) used as a source of strontium. Strontium compounds
give a crimson-red color to a flame, so they are used in fireworks,
tracer bullets, and flares. Perhaps you have seen a red flare set out on
the highway at night to warn motorists that a truck has stalled. The
chances are good that the flare’s red flame was due to a strontium
compound. Some of the Texas celestite has been mined, but most of the
strontium minerals now used in the United States are imported from
England and Mexico.
Cerargyrite. _See_ Silver Minerals.
Chalcedony. _See_ Quartz.
Chalcocite. _See_ Copper Minerals.
Chalcopyrite. _See_ Copper Minerals.
Chalk. _See_ Limestone.
Chert (Flint). _See_ Quartz.
Chrysotile. _See_ Asbestos; Serpentine.
Cinnabar
Cinnabar, which is mercuric sulfide, is the most common mercury mineral.
It has a dark red or a bright yellowish-red color and is transparent to
translucent. When rubbed across a streak plate, it leaves a dark red
streak. If pure, cinnabar has a brilliant, shiny, nonmetallic luster. It
is, however, commonly found mixed with impurities, such as clay,
calcite, iron oxide, or bituminous material, and then it looks dull and
earthy. Cinnabar is quite heavy—it has a specific gravity of 8.10. It is
rather soft, and you can scratch it with a copper penny.
Some prospectors use a quick chemical test to identify cinnabar. They
rub a clean, shiny copper coin with a mineral sample that has been
moistened with a drop or two of dilute hydrochloric acid. If the sample
is cinnabar, a light silvery-gray coating appears on the coin.
Cinnabar occurs as small crystals or as fine-grained or compact
crystalline masses. It is found in veins that fill cracks in rocks and
also occurs as crusts and coatings on rocks. It also may be widely
scattered through rocks, such as limestones.
Cinnabar occurs in the Terlingua area of Brewster and Presidio counties
in west Texas. It has been mined there, off and on, since about 1894,
and during this time, mercury worth many millions of dollars has been
produced.
Most of this west Texas cinnabar is found in cracks, pores, and
breccia-filled cavities of Cretaceous limestones and clays. If you will
look at the Texas geologic map (pp. 4-5), you will see that igneous
rocks occur in this district. Many millions of years ago during the
Tertiary Period, when these igneous rocks were still hot magma, some of
them pushed up under the Cretaceous rocks and emitted fluids containing
mercury. The fluids moved upward through cracks and pores in the
Cretaceous rocks where they deposited the mercury as cinnabar and as
other mercury minerals.
[Illustration: Cinnabar crystals (dark) with calcite crystals (white)
from the Terlingua area of Brewster County, Texas.]
Mercury is an unusual element. Instead of occurring as a solid metal at
ordinary room temperatures, as do gold, silver, and lead, it remains a
liquid until it is cooled to 38 degrees below zero Fahrenheit. Because
the silvery little drops of liquid mercury roll about as if they were
alive, this element long has been called _quicksilver_.
Mercury is used in a variety of ways. In some noiseless light-switches,
a glass tube containing a small ball of mercury tilts when the switch is
turned “on.” The mercury then rolls to the end of the tube that contains
electrical contacts and quietly completes the electrical circuit. In
other uses, mercury is added to silver, tin, and other metals to make
fillings for teeth. Some medicines, such as calomel and mercurochrome,
contain mercury. Fulminate of mercury helps to set off dynamite and
other explosives. Mercury is used in many barometers and thermometers,
and farmers use mercury poisons to control insects and fungi.
Mercury also commonly is used to obtain gold from its ores. One method
of accomplishing this is to pass wet gold-bearing gravel or crushed rock
over metal plates that are coated with mercury. The gold particles
quickly mix with the mercury to form an _amalgam_, which later can be
scraped off the plates. The gold is then recovered by heating the
amalgam to drive off the mercury.
Clay
Clay is a smooth, soft, earthy rock made up of mineral particles no
bigger than specks of dust. Some of the particles are clay minerals,
which consist of aluminum, silicon, and other elements. In addition,
tiny particles of quartz, calcite, and other minerals may also be
present in the clay.
The clay particles are all that remain of rocks and of minerals, such as
feldspar, that have been broken into fragments or altered into clay
minerals by weathering. Some clay remains at the place where it formed,
but some is carried away and deposited elsewhere.
Clay is white, tan, brown, red, green, blue, gray—almost any color. When
moist, it has an earthy odor. You can moisten a piece of clay enough to
notice this just by breathing on it. Most clays, when wet, can be molded
into many different shapes—that is, they are plastic, but when they are
dry, they are firm and solid.
Clay is abundant in Texas and has a number of uses. Some goes to make
portland cement, and some is baked or burned in a kiln to make brick,
tile, sewer pipes, pottery, and other products. This kind of clay is
obtained from Tertiary formations of the Gulf Coastal Plain, from Upper
Cretaceous formations in central Texas, and from Pennsylvanian
formations in north-central Texas. (You can locate Tertiary, Cretaceous,
and Pennsylvanian rocks on the Texas geologic map, pp. 4-5.)
A special kind of white burning clay that can be used to make chinaware
is called _kaolin_ or _china clay_. It contains particles of the clay
mineral _kaolinite_ as well as several other clay minerals. Deposits of
china clay occur in southern Jeff Davis County and in Real County near
Leakey, but none is being produced.
Another kind of clay, _bentonite_, forms from weathered volcanic ash.
Bentonite contains the clay mineral _montmorillonite_ and looks smooth
and soap-like. Fresh samples of this clay are white, pale green, or pale
blue, but dried-out or weathered samples are tan, brown, yellow, or
reddish. When wet, bentonite absorbs water, swells, and then has a
jelly-like appearance.
Surface deposits of bentonite occur chiefly in Eocene Tertiary
formations of the Gulf Coastal Plain, in Cretaceous formations of the
Big Bend area of west Texas, and in Quaternary formations of the High
Plains.
[Illustration: Bentonite is used as a drilling-fluid additive in the
rotary method of drilling for petroleum and gas.]
Some bentonite is used to absorb unwanted coloring material in petroleum
and in vegetable oils. It is then known as a _bleaching clay_. Bentonite
bleaching clay is obtained from some of the Tertiary formations along
the Texas Gulf Coastal Plain. It has been produced in Angelina, Fayette,
Gonzales, Jasper, Walker, and other counties in this area.
Another important use of bentonite, and of other clay, too, is as
_drilling mud_. In the rotary method of drilling for oil and gas, mud is
pumped down into the drilled hole. This mud carries the rock cuttings up
to the surface, it cools the drilling tools, and it coats and seals the
walls of the hole. Along the Gulf Coastal Plain, drilling clay is
obtained from Tertiary formations.
Common Opal. _See_ Opal.
Copper Minerals (Chalcocite, Chalcopyrite, Malachite, Azurite)
A number of minerals containing copper, such as _chalcocite_,
_chalcopyrite_, _malachite_, and _azurite_, occur in small deposits in
Texas. They are found chiefly in the Llano uplift area of central Texas,
in the Van Horn area of Culberson and Hudspeth counties in west Texas,
and in a group of counties in north-central Texas.
Copper is an important element. Because it is an unusually good
conductor of electricity (only silver, which costs much more, is a
better one), it is used for many kinds of wires for switchboards,
generators, motors, telephone and telegraph equipment, and light and
power lines.
Manufacturers commonly combine copper with other elements. For example,
some copper is mixed with zinc to make _brass_ and with tin and a little
zinc to make _bronze_. These mixtures are called _alloys_. Many products
are made from copper alloys, including tubing, pipes, jewelry, pots, and
pans. Even our coins contain copper.
Sometimes, a prospector uses a chemical test to find out if copper is
present in a mineral. First, he crushes a small sample of what he
believes is a copper mineral (such as chalcocite, chalcopyrite, azurite,
or malachite). He then puts the sample in a glass jar or test tube and
pours in a small amount of dilute nitric acid (this acid, like
hydrochloric acid, is poisonous). After the sample has dissolved in the
acid, he adds enough ammonium hydroxide to make the solution alkaline.
If the sample is a copper mineral, the solution turns a deep-blue color.
One of the copper minerals, _chalcocite_, copper sulfide, also is known
as _copper glance_. It is a metallic mineral that commonly tarnishes to
a dull black. By chipping off a fragment to obtain a fresh surface, you
will see that it has a shiny lead-gray color. Chalcocite is rather soft,
and it is sectile, that is, a knife will cut through it as well as
scratch it. When you rub chalcocite across a streak plate, it gives a
grayish-black streak. This mineral commonly occurs as compact masses or
as granular masses.
Chalcocite, with its dark color, does not look at all like copper, which
is a bright reddish brown. Chalcocite, however, is the chief copper
mineral at the most important copper mine in Texas, the Hazel mine,
which is about 15 miles northwest of Van Horn in Culberson County in
west Texas. This mine, although now idle and almost filled with water,
has produced about one and a half million pounds of copper along with
more valuable silver ores. Here, the chalcocite and other minerals occur
in material that fills large cracks in red sandstone of the Precambrian
Hazel Formation. It is thought that long ago, molten igneous rock
material far below the surface sent out hot solutions containing copper
and other elements. These solutions moved upward and deposited minerals
in the fracture zone in the sandstone.
Chalcocite occurs also in north-central Texas. It is found in Archer,
Baylor, Clay, Foard, Hardeman, King, Knox, Stonewall, and several other
counties of this area. Here, it occurs in Permian sedimentary rocks
(called “red beds”) as rounded masses, as scattered grains, and as
petrified wood. Because these deposits are far from any igneous rocks,
they apparently did not form in the same way as those at the Hazel mine.
These north-central Texas deposits have never really been commercially
developed. During the Civil War, however, some copper from this area was
made into percussion caps for the Confederacy.
[Illustration: The Hazel copper-silver mine, Culberson County, Texas, as
it appeared in 1951. Photograph by P. T. Flawn.]
Another copper mineral, _chalcopyrite_, is a copper-iron sulfide. It
also is known as _copper pyrites_ and _yellow copper ore_. This mineral
has a metallic luster and a brass-yellow or a golden-yellow color. When
rubbed across a streak plate, it gives a greenish-black streak.
Chalcopyrite will tarnish and then has bronze, blue, purple, and other
rainbow-like colors. This mineral is fairly soft—you can scratch it with
a pocket knife. Because of chalcopyrite’s yellow color, it has often
been mistaken for gold. For this reason, it, like iron pyrite, is often
called _fool’s gold_. (See Gold, p. 60, for ways to tell them apart.)
Chalcopyrite commonly is found in compact masses that show no crystal
shapes. These masses either are scattered through rocks or occur in
material that fills cracks in rocks.
Some chalcopyrite is found in Precambrian sandstone at the Hazel mine
and in other deposits in the Van Horn area of Culberson and Hudspeth
counties. It also occurs in Precambrian rocks at the Sheridan and
Pavitte prospects in Burnet County. These chalcopyrite localities are in
districts where igneous rocks occur.
It is likely that, long ago, hot solutions containing copper moved
upward, out of deeply buried molten magma. While still far below the
surface, the solutions deposited the chalcopyrite in cracks and other
openings in the nearby rocks.
Two copper minerals of Texas, _azurite_ and _malachite_, are copper
carbonates. Azurite is commonly called _chessylite_ and _blue copper_;
malachite is called _green copper carbonate_. Because these minerals are
carbonates, a drop of dilute hydrochloric acid will fizz and bubble when
placed on either of them.
Azurite has a bright, intense blue color and leaves a blue streak when
rubbed across a streak plate. Malachite has a bright green color and
leaves a green streak. These minerals have a nonmetallic luster and a
glassy to dull appearance. Commonly, they are translucent, although some
specimens of azurite are transparent. Both azurite and malachite are
fairly soft—a pocket knife will scratch them, but a copper penny will
not.
Azurite and malachite occur as individual crystals, but you are more
likely to find them as crusts on rocks and on other minerals. Malachite
is also found in rounded fibrous masses that resemble bunches of grapes
(described then as _botryoidal_).
Both azurite and malachite are formed in the same way. Underground
waters seep through rocks that contain deposits of copper minerals (such
as chalcocite and chalcopyrite) and cause chemical reactions which
change these minerals into malachite and azurite.
Malachite is more plentiful than azurite, but both minerals can be found
together. You can expect to find at least one of them at the same
localities where chalcocite, chalcopyrite, and other copper minerals
occur.
Coquina. _See_ Limestone.
Diatomite. _See_ Opal.
Dolomite
Dolomite is the name given both to a rock and to a mineral. The mineral
is a calcium-magnesium carbonate and has a glassy or a pearly luster. It
is any of a number of colors, such as white, pink, brown, or gray, or it
can be colorless. Dolomite leaves a white streak on a streak plate and
is transparent to translucent. It is not particularly hard and can be
scratched with a pocket knife, although not with a copper penny.
Dolomite cleaves perfectly in three directions, and some of the cleavage
fragments are rhombohedrons. However, the cleavages of the individual
mineral grains in specimens of fine-grained massive dolomite are not
readily distinguishable.
[Illustration: Dolomite rock from the vicinity of Fairland, Burnet
County, Texas.]
Most Texas dolomite occurs as coarse-, medium-, and fine-grained
crystalline masses as the chief mineral in dolomite rock and in
dolomitic marble. It is also found as 6-sided crystals that are
rhomb-shaped; when the faces are curved, they have a saddle-like
appearance.
Crystals of the mineral dolomite commonly occur in cavities in the
dolomite rocks. It is believed that they were deposited there by seeping
underground waters. The waters dissolved some of the dolomite in the
rocks and then re-deposited it as crystals.
Dolomite rock is made up mostly of crystalline grains of the mineral
dolomite. In addition, quartz grains, calcite, and other minerals may be
present. Dolomite rock is almost any color—white, buff, pink brown,
gray. It resembles some limestone, and these two rocks actually are
closely related.
To help tell them apart, dilute hydrochloric acid often is used. A few
drops of this acid will readily fizz and bubble if the rock you put them
on is a limestone. If the rock is dolomite, the acid will effervesce
only very little or not at all. (If, however, the acid is put on
powdered dolomite, it then will fizz readily.) Dolomite is slightly
harder than limestone, and it also is slightly heavier.
Some dolomite rocks formed directly from materials that were dissolved
in sea water, and others are altered limestone rocks. Some limestones
altered into dolomite on the sea floor by the addition of magnesium from
the sea water. Others changed into dolomite much later after the sea had
withdrawn and the limestones had become a part of the land; underground
waters containing magnesium seeped through these limestones and altered
them into dolomite.
Many of the dolomite rocks are found with limestones. In Texas they
occur mostly in Cambrian, Ordovician, Mississippian, Pennsylvanian,
Permian, and Cretaceous formations. The geologic map (pp. 4-5) indicates
where these strata appear at the surface in Texas.
Dolomite is abundant in the Llano uplift area of central
Texas—particularly in the Cambrian and Ordovician rocks. A number of
these central Texas dolomites have been quarried for use as building
stones. Some of them also have been crushed and used as a road-building
material and as a stone aggregate that is mixed with cement to make
concrete. This dolomite is also used as terrazzo chips (terrazzo floors
are described with serpentine on p. 88). In addition, Ellenburger
(Ordovician) dolomite from Burnet County was used during World War II as
a source of the lightweight metal magnesium.
Dravite. _See_ Tourmaline.
Feldspar
Feldspar is the name given to a group of nonmetallic minerals that are
much alike. Several of them are so similar that a petrographic
microscope must be used to tell them apart. Each of the feldspar
minerals is an aluminum silicate. Each of them contains, in addition, at
least one of the following elements: potassium, sodium, calcium, and
barium. The feldspar minerals that are found in Texas include _albite_,
a sodium-aluminum silicate, and _orthoclase_ and _microcline_, which are
both potassium-aluminum silicates.
The feldspar minerals are transparent to translucent and have either
glassy or pearly lusters. They can be white, cream, or a shade of red,
brown, yellow, blue, gray, or green. When you rub a feldspar across a
streak plate, it leaves a white streak. The feldspars are rather hard—a
pocket knife will not scratch them, although a piece of quartz or a
steel file will. These minerals have good cleavage in two directions.
The cleavages meet at an angle of about 90°, so that the cleavage
fragments have square corners.
[Illustration: Feldspar cleavage fragment from Llano County, Texas. The
two directions of good cleavage meet at an angle of about 90°.]
The feldspars are important rock-forming minerals. You can find them in
igneous rocks, such as granite or pegmatite, and in metamorphic rocks,
such as gneiss. They also occur as fragments in sedimentary rocks, such
as some sandstone and conglomerate.
Although the feldspars can originate in other ways, they form mostly
from hot magmas that cool and crystallize into igneous rocks. These
minerals occur in the rocks as grains, as cleavable masses, and as
individual crystals. The crystals may be shaped like prisms, or they may
be flat and slabby.
Good places to look for feldspars are in areas where granites,
pegmatites, and other intrusive igneous rocks appear at the surface. The
pegmatite rocks of Burnet, Gillespie, Llano, and Mason counties in the
Llano uplift area of central Texas, and those of the Van Horn Mountains
in Hudspeth and Culberson counties in west Texas, are especially good
sources of feldspar. Large cleavable masses and crystals that are more
than a foot long are found in some of these rocks.
The feldspars have a number of uses. Some of the pegmatite feldspars
from Llano County in central Texas have been crushed and used as
granules for built-up and composition roofs. In addition, some have been
shipped to Mexico for glass-making. Some of the other uses of feldspar
are in making porcelain, ceramic glazes, and scouring compounds. A few
of the feldspar minerals, such as the variety of microcline known as
_amazonstone_, are used as gemstones.
[Illustration: Microcline feldspar crystals from near Granite Shoals
Lake, Llano County, Texas.]
Fibrous Gypsum. _See_ Gypsum.
Flint. _See_ Quartz.
Fluorite
Fluorite is calcium fluoride. The fluorite that is mined and sent to
market, however, commonly is found mixed with quartz, calcite,
limestone, or other rocks and minerals. Industry calls this mixture
_fluorspar_.
Fluorite is a transparent to translucent mineral that has a glassy
luster. It may be colorless, or it may be white, pink, green, purple,
brown, or blue. Some specimens show more than one color. When you rub
fluorite across a streak plate, it leaves a white streak. This mineral
is not particularly hard—a pocket knife will scratch it, although a
copper penny will not. Fluorite has perfect cleavage in four directions.
By carefully breaking a specimen, you can obtain cleavage fragments that
are shaped like octahedrons.
Fluorite occurs as cleavable masses, as fine or coarse grains, and as
crystals. Most of the crystals are cubes, but some may be octahedrons,
dodecahedrons, or combinations of these.
Fluorite has been found both in west Texas and in central Texas. In the
Llano uplift area of central Texas, it occurs in a number of Precambrian
granite, pegmatite, schist, and gneiss rocks. The most important,
although small, deposit in this area is near Spring Creek a few miles
west of Burnet in Burnet County. Here, prospectors have dug holes and
pits in gneiss and schist rocks and found layers of fluorite in them.
The largest known fluorite deposits in Texas (they are not particularly
large when you compare them with the deposits in Illinois and Kentucky)
are those in the Eagle Mountains of Hudspeth County. This fluorite
occurs in both igneous and sedimentary rocks. Many years ago, probably
during the late part of the Tertiary Period, hot magma far below the
surface gave off liquids and gases containing fluorine. These fluids
moved up through large cracks (called faults) in Cretaceous limestones
and Tertiary igneous rocks and deposited fluorite in them. In places,
beds of limestone have been replaced by fluorite. Some of this west
Texas fluorite has been mined and shipped to market.
[Illustration: Fluorite has octahedral cleavage. The four directions of
perfect cleavage can result in cleavage fragments that are octahedrons.]
Fluorite is extremely important as a flux in steel-making to help the
ingredients of the molten steel blend together. In addition, it combines
with sulfur, phosphorus, and other unwanted substances so that they can
be removed from the steel. Other important uses of fluorite are in
glass-making and in the manufacture of hydrofluoric acid. This acid is
used in the aluminum industry as well as in industries that make
high-octane gasoline, insecticides, and refrigerants for refrigerators
and freezers.
Galena
Galena, lead sulfide, is a shiny, lead-gray, metallic mineral that has a
specific gravity of 7.4 to 7.6. It is soft enough to mark paper, and it
leaves a grayish-black streak on a streak plate. This mineral cleaves
perfectly in three directions, and the cleavage fragments have square
corners—some are cubes.
Galena occurs as cleavable masses, as fine or coarse grains, and as
crystals, most of which are cubes. Galena commonly is associated with
other minerals; for example, some of the west Texas galena either
contains some silver (then called _argentiferous galena_) or occurs with
it. Sphalerite, a zinc mineral, is commonly found with galena.
Galena is an important mineral because it is the chief source of lead.
Compounds of lead, called white lead, red lead, and litharge, are used
as paint pigments. Automobile batteries contain lead plates, and
tetraethyl lead is added to gasoline to keep the car’s motor from
knocking. Some other uses of lead are in bullets, type metal, solder,
and cable coverings.
[Illustration: Galena has perfect cubic cleavage. The three directions
of cleavage are at right angles to each other resulting in cubic
cleavage fragments.]
Galena has been found in several areas of Texas and has been mined in
central and west Texas. None, however, has been produced in recent
years. Most of the galena mined in west Texas was obtained from silver
mines, where the galena was a by-product. Some of the west Texas galena
deposits are at Altuda Mountain east of Alpine in Brewster County, in
the Eagle Mountains and the Quitman Mountains in Hudspeth County, and in
the Chinati Mountains and the Shafter area in Presidio County. Most of
the mining has been from the Shafter area (this area is described with
silver minerals on p. 90).
In central Texas, several small galena deposits have been found in
Blanco, Burnet, and other counties of the Llano uplift area. Some galena
has been mined at Silver Creek in northwestern Burnet County. Here,
galena occurs in cracks and as scattered grains in Cambrian limestones
and sandstones.
It is probable that much of the galena in west Texas and in central
Texas was formed when hot magma forced out solutions containing lead.
These solutions moved up through cracks and other openings in the
subsurface rocks and deposited the galena in them.
Small amounts of galena, which likely had a different origin, have been
found in Fisher, Foard, Hardeman, and Young counties. A little occurs
also in rocks associated with salt in a number of the Gulf Coastal Plain
salt domes.
Garnet
Garnet is not one mineral but is the name given to a group of several
minerals that are very much alike. In fact, it often is impossible to
tell some of them apart without using special laboratory tests.
[Illustration: Garnet crystal forms include: A, trapezohedron; B,
dodecahedron; C and D, combination trapezohedron and dodecahedron.]
The garnet minerals have glassy to resinous lusters and are transparent
or translucent. A pocket knife will not scratch them, and some specimens
are too hard even for quartz to scratch. Two of the garnet minerals most
commonly found in Texas are _almandite_, an iron-aluminum silicate, and
_grossularite_, a calcium-aluminium silicate. Almandite has a deep-red
or a brownish-red color. Grossularite is pale green, brownish yellow,
cinnamon brown, or rose red.
Garnet minerals occur as crystals and as masses that are scattered
through some of the metamorphic and igneous rocks. After they have
weathered out of these rocks, the garnets make up a part of many sands
and sandstones. Because these minerals so commonly occur as crystals, it
is helpful to learn to recognize the crystal shapes.
Garnet minerals are found in the igneous and metamorphic rocks of both
central Texas and west Texas. In central Texas, they occur in ancient
Precambrian schist and pegmatite rocks of the Llano uplift area. Some of
these central Texas garnet localities are in northeastern Mason County,
central and northwestern Llano County, west-central Burnet County, and
northeastern Gillespie County.
In west Texas, garnets occur in metamorphic rocks in the Quitman
Mountains, which are southwest of Sierra Blanca in Hudspeth County, and
in the Mica Mine area, which is south of Van Horn near the
Hudspeth-Culberson County line. Garnets also have been found in igneous
rocks in the Franklin Mountains a few miles north of El Paso in El Paso
County.
Garnets that are found in metamorphic rocks such as schists were formed
when great forces squeezed and heated rocks far below the earth’s
surface. This heat and pressure caused elements in the rocks to join
together into different combinations to form new minerals, such as
garnets. Garnets that occur scattered through igneous rocks, such as
some pegmatites and granites, cooled and crystallized from hot, igneous
fluids when the rocks themselves formed.
Most Texas garnets are not transparent. A few, however, are clear enough
to be used as gemstones. These can be cut, polished, and mounted in
rings, brooches, bracelets, and earrings. Although some garnet is widely
used as an abrasive, none from Texas has been produced for this purpose.
Gneiss
Gneiss is a metamorphic rock that has parallel layers or bands. Some
gneiss is made up of the same minerals (chiefly feldspar and quartz) as
granite, and it is then called _granite gneiss_. Several of the other
kinds of gneiss are known as _mica gneiss_, _conglomerate gneiss_,
_gabbro gneiss_, and _hornblende gneiss_. In order to be a gneiss, a
metamorphic rock has to have bands or layers. These bands may be either
straight or wavy and either wide or narrow. In most gneisses, you will
find a layer made up of long or flat mineral grains next to a layer made
up of the grains of an entirely different mineral. The bands may show
color differences, too. For example, a pink layer made up of feldspar
grains may be found next to a black layer made up of hornblende grains.
The mineral grains interlock as they do in igneous rocks, and they are
generally large enough to be seen without a magnifying glass.
[Illustration: Gneiss from Blanco County, Texas, showing light and dark
bands.]
Gneiss can form from an igneous rock, such as granite, or from a
sedimentary rock, such as sandstone. Heat, fluids, and pressures below
the earth’s surface change these rocks into gneiss.
Gneiss that formed during Precambrian time is now seen at the surface in
both west Texas and central Texas. In west Texas, it occurs principally
in the Van Horn area of Culberson and Hudspeth counties. In central
Texas, it is found in Blanco, Burnet, Gillespie, Llano, and Mason
counties of the Llano uplift area.
One of the Llano uplift rocks is called the Valley Spring Gneiss. It
generally has a light color (much of it is pinkish), and it is believed
to have once been a sandstone. Another gneiss of this area, the Big
Branch Gneiss, which has a medium to dark gray color, occurs in northern
Gillespie and Blanco counties and is an altered igneous rock. Some of
the Texas gneiss rocks are suitable for use as building stones.
Gold
Gold commonly occurs in nature as a single element—gold—but much native
gold has a small amount of some other element, such as silver, copper,
or iron, mixed with it.
Native gold is a shiny, yellow, metallic mineral that does not tarnish,
and it leaves a shiny, golden-yellow streak when you rub it across a
streak plate. If silver is present, the color and streak have a lighter
shade. Pure gold is extremely heavy—its specific gravity is 19.3.
Because it is malleable, this mineral will flatten into a thin sheet
when hammered. It is ductile enough to be drawn out into wires. Gold is
also soft—a pocket knife will scratch it easily. When it is to be used
for ornaments and jewelry, gold is usually mixed with other metal, such
as silver, copper, nickel, or palladium, to make it harder. The amount
of gold that is present is then indicated by _carats_ (or _karats_).
Pure gold is 24 carats. If you have a gold ring that has _14 K_ stamped
inside it, you know that it is made of a mixture of 14 parts gold and 10
parts of other metal.
Gold commonly occurs in nature as plates, scales, or grains. Some of the
grains are large enough to be called _nuggets_. It also is found in a
wire-like shape described as _filiform_, it occurs in a network, called
_reticulate_, and it can have a branching and fern-like shape, described
as _dendritic_. Gold is not often found as individual crystals.
Several other minerals, such as pyrite, chalcopyrite, and mica, are
sometimes mistaken for gold. None of these, however, is malleable and
ductile, and none is nearly as heavy as gold. Pyrite and chalcopyrite
have dark-colored streaks unlike that of gold. Mica cleaves so perfectly
that it can be split into thin, flat sheets, but gold has no cleavage at
all.
The best places to look for gold are in areas near igneous rocks and
along the creeks and rivers that drain these areas. It is thought that
most gold originally was carried up from molten igneous rock by hot
solutions. The solutions moved into cracks and other openings in nearby
rocks and deposited the gold, commonly along with quartz. Later, some of
these gold-bearing rocks weathered away. The gold that the rocks
contained either remained at the spot or was washed into creeks and
rivers. These transported accumulations of loose gold are called _placer
deposits_.
[Illustration: Placer gold in very small quantities has been found in
some of the stream gravels of Texas.]
No really important gold deposit has ever been found in Texas, although
traces and small amounts have been reported in several areas. A little
gold has been found in the Llano uplift area of central Texas. It occurs
in quartz veinlets that cut through some of the Precambrian metamorphic
rocks of Llano, Mason, northeastern Gillespie, and west-central Burnet
counties. Many years ago, a small amount of gold was mined northeast of
Llano in Llano County from the Heath mine. Some gold also has been found
in sands and gravels along streams, such as along Sandy Creek and its
tributaries, in parts of this Llano uplift area.
In the Trans-Pecos country of west Texas, small amounts of gold have
been found in the Van Horn area of Culberson and Hudspeth counties, in
the Quitman Mountains district of Hudspeth County, and in the country
around Shafter in Presidio County. Most of the small quantity of gold
that was mined in west Texas was obtained as a by-product from the
Presidio mine in the Shafter district (described with silver minerals on
p. 90).
Small amounts of gold have been reported from other parts of Texas. Some
of these localities are in Eocene Tertiary sandstones in the Gulf
Coastal Plain, in Cretaceous limestones in Irion, Uvalde, and Williamson
counties, and in sand and gravel in Howard and Taylor counties. None of
these deposits has been found to have any commercial value.
Granite
Granite is an intrusive igneous rock that is made up chiefly of
crystalline grains or crystals of quartz and a feldspar mineral, such as
orthoclase or microcline. Several other minerals, including mica and
hornblende, may also be present.
All of the mineral grains in granite are about the same size, and you
can distinguish them without using a magnifying glass. A granite may be
coarse grained, medium grained, or fine grained. When you examine this
rock, you will see that its grains are not cemented but interlocked like
the pieces of a jigsaw puzzle. The color of granite, which is pink, red,
gray, or brownish, depends chiefly on the color of its feldspar grains.
Most granites formed from hot, molten magma that slowly cooled and
hardened far below the earth’s surface. Because of this slow cooling,
fairly large mineral grains were formed.
[Illustration: Polished section of pink granite from Gillespie County,
Texas.]
Granites are now seen at the surface in several areas of Texas. They
were gradually uncovered as the areas became higher and the overlying
rocks slowly weathered away. One of these areas is the Llano uplift of
central Texas where the granites occur in Blanco, Burnet, Gillespie,
Llano, and Mason counties. These granites formed during Precambrian time
and are believed to be about a billion years old. (Scientists are now
able to determine the age of some rocks accurately by very precisely
measuring the relative amounts of isotopes produced by decay of
radioactive minerals.)
[Illustration: Texas State Capitol building at Austin is made of Burnet
County granite obtained from Granite Mountain near Marble Falls, Texas.]
Granites also appear at the surface in the Trans-Pecos country of west
Texas. Some of these areas include the Franklin Mountains of El Paso
County, the Quitman Mountains of Hudspeth County, the Chisos Mountains
of Brewster County, and the Chinati Mountains of Presidio County.
Red, pink, and gray granites from quarries in the Llano uplift area are
widely used as building stones and monument stones. A large quarry at
Granite Mountain just west of Marble Falls in Burnet County has supplied
pink granite for buildings in many parts of the United States. The Texas
Capitol building and several other State buildings in Austin are made of
this granite.
Graphite
Graphite is a mineral that is made up of a single element—carbon.
(_Diamond_, although it does not look at all like graphite, is a
crystalline form of carbon.) Graphite is a steel-gray or black mineral
that commonly has a metallic luster. It is not heavy and is extremely
soft. Graphite will soil your fingers and leave a black mark on paper.
This mineral cleaves perfectly in one direction and splits into thin
flakes that feel greasy.
To help distinguish graphite from _molybdenite_, a mineral it resembles,
you can use a shiny, glazed surface, such as is found on a saucer or a
plate, to test its streak. When rubbed across this kind of surface,
graphite will leave a black streak, but molybdenite will leave a
greenish one.
Graphite commonly occurs as scales, as sheet-like layers, or as compact
masses. It may be found mixed with clay or other impurities, and it then
looks dull and earthy. Crystals of graphite, which are seldom found, are
6-sided and flat.
Graphite occurs in Llano, Burnet, and other counties in the Llano uplift
area of central Texas. One of the Nation’s most important graphite mines
is located in the Clear Creek area several miles northwest of Burnet in
Burnet County. Some graphite has also been mined near Lone Grove in
Llano County. In addition, a graphite schist, obtained south of Llano in
Llano County, has been used as a filtering material.
[Illustration: Graphite is used in pencil lead, generator brushes, and
lubricants.]
All of this graphite occurs in extremely old Precambrian graphite schist
rocks that we now see at the surface in this part of Texas. It is
believed that the schists were once ancient sedimentary rocks, such as
shales, which contained organic matter. Long ago, great forces below the
earth’s surface altered these rocks. When this happened, the organic
material that they contained changed into the mineral we know as
graphite.
Graphite has a number of uses. It is mixed with clay to make the pencil
lead that we use for writing. It serves as a lubricant, either alone or
mixed with oil, grease, or water. In addition, graphite is used to make
generator brushes, stove and shoe polish, and special paints. Because it
can stand great heat without melting, some graphite is mixed with clay
to make the pots or crucibles that hold molten metals.
Grossularite. _See_ Garnet.
Gypsite. _See_ Gypsum.
Gypsum
Gypsum is a hydrous calcium sulfate. This mineral is normally colorless
or white, but impurities cause it to appear gray, brownish, yellowish,
or reddish. It is transparent or translucent and is not heavy. When you
rub gypsum across a streak plate, it leaves a white streak. This mineral
is so soft that a fingernail scratches it easily. Gypsum occurs in
several varieties.
The colorless, glassy, and transparent variety of gypsum is called
_selenite_. It is found as cleavable masses and as crystals that are
prism-shaped or flat and diamond-shaped. It is not uncommon for two
crystals to be joined together so that they have a swallow-tail
shape—these crystals are _twinned_. Groups of flat selenite crystals
arranged together so that they resemble flowers are called _rosettes_.
Many of these have been found in Nolan County.
Gypsum has four directions of cleavage. One of these directions is so
perfect that some selenite splits into thin, clear sheets that may be
mistaken for mica; other selenite cleavage fragments may be mistaken for
calcite. You can distinguish selenite sheets from calcite by testing
their hardness (selenite is softer) and by putting a drop or two of
dilute hydrochloric acid on them. The acid will fizz and bubble on
calcite but not on the selenite gypsum. There is also a quick way to
distinguish the thin selenite cleavage fragments from mica. After you
carefully bend a thin sheet of mica, it will snap back to its original
shape without breaking. Selenite gypsum, however, is not elastic. It
will bend, but it will break if you try to straighten it again.
[Illustration: Selenite gypsum crystal from Bastrop County, Texas.]
Selenite is found in cracks and cavities in rocks. Good crystals have
been collected at Gyp Hill, a salt dome southeast of Falfurrias in
Brooks County, and some selenite has been mined there. Selenite crystals
also occur scattered through clays, particularly along creek banks, in
Lee, Fayette, Bastrop, and several other counties.
Another variety of gypsum is known as _fibrous gypsum_. It is made up of
slender, brittle, needle-like fibers that fill the cracks in some rocks.
If fibrous gypsum has a silky or pearly luster, it is called _satin
spar_. One of the places where satin spar occurs is in Permian rocks in
Hardeman County.
[Illustration: Selenite gypsum rosettes from Nolan County, Texas.]
Most of the fibrous gypsum and selenite is formed by solutions. Some of
these solutions develop when underground waters, seeping through rocks,
pick up and dissolve minerals that contain sulfur (such as pyrite). This
dissolved material changes the water into very weak sulfuric acid. When
the sulfuric acid meets calcium carbonate (as in limestone or calcite),
it combines with the calcium to form the gypsum.
[Illustration: Fibrous gypsum from Terlingua area, Brewster County,
Texas.]
A massive, fine-grained, and translucent variety of gypsum, known as
_alabaster_, is used for articles such as lamp bases, statuettes, vases,
and book-ends.
A loose, earthy, crumbly variety of gypsum, called _gypsite_, is
ordinarily found mixed with other materials, such as clay, sand, and
soil. It occurs either at or near the surface of the ground. Gypsite is
found in Culberson, Reeves, and other counties in west Texas.
A massive, granular variety of gypsum, called _rock gypsum_, may occur
in large deposits. This is the gypsum that is used for making products
such as plaster, wallboard, and some cements.
Deposits of rock gypsum are found both underground and at the surface in
Texas. Surface deposits occur in Permian rocks in several counties to
the east of the Texas High Plains. They also occur in the area between
the Pecos River and the Delaware and Apache Mountains in Culberson and
Reeves counties. Some of the other surface deposits are found near the
Malone Mountains in Hudspeth County and in Lower Cretaceous rocks in
Gillespie and Menard counties. Rock gypsum has been mined from the
deposits in Fisher, Gillespie, Hardeman, Hudspeth, and Nolan counties.
It also has been produced from the cap-rock at Hockley salt dome in
Harris County.
Gypsum and another mineral, anhydrite, have very nearly the same
composition. Both are calcium sulfates. Gypsum, however, contains water
of crystallization, and anhydrite does not. It is likely that most of
the rock-gypsum deposits of Texas originally were beds of anhydrite. By
absorbing water that seeped through it, the anhydrite changed into
gypsum.
Halite
Halite, sodium chloride, is the table salt you sprinkle on food for
seasoning. This mineral ordinarily is white or colorless, but other
materials cause it to be tinted red, blue, gray, brown, or green. When
you rub halite across a streak plate, it leaves a white streak.
Because halite cleaves in three directions, all at right angles to each
other, the cleavage fragments are shaped like cubes. You can see some of
them by looking at a few grains of table salt through a magnifying
glass.
Halite has a salty taste and dissolves easily in water. It also is
transparent to translucent and has a glassy luster. This mineral is soft
enough for a copper penny to scratch it. Halite commonly occurs as cubic
crystals and as granular or compact masses.
In addition to its use as table salt, much halite goes to make soda ash,
chlorine, and other chemicals. A few of its other uses are in leather
making, meat packing, and food canning.
Texas has large underground deposits of halite. These deposits, known as
_rock salt_, occur in the Permian subsurface basin of west Texas and in
the salt domes of the Gulf Coastal Plain. The Permian basin, which
extends under parts of west Texas, New Mexico, Oklahoma, Colorado, and
Kansas, is now completely filled with sediments. It appears level and
flat when you travel across it and does not look at all like a basin or
a valley. During Permian time, however, this area was covered by a salty
sea. As the sea gradually dried up, the dissolved material that it
contained was deposited as thick beds of halite, anhydrite, and other
minerals. Later, these minerals were covered by sedimentary rocks which
were deposited on top of them. Now, the minerals are found many hundreds
of feet below the surface. In Hutchinson, Mitchell, Ward, and Yoakum
counties, some of this Permian basin salt has been produced (as brine)
from wells that have been drilled into it.
[Illustration: Salt domes, which are huge, underground columns of
halite, occur on the Gulf Coastal Plain.]
The Gulf Coastal Plain salt domes are huge and almost circular columns
of halite, some of which are more than 2 miles wide. Some are less than
300 feet below the surface, but most of them are much deeper. These salt
columns pushed upward many thousands of feet from great, deeply buried
salt deposits. The halite is mined from shafts dug into the Hockley salt
dome in Harris County and into the Grand Saline salt dome in Van Zandt
County. Salt brines are produced from wells drilled into several salt
domes of this area.
At the surface in Texas, halite occurs in salt lakes in Crane and
Hudspeth counties and in alkali lakes on the High Plains. It is found
also on the shores of bays and lagoons in Cameron, Kenedy, Kleberg, and
Willacy counties, and it occurs at springs and seepages in various
places in the State.
Hematite
Hematite, iron oxide, the chief ore of iron, is found in many places in
Texas but not in large deposits. This mineral may have a metallic luster
and appear reddish brown, dark brown, steel gray, or black or it may
occur as a soft, red, earth-like material called _red ocher_.
[Illustration: Specular hematite from Carrizo Mountains, Hudspeth
County, Texas.]
Most metallic hematite is too hard for a pocket knife to scratch, but
quartz or a steel file will scratch it. Hematite is fairly heavy, for it
has a specific gravity of 5.26. This mineral has no cleavage, but some
specimens show three directions of parting that are almost at right
angles to each other. A great help in identifying hematite is the dark
reddish-brown streak it leaves when you rub it across a streak plate.
Some hematite occurs as rounded masses that resemble kidneys or bunches
of grapes (then called _kidney ore_); it also is found as flat crystals.
Most of the Texas hematite occurs as granular or compact masses. One of
these massive varieties is composed of shiny scales or plates and is
called _micaceous_ or _specular hematite_. This variety has been found
in Hudspeth County and in northeastern Mason County. Hematite also
commonly occurs as cementing material in many Texas sandstones.
Some hematite is formed by the alteration of magnetite, another iron
mineral. This hematite is known as _martite_, and some of it still has
the crystal shape (an octahedron or a dodecahedron) that belonged to the
magnetite. Most of the hematite found in the Llano uplift area of
central Texas is believed to be altered magnetite. In this central Texas
area, some massive, granular martite has been mined at the Gamble
prospect, a few miles southeast of Fredonia in northeastern Mason
County, where it occurs as layers in Precambrian gneiss.
Small deposits of hematite occur in other parts of Texas, too. Some of
the west Texas localities include Sierra Blanca, the Quitman Mountains,
and the Carrizo Mountains of Hudspeth County and the area around Shafter
in Presidio County.
Hollandite. _See_ Manganese Minerals.
Hyalite. _See_ Opal.
Jasper. _See_ Quartz.
Kaolin. _See_ Clay.
Limestone
Limestone is a sedimentary rock made up chiefly of calcite, a
calcium-carbonate mineral. This rock also commonly contains grains of
quartz, clay minerals, the mineral dolomite, or other materials. If a
large amount of dolomite is present, the rock is called _dolomitic
limestone_. In some limestones, the mineral grains are too small to be
distinguished from each other without a magnifying glass or a
microscope, but in other limestones, the individual mineral grains are
easily seen.
Pure limestone is white, but if it contains clay or plant or animal
matter it is light gray, dark gray, or black. Limestone also may be some
shade of yellow, brown, or red. It is fairly soft and can be scratched
with a knife. Because this rock contains calcite, an easy chemical test
will help identify it: a drop or two of dilute hydrochloric acid will
quickly fizz and bubble when placed on the limestone.
Limestones form in fresh water, such as in lakes, but most of them form
in the seas. As some earlier-formed rocks are weathered, the calcium
minerals that they contain are dissolved. Creeks and rivers carry this
dissolved material to the sea. There, small animals, such as corals,
crinoids, sponges, and foraminifers, take the dissolved material out of
the water to build their calcium carbonate shells. Plants, such as
algae, can take calcium carbonate out of solution too, and it collects
on them. Shells, shell fragments, and plant remains accumulate on the
sea floor, forming limy deposits that later become limestone.
Limestones also originate in a slightly different way. When the
temperature and chemical composition of the water permit, calcium
carbonate precipitates as millions of tiny grains of calcite and forms a
limy mud that is converted to limestone. Many limestones contain shell
or plant fragments in addition to these tiny grains of calcite.
[Illustration: Polished section of Lower Cretaceous Edwards Limestone
from Travis County, Texas, containing fossil gastropods.]
There are several special kinds of limestone. If the rock is made up of
many little rounded calcite grains that resemble fish eggs, it is called
_öolitic limestone_. Another limestone, _chalk_, is soft, white, and
fine grained. It consists mostly of tiny shell fragments and
fine-grained calcite. _Coquina_ is a porous limestone made up of loosely
cemented shells and shell fragments. Another special kind of limestone,
known as _lithographic limestone_, because it can be used in printing,
is smooth, firm, and hard. Its mineral grains are too small to be
recognized without a microscope. This kind of limestone breaks with a
smooth, sometimes curved, fracture. Still another variety, _pulverulent
limestone_, is loose, soft, powdery, and white. It occurs in the Lower
Cretaceous Edwards Limestone in Williamson and Bell counties of central
Texas. Some of this limestone is used to polish rice grains, and it is
added to livestock feeds to provide calcium for the animals.
[Illustration: Limestone quarry in Lower Cretaceous Edwards Limestone at
Georgetown, Williamson County, Texas.]
Much limestone is found at the surface in Texas in Cambrian, Ordovician,
Mississippian, Pennsylvanian, Permian, and Cretaceous formations. If you
will look at numbers 5, 6, 9, 10, and 11 on the Texas geologic map (pp.
4-5), you will see that these strata appear at the surface in central,
north-central, and Trans-Pecos Texas.
Limestone has many important uses. Much Texas limestone is crushed and
used as a road-building material and as an aggregate that is mixed with
cement to make concrete. Farmers in some areas improve their crops by
adding limestone to the soil. Limestone also is sent to the iron
furnaces in east Texas to be used in the production of pig iron and
steel.
Some of the Texas limestones are heated to a fairly high temperature in
order to change them into _lime_ (calcium oxide). Industry uses a large
amount of lime in making chemicals, steel, glass, paper, and other
products. Builders use it to make plasters, mortars, and stuccos. At
plants in Comal, Johnson, Travis, and Williamson counties, lime is made
from Cretaceous limestones.
Another important use of limestone is in making _portland cement_. The
limestone is mixed with clay or shale, and the mixture is burned in a
kiln until it just begins to melt. Then it is allowed to cool. Next, it
is finely ground and in order to keep the finished cement from hardening
or setting too quickly when it is used, a _retarder_, such as gypsum, is
added. A number of cement-manufacturing plants in Texas use Cretaceous
limestones, shales, and clays.
Many of the Texas limestones make excellent building stones. Some of
them are quarried from Pennsylvanian and Cretaceous formations in
north-central Texas and from Lower Cretaceous formations in counties
near the Llano uplift of central Texas. A large quarry on the
Williamson-Travis County line near Cedar Park in central Texas has
supplied Cretaceous limestone for many buildings and monuments in the
United States and Canada.
Limonite
Limonite is not really a definite mineral but is a mixture of iron
oxides containing water. It is believed to be closely related to an iron
mineral called _goethite_. Some limonite may be dull and earthy with the
appearance of brownish-yellow or rusty brown clay. This variety is so
soft that a fingernail will scratch it easily.
Other limonite has a dark brown or black color and a metallic or almost
metallic luster. A copper penny will not scratch it, but a steel file
will. This kind of limonite may have a shiny black surface that
resembles glossy lacquer. The property that will help you most in
identifying limonite is the rusty, yellowish-brown streak it leaves when
rubbed across a streak plate.
Limonite has no cleavage and no crystal shape of its own. But crystals
of other iron minerals, such as pyrite and magnetite, alter to form
limonite. It then occurs with a crystal shape that originally belonged
to one of these other minerals. (Such false forms of minerals are called
_pseudomorphs_.) Limonite also occurs as layers in rocks, as hollow or
solid concretions, or as coatings on other minerals. It is found mixed
with minerals such as clays and serves as the cementing material in some
sandstones.
Limonite is found in many localities in Texas including Blanco,
Brewster, Burnet, Llano, and San Saba counties. The most important
limonite deposits in Texas, however, are in the eastern part of the
State, particularly in Anderson, Cass, Cherokee, Henderson, Marion,
Morris, Nacogdoches, Smith, and Upshur counties.
The east Texas limonite deposits occur mainly in Weches sedimentary
rocks. These rocks, which were deposited in the sea during Eocene
Tertiary time, contain clay along with greensands. (Greensands are
small, soft grains that contain _glauconite_, a mineral composed of
iron, silicon, and several other elements.) Later, as the sea retreated,
these sediments became a part of the land. Waters seeping through the
sediments changed into weak solutions of carbonic and sulfuric acid that
dissolved the iron out of some of the greensands. When conditions were
favorable, this iron was re-deposited as an iron-carbonate mineral
called _siderite_. Siderite was changed to limonite by weathering. Some
siderite is still found in east Texas, and it is also mined along with
the limonite as an iron ore.
East Texas iron ore has been mined from time to time ever since about
1855, and records show that a number of local iron furnaces once
operated. The brown iron ore (as the limonite is also called) now is
mined from open pits in Cass, Cherokee, and Morris counties.
This ore, after being washed, goes into blast furnaces at Lone Star
(near Daingerfield) and at Houston. In the blast furnaces the ore is
changed into metallic iron by mixing it with coke (made from coal) and
limestone and blowing in blasts of hot air.
To make steel, the iron from the blast furnace (called _pig iron_) is
put into open-hearth furnaces together with scrap iron, limestone, and
other materials. This mixture is heated and melted together to get rid
of unwanted substances. Then other elements, such as molybdenum,
manganese, or nickel, are added to make steel with the right strength
and toughness.
[Illustration: Limonite ore is changed to metallic iron in a blast
furnace.]
Skip car
Hot Gases
Blast Furnace
Iron Ore
Limestone
Coke
Fire Brick lining
Steel Plate covering
3550°F
Hot Air Blast
Slag
Slag Ladle
Molten Iron
Iron Ladle
Steel mills alongside the furnaces in Texas turn out many products, such
as steel plates for oil tanks, ships, and tank cars and steel beams for
framework in buildings and bridges. Some of their other products include
pipes for the oil and chemical industries and wire for nails and fencing
material.
Lithographic Limestone. _See_ Limestone.
Llanite
Llanite is a unique rock that is found only in Llano County in central
Texas. This intrusive igneous rock is made up of easily seen crystals
and grains of quartz and feldspar that are scattered through a
brown-colored mass of extremely small mineral grains. The quartz is
beautiful, sky-blue, and opal-like; the feldspar has a rusty pink color.
(Because the quartz looks like opal, this rock often is called _opaline
granite_.) The mineral grains that make up the brown-colored mass are so
tiny that they can be identified only with a microscope. They are
quartz, feldspar, mica, fluorite, and apatite.
Llanite formed during Precambrian time. Molten rock material forced its
way upward into cracks that cut across granite and schist rocks while
the rocks were still far underground. This hot magma remained in the
cracks where it cooled and hardened to form long, narrow, wall-like
masses (called _dikes_) of llanite. We can see some of the llanite dikes
exposed at the earth’s surface to the north and northeast of Llano in
Llano County because the overlying rocks have weathered away.
Llanite has been quarried from one of the dikes west of Babyhead in
northern Llano County. Because llanite is both attractive and strong, it
has been used as an ornamental stone and as a monument stone.
Magnetite
Magnetite, iron oxide, is a black, metallic mineral with an outstanding
physical property: it is magnetic—fragments of magnetite readily cling
to a magnet. It also leaves a black streak when rubbed across a streak
plate. Although this mineral is too hard to be scratched by the average
pocket knife, a steel file will scratch it. Magnetite is fairly heavy—it
has a specific gravity of 5.18.
Magnetite occurs as compact or granular masses, as scattered grains, and
as crystals. Most of the crystals are octahedrons, but some
dodecahedrons are found. Magnetite helps make up a part of many
metamorphic and igneous rocks, and it also occurs as tiny crystals and
grains in some sands, sandstones, and other sedimentary rocks.
[Illustration: Metallic iron, after leaving the blast furnace, is made
into steel in an open-hearth furnace.]
Open-Hearth Furnace
Scrap metal
Alloying Elements
Limestone
Furnace Interior
Live Fuel Burner
Air pre-heated
Most of the magnetite that has been found in Texas occurs in Precambrian
gneiss and schist rocks of the Llano uplift area of central Texas,
particularly in Llano County and in eastern Mason County. It occurs as
thin layers, as thick lens-shaped deposits, and as scattered grains in
the rocks. Probably at least a billion years ago these gneisses and
schists were sedimentary rocks, such as shales and sandstones. Some
geologists believe that these rocks could have contained iron sediments
(perhaps in the form of _glauconite_). Great forces below the earth’s
surface crumpled and squeezed the sedimentary rocks and changed them
into the metamorphic schist and gneiss rocks we see today. As this
happened, the iron sediments in the rocks were changed into magnetite.
[Illustration: Granular magnetite fragments from northwest of Llano,
Llano County, Texas, are attracted to a magnet.]
At least some of the magnetite in this area (such as the deposit at Iron
Mountain in Llano County) probably had a different sort of origin.
Molten igneous rock material containing iron could have moved up into
cracks in the ancient sedimentary rocks. Then the magnetite formed from
this iron material when the igneous and sedimentary rocks were changed
into the schists and gneisses of today.
None of the Llano and Mason County magnetite deposits is really very
large. Nevertheless, prospecting and a little mining have been carried
on from time to time at several deposits in this area. At Iron Mountain,
which is about 12 miles northwest of Llano in Llano County, magnetite
has been mined from open pits. Although magnetite is commonly used as a
source of iron, the magnetite from this deposit was used as a heavy
concrete aggregate.
Malachite. _See_ Copper Minerals.
Manganese Minerals (Braunite, Hollandite, Pyrolusite)
Although manganese does not occur alone in nature as a native element,
it makes up a part of many minerals and compounds. This element has an
important use in steel making, where it helps rid the steel of unwanted
substances, such as oxygen and sulfur, and, in addition, it is used to
make tough, hard, manganese steel for armor plate, railroad tracks,
safes, and steam shovels. Manganese has various uses outside the steel
industry. It is added to copper and nickel to make alloys, it is used in
the manufacture of dry-cell batteries, and (as manganese sulfate) it is
used as a fertilizer.
Manganese minerals and compounds, such as _braunite_, _hollandite_,
_pyrolusite_, and _wad_, occur in several counties in Texas. No large,
commercial deposits have been found here.
Some manganese compounds and minerals are covered with a soft, sooty
black material that will soil your fingers. This can help you recognize
these minerals; however, a few non-manganese minerals, such as some
chalcocite, also have a black coating that soils your fingers in a
similar way.
One of the manganese minerals, _braunite_, is a complex oxide of
manganese that contains silica. It has a submetallic luster and is dark
steel-gray or black. When rubbed across a streak plate, it leaves a
steel-gray or a black streak. This mineral is too hard to be scratched
by a pocket knife, but a piece of quartz or a steel file will scratch
it. Braunite has a specific gravity of 4.75 to 4.82. It has four
directions of cleavage that are parallel to the faces of a pyramid.
In the Spiller mine, about 15 miles northeast of Mason in Mason County,
masses of braunite occur as lens-shaped layers in Precambrian gneiss and
quartzite rocks. This braunite may have formed from another manganese
mineral (possibly manganese garnet) that was exposed at the earth’s
surface after the overlying rocks eroded away. As this other mineral
weathered, it may have altered into braunite, or the braunite could have
been deposited from solutions emanating from hot magmas before the great
thickness of overlying rock was removed.
The mineral variety _hollandite_ is a rare manganate of manganese and
barium. It has a metallic luster, and its color is silvery gray or
black. When you rub it across a streak plate, hollandite leaves a black
streak. It has a specific gravity of 4.7 to 5. Hollandite is rather
hard, but a steel file will scratch it.
[Illustration: Hollandite from Jeff Davis County, Texas.]
Hollandite occurs in western Jeff Davis County in west Texas at what is
called the Mayfield prospect. Here, it is found as rounded masses that
occur in a vein near a large fault in Lower Cretaceous limestone rocks.
Other manganese compounds, _pyrolusite_ and _wad_, are found in several
important deposits near the Pecos River in western Val Verde County.
_Pyrolusite_ is a manganese dioxide mineral. It is black, opaque, and so
soft that it rubs off on your fingers like soot. Pyrolusite may be
granular and massive or may be powdery. It also occurs as a fern-like
coating on rocks. _Wad_ is not really a mineral but is an impure,
dull-black or brownish-black mixture of manganese oxide, water, and
other substances. It can be soft enough to soil your fingers, or it can
be too hard to scratch with a pocket knife. Wad occurs in earthy or
compact masses or in crusts or stains on rocks.
In Val Verde County, the wad and pyrolusite are found mixed with soil,
clay, gravel, sand, and plant remains. This material fills cracks in
Lower Cretaceous limestones, it is scattered through gravels, and it is
deposited in low places at the surface. The manganese in these deposits
came from limestone rocks that have since weathered away. Rainwater
trickled into these rocks and dissolved the manganese minerals they
contained. This manganese was washed down toward the Pecos River and was
deposited as wad and pyrolusite.
Marble
Marble is a metamorphic rock made up chiefly of sparkling grains of
calcite or dolomite, but other minerals may be present. The marble may
be fine grained, medium grained, or coarse grained; commonly, all the
mineral grains are about the same size.
Marble may be of uniform color, banded, spotted, or streaked. If it is
made up only of pure calcite or dolomite, the marble is white. If,
however, it contains carbonaceous material, such as graphite, it is
grayish or black. Limonite impurities cause the marble to be yellowish
brown, and manganese oxides and hematite give it a brownish, pinkish, or
reddish color.
Marble is a rather soft rock, and you can scratch it easily with a
pocket knife. A few drops of dilute hydrochloric acid will bubble and
fizz readily on calcite marble; on dolomite marble, it may fizz
slightly.
Marble forms from limestone or from dolomite rock. Heat and pressure
below the earth’s surface cause the calcite and dolomite mineral grains
in these rocks to recrystallize. A fine-grained limestone can be changed
into a coarse-grained calcite marble. The marble is not made up of new
and different minerals, but it has a new texture unlike that of the
limestone. (To a builder, the word _marble_ has another meaning. He
considers rocks such as unaltered limestone, unaltered dolomite, or even
serpentine to be marble, if they will take a high polish.)
[Illustration: Polished section of Precambrian metamorphic marble from
Llano County, Texas.]
Metamorphic marbles occur at the surface in central Texas and in west
Texas. Some of the west Texas occurrences are in the Van Horn area of
Culberson and Hudspeth counties and in the Big Bend area of Brewster
County. In central Texas, Precambrian marbles are found in Burnet,
Gillespie, Llano, and Mason counties of the Llano uplift area. Many of
them are suitable for use as monument and building stones. Some of the
Llano County marble is quarried and used as granules for roofs and as
terrazzo chips for making colorful floors (described with serpentine on
p. 88).
Martite. _See_ Hematite.
Mica
Mica is not just one mineral but is the name given to a group of similar
minerals. The mica minerals are easy to recognize. Because they have
perfect cleavage in one direction, they split into thin, flat sheets.
You can see through some mica sheets, and they are elastic enough to be
bent back and forth. (Another mineral, selenite gypsum, also will split
into thin, flat, transparent sheets, but selenite sheets break when you
bend them.)
[Illustration: Mica minerals have perfect cleavage in one direction,
resulting in thin, sheet-like cleavage fragments.]
Basal Cleavage
Two of the mica minerals that you are most likely to find in Texas are
_muscovite_ and _biotite_. Both these minerals are potassium-aluminum
silicates, and biotite, in addition, contains magnesium and iron. In
general, muscovite is light colored, that is, it has a light brown,
yellow, or green tint, or is colorless, and biotite is dark colored,
commonly dark green, brown, or black. These minerals have glassy or
pearly lusters and are rather soft—a copper penny scratches them. The
specific gravity of biotite is 2.8 to 3.2, and that of muscovite is 2.76
to 3.1.
Mica minerals occur in igneous rocks, such as granite and pegmatite, and
in metamorphic rocks, such as schist and gneiss. They also are found as
tiny flakes in some sandstones, limestones, and other sedimentary rocks.
Most of the Texas mica is found in the Llano uplift area (particularly
in Llano County) and in the Mica Mine area. (The Mica Mine area is in
the Van Horn Mountains about 15 miles south of Van Horn in west Texas.)
In both these areas, the mica minerals occur mostly in Precambrian
pegmatites and mica schists.
The gleaming mica schists were once igneous rocks or sedimentary rocks
such as sandstones and shales. Long ago, great forces beneath the
earth’s surface changed the rocks into mica schists. The mica that is
found in pegmatites formed from hot fluids of igneous origin when the
pegmatite rock itself was formed.
Clusters of mica in the pegmatites are called _books_, because the thin
sheets into which the mica splits look like pages. Some muscovite books
up to 8 inches across are found in the Mica Mine area of the Van Horn
Mountains.
The books or sheets of muscovite mica that occur in pegmatites are
especially valuable to industry. Muscovite can stand great heat without
melting, it is tough, it splits into thin sheets, and it lets very
little heat and electricity pass through. Because of these properties,
muscovite is used in fuses and as insulators in heating elements of
electric irons and toasters. (Biotite is not used, because the iron it
contains makes it a conductor of electricity.) Sheet muscovite also is
widely used by the electronics industry as a non-conducting material in
the manufacture of tubes and other products.
Both muscovite and biotite from mica schist rocks, as well as scrap
pieces of sheet mica from pegmatites, are ground into flakes or powder.
This ground-up mica has many uses, ranging from a powder coating for
automobile inner-tubes to Christmas tree “snow.”
Only a small amount of mica has been mined in Texas. A fair grade of
sheet mica occurs in the pegmatites at Mica Mine in west Texas, but the
deposit is not large. In the pegmatites of the Llano uplift area of
central Texas, no sheet mica has been found that is considered good
enough for the requirements of industry. Mica suitable for grinding,
however, is found in both these Texas areas.
Micaceous Hematite. _See_ Hematite.
Microcline. _See_ Feldspar.
Milky Quartz. _See_ Quartz.
Muscovite. _See_ Mica.
Native Silver. _See_ Silver Minerals.
Obsidian and Vitrophyre
Obsidian is a dark, glassy-looking igneous rock. Most obsidian contains
the same chemical elements as granite and rhyolite, since all three of
these rocks can form from the same type of molten rock material.
Obsidian, however, has no separate minerals, because its chemical
elements are not combined in an orderly way. It is a natural glass.
Because it is a glass, we know that obsidian forms very quickly. One way
for it to form is from the sudden cooling of hot, molten lava that flows
out of volcanoes. If the lava cools and hardens before the separate
minerals can crystallize, it becomes a natural glass, such as obsidian.
This rock is smooth and shiny. Most of it is black, but some can be dark
green or dark brown. Obsidian allows light to pass through it, and it
breaks with a curved, conchoidal fracture. The broken edges are very
sharp.
Another glassy igneous rock that forms from fast-cooling lava is
_vitrophyre_. It looks like obsidian except that it has crystals or
crystalline mineral grains (which may be light colored) scattered
through the dark glassy material.
[Illustration: Obsidian was used by the Indians to make arrowheads.]
Obsidian and vitrophyre are found in the Big Bend area of Brewster and
Presidio counties in west Texas. They occur with other igneous rocks
that formed there during Tertiary time.
The Indians who long ago roamed this area used the smooth, shiny
vitrophyre and obsidian to make some of their arrowheads and scrapers.
Today, rock collectors pick up these attractive rocks for their
collections, and some of them cut and polish obsidian and vitrophyre for
use as gemstones.
Onyx. _See_ Quartz.
Öolitic Limestone. _See_ Limestone.
Opal
Opal is like hardened jelly or gelatin. It has no crystalline inner
structure and no crystal shape of its own—it is amorphous. This mineral
has almost the same chemical composition as quartz. Both are silicon
dioxides (silica), but opal, in addition, contains water.
Opal can be almost any color—red, yellow, blue, brown, gray, white—or it
can be colorless. It is transparent or translucent and appears glassy,
resinous, greasy, or dull. Opal has a specific gravity of 1.9 to
2.2—this mineral is a little lighter than quartz. It also is softer than
quartz. A copper penny will not scratch opal, but quartz will. Opal has
a white streak and a curved, conchoidal fracture but no cleavage.
Opal occurs in a number of places in Texas. In the Trans-Pecos country
of west Texas, it fills cracks and cavities in some of the extrusive
igneous rocks. It occurs on the High Plains of northwest Texas, and it
is found in Tertiary formations of the Gulf Coastal Plain where it
occurs as masses that fill cracks and cavities in sedimentary rocks, as
the cementing material in some sandstones (such as in the Catahoula
sandstone), and as opalized wood.
Much opal forms from underground waters that contain silicon. These
solutions move through the rocks and deposit the opal in them.
Opal is found in a number of varieties. Some show a beautiful, lustrous
play of colors that comes from inside the specimens. These varieties are
known as _precious opal_ and are prized as gemstones. In Texas, some
precious opal is found near Alpine in Brewster County. It has a milky
white to bluish-white color, is translucent, and shows a fiery orange,
red, blue, and green play of colors.
The variety known as _common opal_ shows no play of colors. It may be
white, gray, bluish, reddish, greenish, or yellowish, and it is only
slightly translucent. It is found in Brewster, Jeff Davis, Presidio, and
other counties of the Trans-Pecos country of west Texas. It occurs also
around some of the wet-weather (playa) lakes on the Texas High Plains.
In the Gulf Coastal Plain, common opal is found with chalcedony (a
variety of quartz) in Tertiary formations. A south Texas locality
sometimes visited by collectors is near Freer in Duval County.
A clear, commonly rounded, variety of opal that looks like ice is called
_hyalite_. Two areas in which it has been found are in Presidio County
in west Texas and in Llano County in central Texas.
[Illustration: Opalized wood from Washington County, Texas.]
A variety of petrified wood, called _opalized wood_, is opal that
replaced the fibers of a piece of wood. Wood opal is found at a number
of places in the Gulf Coastal Plain. It occurs there in Tertiary
formations within about 20 miles of the boundary line between areas 2
and 3 shown on the geologic map (pp. 4-5).
A soft opaline material called _diatomite_, or _diatomaceous earth_, is
made up chiefly of the skeletons of diatoms—tiny, one-celled plants that
live in fresh or salt water. These little plants are able to take silica
from the water to make opal skeletons for themselves. When the diatom
skeletons collect at the bottom of a lake or sea, they form the light,
crumbly, white, gray, or cream-colored deposit of impure opal known as
diatomite. Industry uses this material as a filter, as insulation, as an
abrasive, and as a filler.
Diatomite formed in ancient lakes on the Texas High Plains during late
Tertiary (Pliocene) and early Quaternary (Pleistocene) times. It is
found in Armstrong, Crosby, Dickens, Ector, Hartley, and Lamb counties.
Opaline Granite. _See_ Llanite.
Orthoclase. _See_ Feldspar.
Pegmatite
Pegmatites occur in igneous rock areas, and most geologists consider
them intrusive igneous rocks. They are made up of crystals and
crystalline mineral grains that fit together—the grains are interlocked.
The crystals and grains in pegmatites are larger than those of
surrounding rocks, and some are huge, even larger than a man. However,
there is a wide range of grain sizes in pegmatite.
Some pegmatites cut through igneous or metamorphic rocks in such a way
that they resemble walls (called _dikes_). Others are found as veins, as
flat masses, or as odd-shaped bodies in rocks. Many pegmatites occur in
granites and contain feldspar, quartz, mica, and other minerals, as
granite does. Some pegmatites occur with other kinds of igneous rocks
and contain the same minerals as these rocks. A few pegmatites contain
rare and unusual minerals.
Many geologists believe that pegmatites form from hot fluids of igneous
origin that are left after other igneous rocks, such as granite, have
already formed. These left-over fluids contain large amounts of
aluminum, potassium, silicon, sodium, and several other elements. While
the granite or other rocks are still far underground, this material
pushes up into them, and may even partly dissolve them. Then it slowly
cools and hardens into pegmatite. It is believed that, later, more
fluids move into cracks in some pegmatites. This new material adds other
minerals to the pegmatites and alters some of those minerals already
there.
[Illustration: Quartz-feldspar pegmatite from Burnet County, Texas.]
Some of the pegmatites we now see at the surface in Texas are probably
about a billion years old. They formed during Precambrian time and occur
with other extremely old rocks. One well-known Texas pegmatite area is
the Mica Mine district of west Texas. It is about 15 miles south of Van
Horn in the Van Horn Mountains of Culberson and Hudspeth counties.
Another pegmatite area is in the Llano uplift of central Texas. These
central Texas pegmatites occur in Burnet, Gillespie, Llano, and Mason
counties.
Large crystals and grains of feldspar, mica, and quartz are found in the
pegmatites of both these areas. A small amount of mica has been mined
from the west Texas pegmatites, and feldspar has been produced from the
central Texas pegmatites.
An extremely rare and unusual pegmatite occurs in the Llano uplift area
at Baringer Hill, which is west of Burnet in Llano County. This
pegmatite was once on the bank of the Colorado River, but when Buchanan
Dam was built, the area was flooded. The Baringer Hill pegmatite now
lies beneath the water of Lake Buchanan. Many rare minerals, which
contain beryllium, cerium, thorium, uranium, yttrium, zirconium, and a
number of other elements, occur in this pegmatite. Some of these
minerals, such as those containing yttrium and zirconium, glow or
incandesce when they are heated. During the early part of this century,
before the area was flooded, several of the yttrium minerals were mined
and used in making lamp mantles.
Pitchblende. _See_ Uranium Minerals.
Precious Opal. _See_ Opal.
Pulverulent Limestone. _See_ Limestone.
Pumicite. _See_ Volcanic Ash.
Pyrite
Pyrite is a shiny, pale golden-yellow or brassy-yellow metallic mineral.
This mineral, an iron disulfide, is so often mistaken for gold that it
is widely known by the nickname _fool’s gold_.
Except for their similar color and luster, pyrite and gold are really
very different. When you rub pyrite across a streak plate, it leaves a
black, a greenish-black, or a brownish-black streak, but the streak of
gold is gold-colored. Pyrite is too hard for the average pocket knife to
scratch, but a knife will scratch gold easily. Pyrite is brittle and
readily breaks, but gold is malleable and flattens out when hit with a
hammer. Pyrite is only about 5 times as heavy as an equal volume of
water, but pure gold is over 19 times as heavy. And pyrite may have a
brown or a multicolored tarnish on it, but gold never tarnishes.
[Illustration: Pyrite veins in white marble from Llano County, Texas.]
[Illustration: Cubic crystals of pyrite.]
Pyrite is a common mineral and is found in many of the igneous,
metamorphic, and sedimentary rocks of Texas. It may be scattered through
the rocks, or it may fill cracks and cavities in them. This mineral
occurs as granular and compact masses, as rounded masses, or as
crystals. The crystals are commonly cubes, pyritohedrons, or
octahedrons. In some crystals, the shapes are combined (such as a cube
with an octahedron or two pyritohedrons grown through each other). You
may notice that the sides of some cubes and pyritohedrons have fine,
parallel grooves (called _striae_ or _striations_) on them.
Pyrite originates in a number of different ways. Some of it forms, along
with other minerals in igneous rocks, from hot magmas. It also forms in
metamorphic rocks by the same processes that produce these rocks. Some
of the pyrite in limestone and other sedimentary rocks is formed when
the rocks themselves are deposited by seas or streams. Pyrite also is
deposited by the hot fluids that are given off by magmas. These fluids
travel up into cracks and other openings in rocks and then form pyrite
as well as other minerals. Much pyrite forms in still another way. As
water seeps through rocks, it dissolves some of the iron minerals that
they contain. When, under certain conditions, these iron solutions mix
with hydrogen sulfide (this is the gas that makes some water smell like
rotten eggs), pyrite is formed.
Pyrite alters easily. Because of this, most builders carefully check the
limestone, granite, marble, or whatever other building stone they plan
to use to be sure that it does not contain large amounts of pyrite. When
exposed to the weather, pyrite changes to limonite and causes an
unsightly rust stain.
Pyrite is used as a source of sulfur, and it is produced for this
purpose in several states. In Texas, however, no pyrite deposits have
been found that are large enough to be mined.
Pyrolusite. _See_ Manganese Minerals.
Quartz
Quartz, silicon dioxide, is one of the most common minerals. It is
glassy, waxy, greasy, or dull and is transparent or translucent. Pure
quartz is colorless, but impurities make some varieties white, black, or
a shade of red, yellow, blue, violet, or brown. Quartz is a hard
mineral. It scratches window glass and cannot be scratched by a pocket
knife or even by a steel file. It has a specific gravity of 2.65. The
curved, conchoidal fracture shown by many specimens helps identify it.
Quartz is plentiful in Texas. It occurs in igneous rocks, such as
granite, llanite, and pegmatite; in metamorphic rocks, such as
quartzite, schist, and gneiss; and in sedimentary rocks, such as some
sandstone, conglomerate, and breccia.
[Illustration: Quartz crystal, with inclusions, from Burnet County,
Texas.]
Quartz is found as crystals and as masses. Some of the masses are
coarsely crystalline, but some are made up of extremely small
crystalline particles called _cryptocrystalline_ quartz. Some of the
cryptocrystalline varieties of quartz found in Texas are chalcedony,
chert, and jasper. Some of the coarsely crystalline varieties found here
are amethyst, milky quartz, rose quartz, smoky quartz, and rock crystal.
[Illustration: Amethyst geode from the Alpine area of Brewster County,
Texas.]
A colorless, glassy variety of quartz, called _rock crystal_, is clear
enough to see through. It is found as crystals that are 6-sided prisms
with pyramid-like faces on the ends. This variety is commonly associated
with igneous rocks, such as those of the Llano uplift area of central
Texas and of the Trans-Pecos country of west Texas. It is commonly used
as a gemstone and is made into necklaces, earrings, and other jewelry.
Some specimens of rock crystal have slender, needle-like crystals of
other minerals, such as tourmaline, actinolite, or rutile, enclosed in
them.
A clear, glassy variety of quartz, _amethyst_, has a purple or violet
color. It, like rock crystal, is commonly found in 6-sided prisms with
pyramid-shaped ends and is also prized as a gemstone. Amethyst has been
found in Precambrian rocks in the Llano uplift area of central Texas.
(Amethyst Hill, a locality well known to collectors for many years, is
in northeastern Gillespie County.) In west Texas, amethyst has been
found in Cenozoic igneous rocks in the Sierra Blanca and Quitman
Mountains of Hudspeth County and in the Alpine area of Brewster County.
[Illustration: Milky quartz from Burnet County, Texas.]
A variety of quartz with a milk-white color and a glassy to greasy
luster is called _milky quartz_. It occurs either as crystals or as
crystalline masses. Very little light will pass through it. In central
Texas, milky quartz occurs abundantly in the Precambrian rocks of the
Llano uplift area in Blanco, Burnet, Gillespie, Llano, and Mason
counties. It also is found in some of the rocks of the Trans-Pecos
country of west Texas, such as in the Carrizo Mountains of Culberson and
Hudspeth counties. Other good places to look for this variety of quartz
are in the sands and gravels along many streams in Texas.
Some quartz has a glassy to a greasy luster and a rose or pink color.
_Rose quartz_, as this variety is called, commonly occurs as masses
rather than as individual crystals. It can be found along some of the
streams in Texas and also in igneous rocks, such as those of the Llano
uplift area of central Texas.
A kind of quartz with a smoky brown, a smoky yellow, or a dark
brownish-black color is called _smoky quartz_. Its luster is glassy, and
it may be either translucent or transparent. Smoky quartz is commonly
found as crystals that are shaped like 6-sided prisms with pyramid-like
ends. It is commonly associated with igneous rocks, and beautiful
specimens have been found in the Lake Buchanan area of Llano and Burnet
counties in central Texas.
[Illustration: Smoky-quartz crystals from Burnet County, Texas.]
A cryptocrystalline variety of quartz, _chalcedony_, has a waxy to dull
luster and a tan, white, gray, or light-blue color. It is translucent
but not transparent. Chalcedony does not have its own crystal shape but
instead is found in masses that line or fill cracks, pores, and other
cavities in rocks. It is formed when water containing silicon slowly
seeps into these openings in the rocks and deposits the silicon dioxide
there as chalcedony.
Chalcedony commonly occurs in some of the Tertiary rocks of the Gulf
Coastal Plain. For example, chalcedony associated with opal is found
near Freer in northern Duval County. In the High Plains of west Texas,
it is found in alkali-lake deposits, such as at Shafter Lake in Andrews
County and at Cedar Lake in Gaines County. In the Trans-Pecos country of
west Texas, it can be found filling small cavities in extrusive igneous
rocks.
[Illustration: Polished agate from Rio Grande gravels of Zapata County,
Texas.]
A variety of chalcedony that generally is made up of more than one color
is called _agate_ (although agates consisting of several shades of a
single color are also found). The colors may be spread out unevenly so
that the agate has a cloudy appearance, or they can be arranged in wavy,
in straight, or in concentric lines or bands. If the bands are straight
and parallel, the specimen is called _onyx_. Agate that has a moss-like
or tree-like design in it is called _moss agate_. Some agates make
attractive gemstones when cut and polished.
[Illustration: Jasper from Uvalde County, Texas. Dark areas are brownish
red; light areas are a yellowish-tan.]
Much agate has been found filling cavities in Cenozoic igneous rocks in
Brewster, Presidio, and other counties in the Trans-Pecos country of
west Texas. It has been found also in an area about 10 to 15 miles wide
along the Rio Grande, mostly in southern Webb County and in Zapata and
Starr counties.
Trees and other plants have been replaced by agate. Many specimens of
_agatized wood_ have been collected from Tertiary formations in Fayette,
Gonzales, Lee, Washington, and other counties of the Gulf Coastal Plain.
(The agatized wood, along with opalized wood, occurs within about 20
miles of the boundary between no. 2 and no. 3 on the geologic map, pp.
4-5.)
A hard, compact, slightly translucent variety of cryptocrystalline
quartz is called _jasper_. It commonly has a red, brown, or yellow color
due to the presence of an iron oxide, such as hematite. Some jasper is
made up of irregular bands of more than one of these colors. This
variety of quartz often is polished to make attractive gem or ornamental
stones. It has been collected at several localities in Texas,
particularly from creek and river gravels. Starr and other nearby
counties along the Rio Grande have furnished a number of good specimens.
A hard, smooth, compact, translucent rock that is made up mostly of
cryptocrystalline quartz is called _chert_ or _flint_. It is white,
black, or some shade of gray, brown, or pink, and its luster is waxy,
slightly glassy, or dull. Chert is found in many creek and river gravels
in Texas. It also occurs with limestone, such as in the Lower Cretaceous
Edwards Limestone of central Texas and in the Ordovician Ellenburger
strata in the Llano uplift area. Chert also is found with the Ordovician
rocks of the Marathon area of Brewster County.
Geologists do not agree on whether chert and flint are two names for one
variety of rock, or whether each is a separate variety. Some, however,
now give _chert_ a geological meaning and _flint_ an archaeological
meaning. They use the word _chert_ to describe geological formations or
rock specimens. They give the name _flint_ to the same rock when it has
been used by Indians in making arrowheads, scribers, scrapers, and
spearheads.
Quartzite
Quartzite is either a metamorphic rock or a sedimentary rock. (The
sedimentary kind of quartzite is described with sand and sandstone on p.
86.) Metamorphic quartzite is made up mostly of quartz. It forms when
heat and fluids below the earth’s surface cause the grains and cement of
a quartz sandstone to recrystallize. When this happens, the grains
interlock and are no longer held together by cement. Metamorphic
quartzite, like sedimentary quartzite, is a hard, firm rock that breaks
through the quartz grains instead of between them.
Ancient Precambrian metamorphic quartzite occurs at the surface in the
Llano uplift area of central Texas, in the Van Horn area of west Texas,
and in the Franklin Mountains north of El Paso in extreme west Texas.
Rhyolite
Rhyolite is a fine-grained or glassy igneous rock that commonly is
extrusive or volcanic. It has a pink, red, tan, white, gray, purple, or
black color. This rock, like granite, is made up chiefly of feldspar and
a silica mineral, such as quartz, but other minerals may be present.
Both rhyolite and granite form from the same kind of molten rock
material. Nevertheless, even though their compositions are the same,
these two rocks do not look alike. Their textures differ because granite
forms slowly and rhyolite forms quickly.
Much of the Texas rhyolite formed from hot, molten lava. This lava
flowed out onto the surface either through volcanic cones or cracks in
the ground. Some of the lava cooled and hardened too quickly for mineral
grains to develop. This rapidly cooled lava formed a rhyolite rock that
is made up, at least partly, of glass. In many of the rhyolites,
crystalline mineral grains were able to form, but these grains are
extremely small, and you may not be able to distinguish them even with a
magnifying glass. Some rhyolite, because it hardened from moving,
flowing lava, has streaks and bands of different colors and textures.
This rhyolite has _flow structure_.
One variety of rhyolite has easily seen crystals and grains of minerals,
such as feldspar, quartz, and mica, scattered through a mass of the tiny
crystalline grains (in much the same way that raisins are scattered
through a cake). The easily seen crystals and grains are called
_phenocrysts_, and the rock itself is called a _rhyolite porphyry_.
Many rhyolites and rhyolite porphyries occur in the Tertiary igneous
rocks of the Trans-Pecos country of west Texas. Just a few of these
localities include the Barrilla Mountains of Jeff Davis and Reeves
counties, the Chisos Mountains of Brewster County, the Chinati Mountains
of Presidio County, and the Davis Mountains of Jeff Davis County.
Rock Crystal. _See_ Quartz.
Rock Gypsum. _See_ Gypsum.
Rock Salt. _See_ Halite.
Rose Quartz. _See_ Quartz.
Salt. _See_ Halite.
Sand and Sandstone
Sand is a loose, uncemented sedimentary deposit made up of fragments of
weathered rocks and minerals. These fragments must be of a certain size
(between ¹/₁₆ millimeter and 2 millimeters in diameter) in order to be
called sand grains. The largest sand grains are about the size of a
pinhead. Sand grains are smaller than the fragments known as _granules_;
they are larger than those known as _silt_.
Many sands are made up chiefly of grains of quartz. This mineral is
plentiful and does not easily weather away. In addition, rock fragments
and many other minerals, such as feldspar, mica, gypsum, magnetite, and
garnet, are found as sand grains.
Rains wash many of the sand grains and other weathered rock and mineral
fragments into creeks and rivers. These streams may carry the sand and
other sediments long distances before depositing them. Today, we find
sands along the banks of many creeks and rivers in Texas and along the
beaches of the Gulf of Mexico. The sand in the rivers is in transit to
the Gulf. In addition, sand occurs at the surface in other Cenozoic
formations and in some of the Paleozoic and Mesozoic formations of
Texas.
Sand has many uses. Much _building sand_, which is used in mortar and
concrete, is produced from numerous sand and gravel pits in Texas. Pure
quartz sand that can be used to make glass is known as _glass sand_.
Some of it is found in north-central Texas in Lower Cretaceous
formations. A large glass sand quarry is located at Santa Anna in
Coleman County. Along the Gulf Coastal Plain, sand that is used in
glassmaking occurs in Eocene Tertiary strata.
A coarse-grained sand, _blast sand_, is used with compressed air to
clean the walls of brick and stone buildings and to carve designs on
monument stones, such as marbles. Some coarse sand is also used as a
_filtering_ sand in purifying water. These types of sand have been
produced from the Gulf Coastal Plain as well as from other areas of
Texas.
Sand grains, when nature cements them together, make up the sedimentary
rock _sandstone_. Some sandstones form when underground water carrying
dissolved mineral matter moves through loose sand. As the dissolved
mineral matter comes out of solution, it forms a cement that binds the
sand grains together.
The cement may be material such as calcite (calcium carbonate), quartz,
chalcedony, or opal, which are silica minerals, and limonite and
hematite, which are iron oxides. Clay also may serve as a cement. It is
either deposited along with the sand or is formed from weathered
feldspar sand grains.
The color of the cementing material helps determine the color of the
rock. Iron oxide cement, for example, causes the sandstone to have a
reddish, yellowish, or brownish color. Sandstones also are white, black,
gray, green, or cream colored.
Ordinarily, sandstones break through the cementing material, not through
the sand grains. Thus, the broken surface of the rock feels rough and
gritty. Some quartz sand grains, however, are tightly cemented with
silica to form an extremely hard and compact rock. If this rock breaks
smoothly through the grains instead of between them, it is known as
_quartzite_. Some of this sedimentary quartzite occurs in the Texas Gulf
Coastal Plain in the Tertiary Catahoula strata. (Another kind of
quartzite is described on pp. 84-85.)
[Illustration: Sandstone from the Eocene Wilcox Group of strata of
northwestern Zavala County, Texas.]
Ordinary sandstones are seen at the surface in many localities in Texas,
and a number of them have been used as building stones. Some of the
places where sandstones occur are in the Cambrian and Pennsylvanian
formations of the Llano uplift area of central Texas and in the
Pennsylvanian, Permian, and Lower Cretaceous formations of north-central
Texas. Tertiary sandstones occur in the Texas Gulf Coastal Plain, and
Triassic sandstones are found along the edges of the Texas High Plains.
Sandstone is also found in many formations of the Trans-Pecos country of
west Texas.
Sandstone. _See_ Sand and Sandstone.
Satin Spar. _See_ Gypsum.
Schist
Schist is a metamorphic rock that splits easily along thin, generally
parallel layers, called _folia_. These layers may be either straight or
curved, and they are made up of crystalline grains of one or more than
one mineral. This structure is called _schistosity_ or _foliation_. When
you examine schist, you will see that many of the mineral grains are
flat or long, and that they are lined up in one direction to form the
layers. Some schists have fairly large crystals (many with perfect
shapes) scattered through them. For example, mica schists may contain
beautiful crystals of garnet.
Each kind of schist is named for an outstanding mineral that it
contains. Mica schist contains a large amount of mica. We also find
hornblende schist, actinolite schist, chlorite schist, talc schist, and
graphite schist. (Graphite schist is discussed with graphite on p. 63.)
Schists form from other rocks, such as granite, gabbro, or shale. The
rocks are changed into schists by fluids and by heat and pressure below
the earth’s surface.
Extremely ancient schists that formed during Precambrian time are
exposed at the surface in the Allamoore—Van Horn area of west Texas and
in the Llano uplift area of central Texas. Geologists believe that the
Packsaddle Schist of the Llano uplift area was once shale. Good
exposures of this schist are seen in the Honey Creek area near
Packsaddle Mountain in Llano County.
Schorl. _See_ Tourmaline.
Sedimentary Quartzite. _See_ Sand and Sandstone.
Selenite. _See_ Gypsum.
Serpentine
Serpentine is the name given both to a rock and to a mineral. The
mineral serpentine (a hydrous magnesium silicate) is found in two
different forms. If it is fibrous, it is called _chrysotile_; if it is
layered and platy, it is known as _antigorite_. Antigorite is brownish
green and smooth and waxy looking. Some of it can be split into thin
sheets. Chrysotile is made up of greenish, silky fibers, which may be
brittle and break apart in large pieces. If, however, the fibers can be
pulled apart into soft flexible, little threads, the mineral is called
_chrysotile asbestos_.
Light will pass through both these varieties of serpentine, and both are
soft enough to be scratched by a pocket knife. When rubbed across a
streak plate, they leave white streaks. Antigorite and chrysotile have
no crystal shapes of their own, but several other minerals can alter to
form these two varieties of serpentine. Thus antigorite and chrysotile
may be found as _pseudomorphs_ in a crystal shape that originally
belonged to another mineral.
Antigorite and chrysotile are commonly found closely mixed with
dolomite, talc, magnetite, calcite, pyrite, and several other minerals.
These minerals make up serpentine rock (also called _serpentinite_).
This rock ordinarily is some shade of green (such as whitish, yellowish,
brownish, bluish, or dark blackish green), and it may be mottled. It is
brittle or tough and generally is massive. Serpentine rock, like the
serpentine minerals, is fairly soft—you can scratch it with a pocket
knife.
In the Llano uplift area of central Texas, serpentine rock is found
among Precambrian metamorphic rocks, such as gneiss and schist. An
especially large deposit in this area is known as the Coal Creek
serpentine mass. It is over 3½ miles long, and at one place, it is
almost 1½ miles wide. This mass of serpentine extends across the
Blanco-Gillespie County line in the extreme northern parts of these two
counties. (A little fibrous chrysotile is found here, but it will not
break into flexible enough threads to be called chrysotile asbestos.)
Several other deposits of serpentine occur in northeastern Gillespie
County and in southern Llano County.
It is believed that the Coal Creek serpentine was formed from an igneous
rock such as _peridotite_, which is made up chiefly of grains of the
mineral _olivine_. The peridotite may have been altered into serpentine
by underground waters that seeped through it. It is possible, however,
that other serpentines in the area were formed when rocks were altered
by hot fluids and great pressures far below the earth’s surface.
The Llano area serpentine has been widely used in terrazzo floors. To
make these floors, small pieces of serpentine and other colored rocks
are put into cement that is spread over a concrete slab. Then, after the
cement has hardened, it is ground to a flat, smooth surface and
polished. The resulting terrazzo floor is both colorful and durable.
Serpentine rock also is cut into slabs, polished, and used as indoor
building stones. _Verde antique_, a variety often seen in the lobbies of
office buildings, consists of green serpentine rock with streaks of
white calcite or dolomite in it.
In the Balcones fault zone area (shown on the Texas physiographic
outline map, p. 42) from Uvalde County to Williamson County, serpentine
occurs with Upper Cretaceous rocks. The serpentine rock is seen at the
surface in a few places (such as in Travis and Uvalde counties), but
much of it is underground. In several oil fields of this area (as at
Thrall field in Williamson County and at Lytton Springs field in
Caldwell County), the serpentine rocks contain oil.
Serpentinite. _See_ Serpentine.
Shale
Shale is a sedimentary rock made up of tightly packed clay and mud
particles. It has a smooth appearance because it is so fine grained. In
fact, most of the particles in it are too small to be distinguished with
a magnifying glass. These particles are the weathered remains of earlier
rocks. They were carried by creeks and rivers to other parts of the land
or to the sea, where they formed layers of clay and mud. Later, other
sediments were deposited on top of them. The weight of these new
sediments squeezed the clays and muds together to form firm, compact
shale.
Shale looks very much like some clays. It, like clay, can be almost any
color. If the shale contains animal or plant matter, it is black, gray,
or blue. If it contains iron oxide (many minerals containing iron alter
to this material), it is a shade of red, yellow, or brown. Shale is soft
and can be easily scratched by a knife. It also is brittle and crumbles
easily. This rock has a property that will help you to distinguish it
from clay: the particles that make up the shale were deposited in
layers, and the shale splits into flat, thin flakes along these layers,
which clay will not do.
Shale is fairly abundant in Texas, especially in Mississippian,
Pennsylvanian, and Cretaceous formations. For example, Pennsylvanian
shales are found at the surface in north-central Texas, in the area
around the Llano uplift of central Texas, and in the Marathon and
Solitario uplifts of west Texas.
Many of shale’s uses are the same as those of clay. Some of it can be
used to make brick, tile, and other products, and some is often used
instead of clay in making portland cement. Cement plants at Dallas, El
Paso, Fort Worth, and Waco are located at places where Cretaceous
limestones, which also are used in cement making, and Cretaceous shales
are found near each other at the surface.
Oil shale, from which petroleum can be obtained by heating, has been
found in central Texas. It occurs in Mississippian formations in
Lampasas, McCulloch, and San Saba counties. Because oil is much less
expensive to obtain from wells, it is not produced from these shales.
Silver Minerals (Argentite, Cerargyrite, Native Silver)
Silver has many uses. Like gold, it is a beautiful metal that long has
been used for coins and ornaments. A large amount of silver goes to make
articles such as spoons, forks, platters, and trays. The photographic
industry uses silver—much of the film for cameras is coated with a
silver halide. Doctors and dentists use silver, too. The mixture that a
dentist uses to fill teeth contains silver along with several other
metals. Doctors sometimes use silver wire to fasten broken bones, and
silver compounds and solutions, such as silver nitrate, are used in some
kinds of medical treatment.
Perhaps more people have heard of legendary, lost silver mines of Texas
than of the actual and important silver deposits found in the
Trans-Pecos country of west Texas. Some of the west Texas silver
minerals include _argentite_, _cerargyrite_, and _native silver_.
Although the argentite and native silver commonly found there are mixed
with galena, a lead mineral, or with chalcocite, a copper mineral, they
also occur separately.
The element silver is found alone as _native silver_. When pure, it is
rather easy to recognize. It is metallic and has a silver-white color
that may tarnish to gray, black, or yellowish brown. Native silver is
heavy (it has a specific gravity of 10.5) and soft (a pocket knife
scratches it easily). When you rub it across a streak plate, native
silver, unless it is tarnished, leaves a shiny, silver-white streak.
This metal is so ductile that it can be drawn into a wire. It is also
malleable and flattens when hit with a hammer.
Silver occurs as crystals, which are poorly shaped cubes and
octahedrons, or as irregular masses. It may have a net-like appearance
(called _reticulate_), or it may be shaped like little needles
(described then as _acicular_). It occurs in wires (then called
_filiform_) or as scales or plates.
[Illustration: Prospector.]
Two of the Texas silver minerals, _argentite_ and _cerargyrite_, do not
resemble silver at all. _Argentite_, a silver sulfide, is also called
_silver glance_. It is a dark, lead-gray mineral with a metallic luster
that weathers to a dull black. When you rub it across a streak plate,
argentite gives a shiny, blackish to lead-gray streak. This mineral is
soft enough to leave a mark on paper. It has a specific gravity of 7.3,
and it is sectile enough to be cut smoothly (like soap) with a knife. In
some places argentite is found as irregular masses or as a coating on
rocks and other minerals.
Another silver mineral, _cerargyrite_ (or _horn silver_) is a silver
chloride. This mineral has a nonmetallic luster and is transparent to
translucent. It resembles pearl-gray, white, greenish, or colorless wax.
When exposed to the light it turns violet brown or black. Cerargyrite is
soft—you can scratch it with a fingernail. Like argentite, it is
sectile. This mineral has a specific gravity of 5.5, and it commonly
occurs as irregular masses and as crusts.
These silver minerals have been mined at a number of places in
Trans-Pecos Texas. The largest silver mine in Texas, the Presidio mine,
is located near Shafter in south-central Presidio County. It contains
argentite, cerargyrite, and native silver, along with galena and several
other minerals. This mine is not open now, but in the years between 1885
and 1942, it produced a large amount of silver along with some lead and
gold. There are several other lead-silver mines in this Shafter area,
but none has produced as much as the Presidio mine.
In this mine, the silver minerals occur mostly in large, flat deposits
in Permian limestone and other sedimentary rocks. The minerals are
believed to have been deposited there—probably during Tertiary time—by
solutions that came from hot magma far below the rocks. As they moved in
along the layers of limestone, the solutions replaced portions of this
rock with minerals containing silver, lead, and other elements. Later,
water seeped into these deposits and dissolved some of the minerals.
This dissolved material was then re-deposited, and it formed most of the
minerals we now find there.
No silver is being mined in Texas at present, but it has, in the past,
been produced from other Trans-Pecos mines. Galena that contains silver
(called _argentiferous galena_) has been mined at the Bird mine at
Altuda Mountain (about 14 miles east of Alpine) in northern Brewster
County. It also has been obtained from mines in the Quitman Mountains
and in the Eagle Mountains of Hudspeth County. Some cerargyrite has been
mined at the Plata Verde mine near the Culberson-Hudspeth County line.
Several mines in the Van Horn area of Culberson and Hudspeth counties
have produced silver along with copper. An important silver mine in this
area is the now idle and flooded Hazel mine. (This mine is described
with copper minerals on p. 52.)
Smoky Quartz. _See_ Quartz.
Soapstone. _See_ Talc and Soapstone.
Specular Hematite. _See_ Hematite.
Sulfur
Sulfur is one of Texas’ most valuable minerals. It consists of only a
single element, sulfur. This mineral has a resinous luster and is
transparent to translucent. Sulfur ordinarily is yellow, but impurities
cause it to look greenish, brownish, reddish, or grayish. When you rub
it across a streak plate, it leaves a white or a pale-yellow streak.
Sulfur has a specific gravity of 2.04 to 2.09 and is soft enough to be
scratched by a copper penny. It breaks with a conchoidal to uneven
fracture. When it gets hot enough (478° Fahrenheit), sulfur will burn.
For this reason, it often is called _brimstone_.
Sulfur does not conduct electricity and is a poor conductor of heat. You
can test how poorly heat passes through it by holding a fragment of
sulfur up to your ear. You may be able to hear a crackling sound. The
sound results when the outer part of the fragment expands (due to the
heat from your hand) while the inner part (which has received no heat)
remains unchanged.
Crystals of sulfur are sometimes found, and most of them have either a
double-pyramid shape or a flat, tabular shape. Sulfur also occurs as
compact masses, as crusts, and as scattered grains.
Native sulfur deposits are found in two widely separated areas of
Texas—one in west Texas and the other along the Gulf Coast in southeast
Texas, extending over into Louisiana. In the Gulf Coast area, native
sulfur is found on some of the salt domes.
The salt domes are huge (from about half a mile to more than 2 miles
across), column-shaped masses made up of halite and some anhydrite.
These masses have pushed up toward the surface through thousands of feet
of sand, clay, and other sedimentary rocks. On top of many of the salt
columns is a covering of limestone (calcite), anhydrite, and gypsum
known as the _cap-rock_. It is in this cap-rock that the sulfur is
found.
It is thought that when the masses of halite and anhydrite pushed toward
the earth’s surface, some of the upper part of the halite dissolved. The
anhydrite, however, did not dissolve, and it remained on top of the salt
column. Then, a part of this anhydrite was altered into the gypsum,
limestone, and sulfur that now are found in some of the cap-rocks.
Laboratory experiments have shown that the sulfur in the cap-rocks
likely formed through the action of sulfate-reducing bacteria. These
bacteria, in the presence of petroleum, converted the sulfate in some of
the anhydrite into hydrogen sulfide. Later, hydrogen sulfide was
oxidized—perhaps by reaction with more of the anhydrite—to form the
sulfur.
Most of the large cap-rock sulfur deposits are about 1,500 to 2,400 feet
underground. At first, an attempt was made to get this sulfur out of the
ground by digging shafts down to it, but loose, wet, caving sands and
poisonous gases, such as hydrogen sulfide, made this mining method
almost impossible. Finally, a chemist, Herman Frasch, found a way to
obtain the sulfur by making use of sulfur’s low melting point. When
sulfur gets slightly hotter than boiling water (235° to 247°
Fahrenheit), it melts and becomes a dark, yellowish-brown liquid.
In the Frasch method of sulfur mining, a well is drilled into the
salt-dome cap-rock, and three pipes, one inside the other, are put into
the well. Superheated water under pressure (hotter than 212° Fahrenheit,
the temperature at which water ordinarily turns into steam) is sent down
one of the pipes to melt the sulfur in the cap-rock around the bottom of
the well. Then, compressed air is sent down another of the pipes. This
air presses against the liquid sulfur and forces it up to the surface
through the third pipe. At the surface, the sulfur is poured into bins,
where it cools and becomes a solid again, or it is transported molten,
in pipelines and tankers.
Sulfur has been obtained from a number of the Texas Gulf Coast salt
domes including Bryan Mound, Clemens dome, Damon Mound, and Hoskins
Mound in Brazoria County; Palangana dome in Duval County; Long Point
dome, Nash dome, and Orchard dome in Fort Bend County; High Island dome
in Galveston County; Fannett dome and Spindletop dome in Jefferson
County; Moss Bluff dome in Liberty County; Gulf dome in Matagorda
County; and Boling dome in Wharton County.
In west Texas, sulfur occurs in Permian rocks both at the surface and
underground. A small amount of sulfur has been mined in the Rustler
Springs area of northeastern Culberson County and northwestern Reeves
County, about 50 miles northwest of Pecos. There, scattered grains,
crystals, and irregular masses of sulfur occur in cracks and in
dissolved-out openings in the Castile Gypsum and in the surface gravel,
gypsum, sand, and clay that cover most of this formation.
Sulfur has many uses. It is used as an insect-killer, thus helping our
food crops to grow. It is used in pulp and paper manufacturing and in
the vulcanizing of rubber. Some other uses are in the making of paints,
dyes, and explosives. A large amount of sulfur goes to make sulfuric
acid, which itself has numerous uses in the chemical, steel, oil
refining, and other industries.
[Illustration: Sulfur is obtained from the cap-rock of Gulf Coastal
Plain salt domes by the Frasch process.]
Sulfur
Uncemented Sediments
Limestone
Sulfur-Bearing Limestone
Hot Water
Melted Sulfur
Anhydrite
Talc and Soapstone
Talc, a hydrous magnesium silicate, is an extremely soft mineral—your
fingernail scratches it easily. It has a greasy or a pearly luster, and
its color is white, light green, or gray. When rubbed across a streak
plate, it leaves a white streak.
Talc cleaves perfectly in one direction, and the cleavage fragments are
thin, flat, and sheet-like. Its fracture is uneven. This mineral has a
soaplike or greasy feel, and it is sectile—a knife will cut through it.
Talc is not particularly heavy—it has a specific gravity of 2.7 to 2.8.
This mineral seldom occurs with a crystal shape. More commonly it is
massive and is granular or layered.
Talc is not always found as a single, pure mineral. In nature, it
commonly occurs mixed with one or more other minerals, such as
tremolite, anthophyllite, chlorite, and magnetite. This combination of
talc with other minerals forms a soft, greasy or soapy-feeling
metamorphic rock called soapstone. The talc in this rock may be
difficult to identify without special laboratory tests.
In Texas, talc and soapstone are found in Precambrian metamorphic rocks.
In west Texas, talc occurs in an area about 20 miles long (just north of
U. S. Highway 80 in the vicinity of Allamoore, Eagle Flat siding, and
Talc Rock siding) in Hudspeth County. Some of this talc is mined from
open pits and used by the ceramic industry to make wall tile. Some of it
is finely ground, mixed with insect poison, and used as insect powders
and dusts.
[Illustration: Talc schist from the Allamoore area of Hudspeth County,
Texas.]
Deposits of soapstone, containing talc, occur in the Llano uplift area
of central Texas with schist, gneiss, and serpentine rocks in
northeastern Gillespie, northwestern Blanco, and southern Llano
counties. Smaller deposits occur in northeastern Mason County and in
northwestern and southeastern Llano County.
The Llano uplift area soapstones are light green to light buff. It is
thought that some of them were once igneous rocks that contained
magnesium minerals. Fluids, along with great heat and pressures below
the earth’s surface, changed these igneous rocks into soapstone.
Some of this Llano uplift area soapstone is mined from open pits near
Willow City in Gillespie County. It is used mostly in making insect
powders and roofing granules. In addition, some of the central Texas
soapstones have been used for hearths and for fireplace linings.
Topaz
Topaz, an aluminum fluorosilicate, is a mineral especially prized by
collectors because many specimens are gemstones. Topaz is transparent,
has a glassy luster, and is quite hard (neither quartz nor a steel file
will scratch it). The topaz that has been found in Texas is either
colorless, pale blue, or sky blue. This mineral is fairly heavy—its
specific gravity is 3.4 to 3.6. It cleaves perfectly in one direction
(called basal cleavage), and some of the cleavage fragments have a flat,
slabby appearance.
Topaz is commonly found as prism-shaped crystals, as cleavage fragments,
and as irregular grains. Some fragments of topaz look like quartz.
Topaz, however, is harder and heavier than quartz, and it has perfect
basal cleavage, which quartz does not have.
In Texas, crystals, grains, and cleavage fragments of topaz occur in the
Llano uplift area of central Texas. They are found near Streeter and
Grit in west-central Mason County and near Katemcy in northern Mason
County. Here, some of the topaz occurs in Precambrian pegmatite veins
that cut through granite rocks. Most of the topaz, however, is found as
pebbles in the gravels of nearby creeks, where it has washed after
weathering out of the rocks.
[Illustration: Topaz crystal from near Streeter, Mason County, Texas.]
Topaz probably originates when hot fluids move up out of molten magma
into cracks and cavities in the surrounding rocks. There, the fluids
react with elements in the rocks to form the topaz.
Topaz is a good gemstone because, in addition to its beauty, it is hard
and is not easily marred by scratches. The Mason County topaz makes
excellent gemstones. Most of it is beautiful and clear and is either
colorless or of a pleasing blue color. These stones are cut, polished,
and mounted in rings and other jewelry. A number of specimens of this
Mason County topaz are displayed in museums.
Tourmaline
Tourmaline is a complex silicate of boron and aluminum. Other elements,
such as magnesium, sodium, lithium, calcium, iron, or fluorine, also may
be present. This mineral has a glassy to resinous luster. Only the
dark-colored varieties of tourmaline have been found in Texas. One is a
black variety called _schorl_, and another is a brown variety called
_dravite_. Other kinds of tourmaline, although not found in Texas, are
colorless or some shade of blue, yellow, red, pink, or green. Some
crystals even show more than one color.
Tourmaline is too hard to scratch with a steel file, it has a specific
gravity of 3 to 3.25, and it has a conchoidal to uneven fracture. Very
little light passes through the dark varieties, and some fragments of
schorl look like shiny, black coal.
Tourmaline occurs as masses without crystal shapes, but crystals are
commonly found. The crystals are prism-shaped and have small vertical
grooves, called _striations_, on the prism faces. When you look at some
crystals from an end, you will see that the cross section is a triangle
with the sides bowed outward.
[Illustration: Black tourmaline crystals with milky quartz from north of
Llano, Llano County, Texas.]
Both the black and the brown varieties of tourmaline have been found at
several places in the Llano uplift of central Texas. One well-known
locality is at Town Mountain north of Llano in Llano County. Here, the
tourmaline occurs in milky quartz that is associated with Precambrian
granite rocks. In west Texas, in Culberson and Hudspeth counties, black
tourmaline occurs in pegmatite rocks in the Van Horn Mountains, the
Carrizo Mountains, and the Wylie Mountains. In the Eagle Mountains of
Hudspeth County, it is found in metamorphic rocks as well as in
pegmatites.
Some tourmaline formed from hot fluids containing boron that were given
off by magmas far below the earth’s surface. These fluids traveled up
through cracks and other openings in overlying rocks. As the fluids
reacted with other elements and compounds, the tourmaline formed.
The clear, light-colored varieties of tourmaline are much admired, and
they are more widely used as gemstones than are the dark-colored
varieties. Some collectors, however, find that the dark-colored Texas
tourmalines, when cut and polished, make shiny, attractive gemstones.
Some tourmaline is used as grinding material, but no Texas tourmaline is
produced for this purpose.
Travertine. _See_ Calcite.
Uranium Minerals (Carnotite, Uranophane, Pitchblende)
In 1945, the world suddenly became aware of the awesome power of atomic
energy when the element _uranium_ was used to produce some of the first
atomic bombs. Uranium does not occur alone in nature but is found
combined with other elements in a number of minerals.
All of the uranium minerals are radioactive. The uranium they contain is
gradually breaking down and changing into a series of 13 other elements,
called _daughter_ elements. Each daughter element breaks down and
changes into the next daughter element of the series. While breaking
down, these elements give off particles and rays of energy.
This energy or _radioactivity_ is made up of what are called alpha
particles, beta particles, and gamma rays. You cannot see, hear, taste,
smell, or feel them. The alpha and beta particles are weak and do not
travel far. The gamma rays, however, can travel farther and can pass
through seemingly solid material. Scientists have found that these rays
can move through about 1 foot of rock, 2½ feet of water, and several
hundred feet of air.
Prospectors searching for uranium minerals carry instruments that are
able to detect this radioactivity. The uranium itself gives off only
alpha particles, but some of its daughter elements give off gamma rays.
These daughter elements are normally found with the uranium, and it is
their strong gamma rays that the instruments are most apt to detect.
[Illustration: A Geiger counter is used to detect radioactivity.]
One of the instruments used is the _Geiger counter_. It indicates
radioactivity by means of a meter, a flashing light, or a clicking
sound, which can be heard through earphones. Another instrument for
detecting radioactivity is the _scintillation counter_. It is more
sensitive than the Geiger counter and it can detect radioactivity from a
greater distance. The scintillation counter can be used from an
automobile or an airplane, but the Geiger counter must be quite close to
the source of radioactivity to be of use.
Various uranium minerals have been found, mostly in small amounts, in a
number of places in Texas. Some of these minerals, such as uraninite or
pitchblende, are heavy and dark colored. Others, including carnotite,
tyuyamunite, autunite, and uranophane, are a shade of yellow or green.
They are quite soft. Deposits of the light-colored uranium minerals have
been mined from two areas of Texas. One of these areas is in Garza
County on the Texas High Plains, and the other is in Karnes and Live Oak
counties in the Gulf Coastal Plain.
One of the light-colored uranium minerals, _carnotite_, is a
potassium-uranium vanadate, which has a bright canary-yellow or
lemon-yellow color. This mineral is transparent to translucent and has
an earthy or a pearly luster. Carnotite usually is found as crusts and
as powdery masses. It is quite soft and can be scratched with a
fingernail.
Carnotite, along with tyuyamunite, autunite, and several other soft,
yellowish or greenish uranium minerals, is found in the Texas Gulf
Coastal Plain. These minerals occur in the Jackson, Catahoula, and
Oakville strata (which are Tertiary in age) in an area extending from
Gonzales County to the Rio Grande (in parts of the area indicated by no.
2 and no. 3 on the geologic map, pp. 4-5). The largest deposits in this
district have been found in the Karnes County area.
The Gulf Coastal Plain uranium minerals occur mostly with sandstones and
clays in a sequence of strata that contains volcanic ash. It is believed
that small scattered amounts of uranium compounds that were present in
the volcanic ash sediments were dissolved by seeping underground water.
These waters then moved into the sandstones and clays where they
deposited the uranium as carnotite and as other uranium minerals.
Another uranium mineral, _uranophane_ (calcium-uranium silicate), also
occurs in Texas. Uranophane has a yellow to yellow-orange color and a
pearly to greasy luster. When rubbed across a streak plate, it leaves a
light yellow to a light yellow-orange streak. It is soft enough to be
scratched by a copper penny. Uranophane has been found in extrusive
igneous rocks in northwestern Presidio County in west Texas.
A dark-colored uranium mineral, _pitchblende_, is a variety of the
mineral _uraninite_, uranium dioxide. Pitchblende does not occur with a
crystal shape but rather as rounded and irregular-shaped masses. It is
brownish black, greenish black, or black. If you rub it across a streak
plate, pitchblende leaves a brownish-black streak. This mineral is heavy
(it has a specific gravity of 6.5 to 8.5) and hard (a pocket knife will
not scratch it, although a steel file will). Pitchblende has a
submetallic luster and looks dull, greasy, or like pitch or tar.
Small amounts of pitchblende have been found at several places in Texas.
One of these localities is a few miles west of Burnet in Burnet County
in central Texas. Here, the pitchblende occurs in Precambrian igneous
rocks that are associated with gneiss. In south Texas, some fine,
scattered particles of pitchblende have been found about 325 feet below
the surface in Tertiary (Pliocene) sediments that cover the Palangana
salt dome in Duval County. No pitchblende is mined in Texas.
Uranophane. _See_ Uranium Minerals.
Vitrophyre. _See_ Obsidian and Vitrophyre.
Volcanic Ash (Pumicite)
Volcanic ash deposits, which also are known as _pumicite_, are loose and
powdery. They are made up mostly of material that is thrown into the air
when volcanoes erupt. If a volcano erupts with a violent explosion, the
nearby rocks are blown into powder. Molten lava also is hurled into the
air, where some of it immediately cools to become tiny bubbles and
particles of glass. The winds may carry some of this fine material far
away before depositing it.
Deposits of volcanic ash are white, bluish, greenish, yellowish, or
grayish, and some of them glisten like snow in the sunlight. They feel
rough and gritty. When examined under a microscope, this material shows
the tiny curved and sharp-cornered particles of the broken volcanic
glass. Deposits of volcanic ash may also contain clay, silt, sand, or
other impurities.
Volcanoes, which may have been located in the Davis Mountains and in
other areas of west Texas and in northern Mexico, erupted during
Tertiary time. The volcanic ash that we find at the surface today in
some of the Tertiary formations in Texas could have come from these
volcanoes. Tertiary volcanic ash deposits occur in the Texas Gulf
Coastal Plain (such as in Brazos, Fayette, Karnes, Polk, Starr, Trinity,
and other counties) and in the Trans-Pecos country of west Texas.
Volcanic ash deposits of Quaternary (Pleistocene) age, which are less
than a million years old, are found in a number of counties on the Texas
High Plains. Farther to the east, ash deposits occur in Baylor, Dickens,
Kent, and Wilbarger counties. This volcanic ash may have come from a
volcano that erupted in northern New Mexico during Quaternary time.
Volcanic ash or pumicite has several commercial uses. Some is used to
make pozzolan cement, and some is used in sweeping compounds, cleansing
and scouring powders, and abrasive soaps. Pumicite has been mined in
Dickens, Scurry, Starr, and several other counties of Texas.
Wad. _See_ Manganese Minerals.
Wood Opal. _See_ Opal.
COMPOSITION, HARDNESS, AND SPECIFIC GRAVITY OF SOME TEXAS MINERALS
For convenient reference, the Texas minerals described in this book are
listed below, together with their chemical compositions, specific
gravities, and hardness. You will be able to find similar information
about additional minerals in mineralogy textbooks such as those noted on
page 24.
_Mineral_ _Composition_ _Specific Gravity_ _Hardness_
Albite NaAlSi₃O₈ 2.62 6
Almandite Fe₃Al₂ (SiO₄)₂ 4.2 7
Amphibole Ca₂Mg₅Si₈O₂₂(OH)₂ 3.0-3.3 1-2½
asbestos
Anhydrite CaSO₄ 2.9 3-3½
Argentite Ag₂S 7.3 2-2½
Azurite Cu₃(CO₃)₂(OH)₂ 3.77 3½-4
Barite BaSO₄ 4.5 3-3½
Biotite K(Mg, 2.8-3.2 2½-3
Fe)₃AlSi₃O₁₀(OH)₂
Braunite 3MnMnO₃MnSiO₃ 4.75-4.82 6-6½
Calcite CaCO₃ 2.72 3
Carnotite K₂O·2UO₃·V₂O₅·nH₂O 5.03 2
Cassiterite SnO₂ 6.8-7.1 6-7
Celestite SrSO₄ 3.95-3.97 3-3½
Cerargyrite AgCl 5.5 1-1½
Chalcocite Cu₂S 5.5-5.8 2½-3
Chalcopyrite CuFeS₂ 4.1-4.3 3½-4
Cinnabar HgS 8.10 2½
Dolomite CaMg(CO₃)₂ 2.85 3½-4
Feldspar (_see_ Albite, Microcline, Orthoclase)
Fluorite CaF₂ 3.18 4
Galena PbS 7.4-7.6 2½
Garnet (_see_ Almandite, Grossularite)
Gold Au 15.0-19.3 2½-3
Graphite C 2.2 1-2
Grossularite Ca₃Al₂(SiO₄)₃ 3.53 6½
Gypsum CaSO₄·2H₂O 2.32 2
Halite NaCl 2.16 2½
Hematite Fe₂O₃ 5.26 1-6½
Hollandite MnBaMn₁₆O₁₄ 4.7-5 4-6
Limonite FeO(OH)·nH₂O 3.6-4.0 1-5½
Magnetite Fe₃O₄ 5.18 6
Malachite Cu₂CO₃(OH)₂ 3.9-4.03 3½-4
Mica (_see_ Muscovite, Biotite)
Microcline KAlSi₃O₈ 2.54-2.57 6
Muscovite KAl₃Si₃O₁₀(OH)₂ 2.76-3.1 2-2½
Opal SiO₂·nH₂O 1.9-2.2 5-6
Orthoclase KAlSi₃O₈ 2.57 6
Pitchblende UO₂ 6.5-8.5 5½
Pyrite FeS₂ 5.02 6-6½
Pyrolusite MnO₂ 4.75 1-2
Quartz SiO₂ 2.65 7
Serpentine Mg₃Si₂O₅(OH)₄ 2.48 3-4
Silver Ag 10.5 2½-3
Sulfur S 2.05-2.09 1½-2½
Talc Mg₃Si₄O₁₀(OH)₂ 2.7-2.8 1
Topaz Al₂SiO₄(F,OH)₂ 3.4-3.6 8
Tourmaline Complex silicate of 3.0-3.25 7-7½
boron and aluminum
Uranophane CaO·2UO₃·2SiO₂·7H₂O 3.8-3.9 2-3
BOOKS ABOUT ROCKS AND MINERALS
Many books have been written about rocks and minerals. Some are listed
below, and it is likely that your librarian will be able to suggest
others.
Nontechnical Books for Beginners
Getting Acquainted With Minerals, by George L. English and David E.
Jensen. McGraw-Hill Book Company, Inc., New York, N. Y. (second edition,
1958).
The Rock Book, by Carroll L. Fenton and Mildred A. Fenton. Doubleday &
Company, Inc., Garden City, N. Y. (1940).
Mineral Collector’s Guide, by David E. Jensen. Ward’s Natural Science
Establishment, Inc., Rochester, N. Y. (1953).
My Hobby is Collecting Rocks and Minerals, by David E. Jensen. Hart Book
Company, New York, N. Y. (1955).
Rocks and Minerals, by Richard M. Pearl. Barnes & Noble, New York, N. Y.
(1956).
1001 Questions Answered About the Mineral Kingdom, by Richard M. Pearl.
Dodd, Mead & Company, New York, N. Y. (1959).
Rocks and Minerals, by Herbert S. Zim and Paul R. Schaffer. Simon and
Schuster, Inc., New York, N.Y. (1957).
Textbooks and Other Reference Books
Economic Mineral Deposits, by Alan M. Bateman. John Wiley & Sons, Inc.,
New York, N. Y. (second edition, 1950).
A Textbook of Mineralogy, by Edward S. Dana, revised by William E. Ford.
John Wiley & Sons, Inc., New York, N. Y. (fourth edition, 1932).
Industrial Minerals and Rocks (Nonmetallics Other Than Fuels), Joseph L.
Gillson, Editor-in-Chief. The American Institute of Mining,
Metallurgical, and Petroleum Engineers, New York, N. Y. (third edition,
1960).
Dana’s Manual of Mineralogy, revised by Cornelius S. Hurlbut, Jr. John
Wiley & Sons, Inc., New York, N. Y. (seventeenth edition, 1959).
Mineralogy, by Edward H. Kraus, Walter F. Hunt, and Lewis S. Ramsdell.
McGraw-Hill Book Company, Inc., New York, N. Y. (fifth edition, 1959).
Nonmetallic Minerals, by Raymond B. Ladoo and W. M. Meyers. McGraw-Hill
Book Company, Inc., New York, N. Y. (second edition, 1951).
Rocks and Rock Minerals, by Louis V. Pirsson, revised by Adolph Knopf.
John Wiley and Sons, Inc., New York, N. Y. (third edition, 1947).
A Field Guide to Rocks and Minerals, by Frederick H. Pough. Houghton
Mifflin Company, Boston, Mass. (third edition, 1960).
Mineral Facts and Problems, by the Staff of the Bureau of Mines. U. S.
Bureau of Mines Bulletin 585. U. S. Government Printing Office,
Washington, D. C. (1960).
Selected References on Texas Rocks and Minerals
Entries marked with an asterisk are published by the Bureau of
Economic Geology, The University of Texas, Austin. Those not out of
print are distributed at nominal sale price, and a list of
publications will be sent on request. These publications can be
consulted at many public libraries and Chamber of Commerce offices.
*Report on the Pavitte Silver-Copper Prospect in Burnet County, Texas,
by V. E. Barnes. Univ. Texas, Bureau Econ. Geol. Mineral Resource Survey
Circ. 5 (1936).
*Report on the Sheridan Copper Prospect in Burnet County, Texas, by V.
E. Barnes. Univ. Texas, Bur. Econ. Geol. Mineral Resource Survey Circ. 9
(1936).
*Building Stones of Central Texas, by V. E. Barnes, R. F. Dawson, and G.
A. Parkinson. Univ. Texas Pub. 4246 (1947).
*Iron Ore in the Llano Region, Central Texas, by V. E. Barnes. Univ.
Texas, Bur. Econ. Geol. Rept. Inves. No. 5 (1949).
*Utilization of Texas Serpentine, by V. E. Barnes, D. A. Shock, and W.
A. Cunningham. Univ. Texas Pub. 5020 (1950).
*Lead Deposits in the Upper Cambrian of Central Texas, by V. E. Barnes.
Univ. Texas, Bureau Econ. Geol. Rept. Inves. No. 26 (1956).
*Mineral Resources of the Colorado River Industrial Development
Association Area, by J. W. Dietrich and J. T. Lonsdale. Univ. Texas,
Bureau Econ. Geol. Rept. Inves. No. 37 (1958).
*Some Uranium Occurrences in West Texas, by D. H. Eargle. Univ. Texas,
Bureau Econ. Geol. Rept. Inves. No. 27 (1956).
*A Preliminary Report on the Stratigraphy of the Uranium-Bearing Rocks
of the Karnes County Area, South-Central Texas, by D. H. Eargle and J.
L. Snider. Univ. Texas, Bureau Econ. Geol. Rept. Inves. No. 30 (1957).
The Brown Iron Ores of Eastern Texas, by E. B. Eckel. U. S. Geol. Survey
Bull. 902 (1938).
*The Rustler Springs Sulphur Deposits as a Source of Fertilizer, by G.
L. Evans. Univ. Texas, Bureau Econ. Geol. Rept. Inves. No. 1 (1946).
Origin of the Gulf Coast Salt-Dome Sulphur Deposits, by Herbert W. Feely
and J. Lawrence Kulp. Bull. Amer. Assoc. Petrol. Geol., vol. 41, pp.
1802-1853 (1957).
*Pegmatites of the Van Horn Mountains, Texas, by P. T. Flawn. Univ.
Texas, Bureau Econ. Geol. Rept. Inves. No. 9 (1951).
*The Hazel Copper-Silver Mine, Culberson County, Texas, by P. T. Flawn.
Univ. Texas, Bureau Econ. Geol. Rept. Inves. No. 16 (1952).
*Basement Rocks of Texas and Southeast New Mexico, by P. T. Flawn. Univ.
Texas Pub. 5605 (1956).
*Texas Miners Boost Talc Output, by P. T. Flawn. Univ. Texas, Bureau
Econ. Geol. Rept. Inves. No. 35 (1958).
*Geology and Mineral Deposits of Pre-Cambrian Rocks of the Van Horn
Area, Texas, by P. B. King and P. T. Flawn. Univ. Texas Pub. 5301
(1953).
*Igneous Rocks of the Balcones Fault Region of Texas, by J. T. Lonsdale.
Univ. Texas Bull. 2744 (1927).
Mineral Resources of the Llano-Burnet Region, Texas, with an Account of
the Pre-Cambrian Geology, by Sidney Paige. U. S. Geol. Survey Bull. 450
(1911).
*Mineral Resources of the Texas Coastal Plain (Preliminary Report), by
J. M. Perkins and J. T. Lonsdale. Univ. Texas, Bureau Econ. Geol.
Mineral Resource Circ. 38 (1955).
Geology and Ore Deposits of the Shafter Mining District, Presidio
County, Texas, by C. P. Ross. U. S. Geol. Survey Bull. 928-B (1943).
*The Geology of Texas, Vol. II, Structural and Economic Geology, by E.
H. Sellards, C. L. Baker, and others. Univ. Texas Bull. 3401 (1935).
*Texas Mineral Resources, by E. H. Sellards and others. Univ. Texas Pub.
4301 (1946).
*Geological Resources of the Trinity River Tributary Area in Texas and
Oklahoma, by H. B. Stenzel, A. E. Weissenborn, and others. Univ. Texas
Pub. 4824 (1948).
Uranium at Palangana Salt Dome, Duval County, Texas, by A. D. Weeks and
D. H. Eargle. _In_ U. S. Geol. Survey Prof. Paper 400-B (1960).
Geology of the Quicksilver Deposits of the Terlingua District, Texas, by
R. G. Yates and G. A. Thompson. U. S. Geol. Survey Prof. Paper 312
(1959).
GLOSSARY
Amorphous—without crystalline structure and therefore without regular
form.
Balcones fault zone—a system of faults extending from north of Waco in
McLennan County, through Travis and Bexar counties, to near Del Rio in
Val Verde County (_see_ p. 42).
Boulder—a large rock or mineral fragment that has a diameter greater
than 256 millimeters (about 10 inches).
Breccia—a rock made up of sharp-cornered, cemented fragments with
diameters greater than 2 millimeters (about ⁸/₁₀₀ of an inch).
Cambrian—the earliest period of the Paleozoic Era (_see_ p. 3).
Cenozoic—the present era, one of the great divisions of geologic time
(_see_ p. 3). This era began about 63 million years ago.
Clastic—made up of broken fragments of rocks or minerals.
Cleavage—occurs when minerals split along smooth flat surfaces that are
parallel to possible crystal faces. These planes as well as crystal
faces are controlled by the crystal lattice or atomic structures of the
minerals.
Cleavage fragment—a mineral specimen that has been broken along its
planes of cleavage.
Cobble—a rock or mineral fragment that has a diameter between 64 and 256
millimeters (about 2½ and 10 inches).
Conchoidal—a curved fracture surface shaped like the inside of a shell
or spoon.
Conglomerate—a rock composed of cemented, rounded rock or mineral
fragments, most of which are of gravel size.
Cretaceous—the third and latest period of the Mesozoic Era (_see_ p. 3).
Cryptocrystalline—made up of tiny crystalline particles that are too
small to be distinguished even under high magnification.
Crystalline—having a definite, orderly internal structure.
Cube—a solid that has six equal, square sides.
Dodecahedron—a solid that has twelve plane, four-sided faces.
Element—a basic building block of all matter, which cannot be separated
into different substances by ordinary chemical means.
Eocene—the second epoch of the Tertiary Period (_see_ p. 3).
Epoch—a unit of geologic time that is a subdivision of a period.
Era—a major division of geologic time, which consists of several
periods.
Extrusive rocks—igneous rocks formed from magma that was extruded on the
earth’s surface.
Fault—a break in the rocks or strata of the earth’s crust along which
movement or slippage has taken place.
Fluid—a substance made up of particles that can move freely about; it
can be a liquid or a gas.
Formation—rocks or strata that are recognized and mapped as a unit.
Fracture—the kind of surface obtained if a mineral is broken in a
different direction from that of the cleavage or parting. Commonly,
fracture surfaces are rough, uneven, or curved, whereas cleavage
surfaces are smooth.
Geologic map (areal)—shows the extent and distribution of formations
exposed at the earth’s surface.
Granular—the texture of a rock or mineral that is made up of visible
grains. If all the grains are about the same size, the term
_equigranular_ is used.
Granule—a rock or mineral fragment that has a diameter of from 2 to 4
millimeters (about ⁸/₁₀₀ to ¹⁵/₁₀₀ of an inch).
Gravel—uncemented rock or mineral fragments that have diameters greater
than 2 millimeters (about ⁸/₁₀₀ of an inch).
Gulf Coastal Plain—an area that extends, in Texas, from the Gulf of
Mexico to the Balcones fault zone and in which Quaternary, Tertiary, and
Upper Cretaceous strata crop out at the surface (_see_ p. 42).
High Plains—an area in northwest Texas extending from the Pecos River
valley north to the Oklahoma-Texas boundary (_see_ p. 42).
Igneous rocks—rocks formed by the cooling and hardening of hot, molten
rock material.
Intrusive rocks—igneous rocks that have formed below the surface of the
earth.
Lava—molten rock material that has poured out onto the earth’s surface
from volcanoes; also the rock that is formed after the molten material
has cooled and hardened.
Llano uplift—an area in central Texas where Precambrian and early
Paleozoic rocks occur at the earth’s surface (_see_ p. 42).
Magma—hot, molten rock material from which igneous rocks form.
Massive—in a mass, without a regular or complete form.
Mesozoic—an era, one of the great divisions of geologic time (_see_ p.
3). This era began about 230 million years ago and lasted until about 63
million years ago.
Metamorphic rock—rock formed from igneous or sedimentary rocks that are
altered by heat, pressure, and fluids below the earth’s surface.
Miocene—the fourth epoch of the Tertiary Period (_see_ p. 3).
Mississippian—the fifth period of the Paleozoic Era (_see_ p. 3).
Nodule—a small, rounded mass or lump.
Octahedron—a solid that has eight triangular faces.
Opaque—no light can pass through.
Ordovician—the second period of the Paleozoic Era (_see_ p. 3).
Paleozoic—an era, one of the great divisions of geologic time (_see_ p.
3). This era began at the end of Precambrian time and lasted until about
230 million years ago.
Parting—occurs when a mineral breaks along a flat surface that is not a
true cleavage plane.
Pebble—a rock or mineral fragment that has a diameter between 4 and 64
millimeters (about ¹⁵/₁₀₀ and 2½ inches).
Pennsylvanian—the sixth period of the Paleozoic Era (_see_ p. 3).
Period—a unit of geologic time, a subdivision of an era.
Permian—the last period of the Paleozoic Era (_see_ p. 3).
Physiographic outline map—shows location of natural regions (p. 42).
Playa lake—a temporary shallow lake in a nearly level, closed basin,
which has no drainage outlet.
Pleistocene—the first epoch of the Quaternary Period (_see_ p. 3).
Pliocene—the last epoch of the Tertiary Period (_see_ p. 3).
Precambrian—comprises the Early and the Late Precambrian Eras, the
earliest great divisions of geologic time. Rocks that formed more than
600 million years ago are known as Precambrian rocks.
Pyritohedron—a solid that has twelve 5-sided faces.
Quaternary—the present period of geologic time; the second period of the
Cenozoic Era (_see_ p. 3).
Recent—the present epoch of geologic time; the second epoch of the
Quaternary Period (_see_ p. 3).
Sectile—describes material, such as soap, that can be cut smoothly with
a knife.
Sediments—material deposited by water, wind, or ice on the earth’s
surface.
Sedimentary rocks—rocks made up of sediments.
Series—a subdivision of a system that includes all rocks formed during
an epoch.
Specific gravity—the ratio of the weight of a substance to the weight of
an equal volume of water.
Streak—the color of the powder of a mineral.
System—all rocks formed during a period.
Tertiary—the first period of the Cenozoic Era (_see_ p. 3).
Translucent—light will pass through, but objects cannot be seen.
Transparent—light will pass through, and objects can be seen.
Trans-Pecos—area of Texas located west of the Pecos River (_see_ p. 42).
Volcanic rocks—igneous rocks that have formed on the earth’s surface;
extrusive rocks.
Index
A
acid: 18, 22
acid tests (_see also_ chemical tests): 18
actinolite: 82
schist: 87
agate: 21, 83
agatized wood: 20, 84
age, earth’s crust: 2
alabaster: 65
albite: 55
alkali lakes: 66, 83
Allamoore: 87, 93
alloys: 52
almandite: 58
Alpine: 57, 78, 82, 90
Altuda Mountain: 57, 90
amalgam: 51
amazonstone: 56
amethyst: 37, 82
Amethyst Hill: 82
amorphous minerals, definition of: 15
amphibole asbestos: 29, 33, 34, 44
Anderson County: 70
Andrews County: 83
Angelina County: 52
anhydrite: 28, 29, 30, 35, 36, 43, 65, 66, 91
anthophyllite: 93
antigorite: 87
Apache Mountains: 65
apatite: 72
Archer County: 52
argentite: 26, 27, 89, 90
argentiferous galena: 57, 90
Armstrong County: 79
asbestos: 43
amphibole: 29, 33, 34, 44
chrysotile: 44, 87
ash, volcanic: 41, 97
augite: 45
Austin: 46, 62
autunite: 97
azurite: 35, 52, 53, 54
B
Babyhead: 72
Balcones fault zone: 10, 42, 45, 88
Bandera County: 45
Baringer Hill pegmatite: 80
barite: 15, 28, 31, 32, 35, 36, 38, 44, 48
barium: 45
Barrilla Mountains: 85
Baylor County: 45, 52, 98
basal cleavage: 76, 93, 94
basalt: 40, 41, 45
basement: 2, 9
Bastrop County: 64
Bell County: 69
bentonite: 51, 52
beryllium: 80
Big Bend area: 9, 42, 51, 75, 77
Big Branch Gneiss: 59
biotite: 32, 33, 34, 76
Bird mine: 90
Blanco County: 44, 58, 59, 61, 70, 83, 87, 94
black mica (_see_ biotite)
blast furnace: 70, 71
blast sand: 86
bleaching clay: 51
blue copper: 53
Boerne: 18
Boling salt dome: 91
books about rocks and minerals: 24, 39, 100
brass: 52
braunite: 26, 27, 73
Brazoria County: 91
Brazos County: 98
Brazos River, Double Mountain Fork and Salt Fork of: 43
breccia: 12, 41, 81
Brewster County: 45, 46, 47, 49, 50, 57, 65, 70, 76, 77, 78, 82,
84, 85, 90
brimstone: 90
bronze: 52
Brooks County: 64
Brown, Thomas E.: vii
Brown County: 45, 49
brown iron ore (_see also_ limonite): 70
Bryan Mound salt dome: 91
building sand: 85
Burnet: 56, 80, 97
Burnet County: 18, 53, 54, 55, 56, 58, 59, 60, 61, 62, 63, 70, 76,
79, 80, 81, 82, 83, 97
C
calcareous sinter and tufa: 46
calcite: 7, 14, 18, 19, 20, 21, 28, 30, 31, 32, 34, 35, 36, 38,
45, 46, 49, 50, 51, 54, 56, 64, 68, 75, 86, 87, 88, 91
dog-tooth spar: 46, 47
Caldwell County: 88
caliche: 46, 47
Cambrian: 2, 3, 55, 58, 69, 86
Cameron County: 66
Capitan reef limestone: 1
Capitol building, Texas State: 62
cap-rock: 91, 92
carats: 60
carbon: 62, 63
carbonized wood: 20
carnotite: 30, 95, 97
Carrizo Mountains: 67, 83, 95
Casey, Miss Josephine: vii
Cass County: 70
cassiterite: 27, 32, 47
Castile Gypsum: 91
Catahoula strata: 78, 86, 97
cave deposits: 18
caves: 18, 19, 49
Caverns of Sonora: 19
Cedar Lake: 83
Cedar Park: 70
celestite: 20, 28, 30, 35, 38, 48
cementing materials: 12, 46, 67, 70, 86
cerargyrite: 28, 29, 34, 37, 89, 90
cerium: 80
chalcedony (_see also_ agate): 16, 21, 45, 78, 81, 83, 86
chalcocite: 21, 26, 27, 52, 54, 73, 89
chalcopyrite: 26, 52, 53, 54, 60
chalk: 40, _69_
chemical elements: 7
chemical sediments: 12
chemical tests: 18, 43, 46, 47, 49, 52, 53, 54, 64, 68, 75
Cherokee County: 70
chert: 19, 40, 81, 84
chessylite: 53
china clay: 51
Chinati Mountains: 45, 57, 62, 85
Chisos Mountains: 45, 62, 85
chlorite: 93
schist: 87
chrysotile: 44, 87, 88
asbestos: 44, 87
cinnabar: 36, 49
clastic rocks: 12
clay: 40, 45, 49, 51, 70, 86, 88, 91
bentonite: 45, 51, 52
bleaching clay: 51
china clay: 51
kaolin: 51
clay minerals: 51
Clay County: 52
Clear Creek area: 63
cleavage: 17
basal: 76, 93, 94
cubic: 17, 57, 65
octahedral: 56, 57
pyramidal: 74
rhombohedral: 46, 54
Clemens salt dome: 91
Coal Creek serpentine: 87, 88
Coke County: 20, 49
Coleman County: 86
color of minerals: 16
columns: 18
Comal County; 45, 69
Comanche County: 49
common opal: 78
composition of minerals: 99
concretions: 19
conglomerate: 11, 12, 41, 55, 81
gneiss: 59
contact metamorphism: 13
copper: 52, 59, 90
blue copper: 53
glance: 52
green copper carbonate: 53
minerals: 52
pyrites: 53
yellow copper ore: 53
coquina: 41, 69
Crane County: 66
Cretaceous: 2, 3, 20, 21, 45, 46, 49, 51, 55, 57, 60, 65, 68, 69,
70, 74, 75, 84, 86, 88
cryptocrystalline quartz: 81, 84
crystal, definition of: 14
crystals, twinned: 46, 63
crystalline minerals, definition of: 14
Crosby County: 79
crust, earth’s: 2
cubic cleavage: 17, 57, 65
Culberson County: 1, 45, 52, 53, 58, 59, 60, 65, 75, 79, 83, 90,
91, 95
D
Daingerfield: 70
Dallas: 88
Damon Mound salt dome: 91
daughter elements: 95, 96
Davis Mountains: 45, 85, 98
Delaware Mountains: 65
diamond: 62
diatomaceous earth: 78
diatomite: 40, 78
Dickens County: 79, 98
Dietrich, John W.: vii
dikes: 72, 79
dog-tooth spar: 46, 47
dolomite: 29, 30, 32, 36, 38, 40, 43, 54, 68, 75, 87, 88
dolomitic limestone: 68
Double Mountain Fork of the Brazos River: 43
double refraction: 46
dravite: 95
Duval County: 78, 83, 91, 97
dynamic metamorphism: 14
E
Eagle Flat: 93
Eagle Mountains: 56, 57, 90, 95
earth’s crust: 2
east Texas: 70
Ector County: 79
Edwards Limestone: 68, 69, 84
effervescence in acid: 18
El Capitan Peak: 1
elements: 7, 8, 95, 96
Ellenburger strata: 55, 84
El Paso: 48, 58, 85, 88
El Paso County: 48, 58, 62
epoch: 2
era: 2
erosion: 11
extrusive igneous rocks, definition of: 9
F
Fairland: 54
Falfurrias: 64
Fannett salt dome: 91
Fayette County: 52, 64, 84, 98
feldspar: 8, 29, 30, 31, 33, 34, 35, 37, 45, 51, 55, 59, 61, 72,
79, 80, 85, 86
albite: 55
amazonstone: 56
microline: 55, 56, 61
orthoclase: 55, 61
fibrous gypsum: 64, 65
filtering sand: 86
Fisher County: 20, 49, 58, 65
Flawn, Peter T.: vii
flint: 84
flowstone: 18
flow structure: 85
fluorite: 29, 32, 34, 35, 37, 38, 56, 72
Foard County: 52, 58
foliation: 87
fool’s gold: 53, 80
formation: 6
Fort Bend County: 91
Fort Worth: 88
fracture of minerals: 17
fragmental rocks: 12
Franklin Mountains: 48, 58, 62, 85
Frasch method of sulfur mining: 91, 92
Fredonia: 67
Freer: 78, 83
furnace (_see_ blast furnace; open-hearth furnace)
G
gabbro: 87
gneiss: 59
Gaines County: 83
galena: 17, 26, 57, 89, 90
argentiferous: 57, 90
Galveston County: 91
Gamble prospect: 67
gamma rays: 96
garnet: 31, 33, 34, 37, 58, 85, 87
Garza County: 97
Geiger counter: 96
gemstones: 56, 58, 77, 82, 94, 95
geode: 19, 46, 49, 82
geologic map: 4, 5
geologic time scale: 2, 3
geologists: 2
geology: 2
Gillespie County: 44, 45, 55, 58, 59, 60, 61, 65, 76, 80, 82, 83,
87, 88, 94
glass, natural: 9, 77, 85, 97
glass sand: 85-86
glauconite: 70, 73
glossary: 102-103
gneiss: 13, 41, 55, 56, 59, 67, 72, 73, 74, 76, 81, 87, 93, 97
conglomerate: 59
gabbro: 59
granite: 59
hornblende: 59
mica: 59
goethite: 70
gold: 26, 51, 59, 80, 90
fool’s: 53, 80
Gonzales County: 52, 84, 97
Grand Saline salt dome: 66
granite: 2, 8, 9, 13, 41, 48, 55, 56, 59, 61, 72, 76, 79, 81, 85,
87, 94, 95
gneiss: 59
opaline: 72
Granite Mountain: 62
Granite Shoals Lake: 56
granules: 85
graphite: 26, 27, 62, 75
schist: 63, 87
gravel: 12, 41, 91
green copper carbonate: 53
greensands: 70
Grit: 94
grossularite: 58
group: 6
Guadalupe Mountains, Peak: 1
Gulf Coast: 91
Gulf Coastal Plain: 20, 42, 43, 46, 51, 52, 58, 60, 65, 66, 78,
83, 84, 86, 87, 92, 97, 98
Gulf salt dome: 91
Gyp Hill salt dome: 64
gypsite: 65
gypsum: 28, 29, 30, 32, 36, 37, 43, 63, 70, 85, 91
alabaster: 65
fibrous gypsum: 64, 65
rock gypsum: 40, 65
rosettes: 63, 64
satin spar: 64
selenite: 63, 64, 76
H
halite: 28, 32, 34, 35, 36, 38, 40, 43, 65, 91
hand lens: 22
Hardeman County: 52, 58, 64, 65
hardness of minerals: 16, 99
Harris, Bill M.: vii
Harris, John S.: vii
Harris County: 65, 66
Hartley County: 79
Hays County: 45
Hazel Formation: 52
Hazel mine: 52, 53, 90
Heath mine: 60
hematite: 26, 27, 36, 66, 75, 84, 86
specular: 67
Henderson County: 70
High Island salt dome: 91
High Plains: 42, 46, 51, 65, 66, 78, 79, 83, 87, 97, 98
Hockley salt dome: 65, 66
hollandite: 26, 27, 28, 73, 74
Honey Creek area: 87
hornblende: 8, 59, 61
gneiss: 59
schist: 87
horn silver: 90
Hoskins Mound salt dome: 91
Houston: 70
Howard County: 45, 60
Hudspeth County: 45, 52, 53, 55, 56, 57, 58, 59, 60, 62, 65, 66,
67, 75, 79, 82, 83, 90, 93, 95
Humphreys, Alan: vii
Hutchinson County: 66
hyalite: 78
hydrochloric acid (_see also_ chemical tests): 18, 22
I
identification charts: 24
igneous rocks, definition of: 9
intrusive igneous rocks, definition of: 9
Irion County: 60
iron: 59, 66, 67, 69, 70, 72, 73
Iron Mountain: 73
J
Jackson strata: 97
jasper: 81, 84
Jasper County: 52
Jeff Davis County: 45, 51, 74, 78, 85
Jefferson County: 91
Johnson County: 69
K
kaolin: 51
kaolinite: 51
karats: 60
Karnes County: 97, 98
Katemcy: 94
Kendall County: 18
Kenedy County: 66
Kent County: 98
kidney ore: 67
King, Elbert A., Jr.: vii
King County: 52
Kinney County: 45
Kleberg County: 66
Knox County: 52
L
labeling rock and mineral specimens: 23
labradorite: 45
Lake Buchanan: 80, 83
Lamb County: 79
Lampasas County: 20, 48, 49, 88
lava: 9
lead: 57, 90
Leakey: 51
Lee County: 64, 84
lens: 22
Liberty County: 91
lime: 69
limestone: 12, 13, 18, 19, 40, 43, 45, 46, 49, 54, 55, 56, 57, 58,
60,68, 74, 75, 76, 84, 88, 90, 91
chalk: 69
dolomitic: 68
lithographic: 69
öolitic: 69
pulverulent: 41, 69
limonite: 19, 27, 28, 30, 32, 70, 75, 81, 86
lithographic limestone: 69
Live Oak County: 45, 97
llanite: 41, 71, 81
Llano: 60, 63, 72, 73, 95
Llano County: 44, 45, 55, 56, 58, 59, 60, 61, 63, 70, 71, 72, 73,
76, 78, 80, 83, 87, 88, 94, 95
Llano uplift: 10, 18, 42, 44, 52, 55, 56, 58, 59, 60, 61, 62, 63,
67, 70, 72, 76, 77, 79, 80, 82, 83, 84, 85, 86, 87, 88,
93, 94, 95
Lone Grove: 63
Lone Star: 70
Longhorn Cavern: 18
Long Point salt dome: 91
Lonsdale, John T.: vii
luster of minerals: 16
Lytton Springs oil field: 88
M
Macon, James W.: vii
magma: 9
magnesium, source of: 55
magnetite: 28, 67, 70, 72, 85, 87, 93
malachite: 34, 52, 53, 54
Malone Mountains: 65
manganese: 73, 75
map—
geologic: 4-5
physiographic outline: 42
Marathon area: 42, 84, 88
marble: 13, 14, 40, 46, 54, 75
Marble Falls: 62
Marion County: 70
martite: 67
Mason: 74
Mason County: 48, 55, 58, 59, 60, 61, 67, 72, 73, 74, 76, 80, 83,
94
masses, crystalline, definition of: 14
Matagorda County: 91
Mayfield prospect: 74
McCulloch County: 88
Medina County: 45
Menard County: 65
mercury: 8, 49, 50, 51
metamorphic rocks, definition of: 12
metamorphism, definition of: 13, 14
mica (_see also_ biotite; muscovite): 8, 60, 61, 63, 64, 72, 76,
79, 80, 85, 87
mica gneiss and/or schist: 59, 76, 87
Mica Mine area: 58, 76, 77, 79
microcline: 55, 56, 61
milky quartz: 82
Mills County: 49
mineral identification charts: 24
mineralogists: 2
minerals, definition of: 7, 14, 15
Mississippian: 3, 55, 69, 88
Mitchell County: 66
Mohs scale of hardness: 16, 17
molybdenite: 63
montmorillonite: 51
Morris County: 70
moss agate: 84
Moss Bluff salt dome: 91
muriatic acid: 22
muscovite: 31, 32, 34, 37, 38, 76
N
Nacogdoches County: 70
Nash salt dome: 91
native elements: 8
native mercury: 8
native silver: 26, 89
nodule: 19, 44, 49
Nolan County: 20, 49, 63, 64, 65
north-central Texas: 43, 51, 52, 69, 70, 86, 88
O
Oakville strata: 97
obsidian: 40, 77
occurrence of minerals: 14
ocher, red: 67
octahedral cleavage: 56, 57
oil shale: 88
olivine: 88
onyx: 84
öolitic limestone: 69
opal: 21, 29, 30, 31, 35, 37, 38, 78, 86
opaline granite: 72
opalized wood: 20, 78, 84
open-hearth furnace: 70, 72
Orchard salt dome: 91
Ordovician: 3, 55, 69, 84
orthoclase: 55, 61
P
Packsaddle Mountain: 87
Packsaddle Schist: 87
Palangana salt dome: 91, 97
paleontologists: 2
parting of minerals: 17
Pavitte prospect: 53
Pecos County: 9
Pecos River: 65, 74, 75
pegmatite: 41, 55, 56, 58, 76, 77, 79, 81, 94, 95
Baringer Hill: 80
Pennsylvanian: 3, 45, 51, 55, 69, 70, 86, 88
peridotite: 88
period: 2
Permian: 1, 3, 20, 21, 43, 45, 49, 52, 55, 64, 65, 69, 86, 90
Permian basin: 42, 43, 65, 66
petrified wood: 20, 52, 78, 84
petrologists: 2
phenocrysts: 85
physiographic outline map: 42
pitchblende: 28, 95, 97
placer deposits: 60
Plata Verde mine: 90
Polk County: 98
porphyritic rocks: 45, 85
portland cement: 51, 69, 88
Precambrian: 2, 3, 44, 45, 48, 52, 53, 56, 58, 59, 60, 61, 63, 67,
72, 74, 75, 76, 79, 82, 85, 87, 93, 94, 95, 97
precious opal: 78
Presidio County: 45, 49, 57, 60, 62, 67, 77, 78, 84, 85, 90, 97
Presidio mine: 60, 90
properties of minerals: 14
pseudomorphs: 70, 87
pulverulent limestone: 41, 69
pumicite: 97, 98
pyramidal cleavage: 74
pyrite: 19, 26, 60, 70, 80, 87
pyrolusite: 27, 73, 74
pyroxene: 45
Q
quartz: 8, 20, 29, 30, 31, 33, 36, 37, 38, 48, 51, 54, 56, 59, 60,
61, 68, 72, 78, 79, 80,81, 84, 85, 86, 94
agate: 21, 83, 84
amethyst: 37, 82
chalcedony: 16, 21, 45, 78, 81, 83, 86
chert: 19, 40, 81, 84
cryptocrystalline: 81, 84
jasper: 81, 84
milky: 82
onyx: 84
rock crystal: 82
rose: 82, 83
smoky: 82, 83
quartzite: 41, 74, 84, 86
Quaternary: 2, 3, 51, 79, 98
quicksilver (_see also_ mercury): 50
Quitman Mountains: 57, 58, 60, 62, 67, 82, 90
R
radioactivity: 95, 96
Real County: 49, 51
Recent: 2, 3
“red beds”: 52
red ocher: 67
reef limestone: 1, 12
Reeves County: 65, 85, 91
reference books: 24, 39, 100
refraction, double: 46
rhombohedral cleavage: 46, 54
rhyolite: 40, 85
porphyry: 41, 85
Rio Grande: 83, 84, 97
rock—
crystal: 82
gypsum: 40, 65
identification charts: 39
salt: 40, 65
units: 2
rocks, definition of: 8
Rodda, Peter U.: vii
rose quartz: 82, 83
rosettes: 63, 64
Rustler Springs area: 91
rutile: 82
S
salt (_see also_ halite): 40, 65, 66
domes: 43, 58, 64, 66, 91, 97
lakes: 66
rock: 40, 65
Salt Fork of the Brazos River: 43
sand: 41, 58, 72, 85, 91
blast: 86
building: 85
filtering: 86
glass: 85-86
sandstone: 41, 52, 55, 58, 59, 72, 76, 81, 84, 86
Sandy Creek: 60
San Saba County: 70, 88
Santa Anna: 86
satin spar: 64
Satorsky, Cyril: vii
scale, geologic time: 2, 3
scintillation counter: 96
schist: 41, 56, 58, 72, 73, 76, 81, 87, 93
actinolite: 87
graphite: 63, 87
hornblende: 87
mica: 76, 87
talc: 87, 93
schistosity: 87
schorl: 95
Scurry County: 98
sedimentary rocks, definition of: 10
sediments: 8, 11
chemical: 12
selenite gypsum: 63, 64, 76
series: 2
serpentine: 34, 40, 44, 75, 87, 93
serpentinite: 87
Seven Heart Gap: 45
Shafter: 57, 58, 60, 67, 90
Shafter Lake: 83
shale: 12, 13, 19, 40, 45, 70, 87, 88
oil: 88
Shelby, Cader A.: vii
Sheridan prospect: 53
siderite: 70
Sierra Blanca: 58, 67, 82
silicified wood (_see also_ petrified wood): 20
silt: 12, 85
siltstones: 12
silver: 26, 52, 57, 59, 89
glance: 90
horn: 90
minerals: 89
native: 26, 89
Silver Creek: 58
sinter, calcareous: 46
slate: 13
Smith County: 70
smoky quartz: 82, 83
soapstone: 40, 93
soils: 10
Solitario uplift: 42, 88
Sonora: 18
Caverns of: 19
specific gravity: 18, 99
specular hematite: 67
sphalerite: 57
Spiller mine: 74
Spindletop salt dome: 91
Spring Creek: 56
stalactites: 18, 19
stalagmites: 18, 19
Starr County: 84, 98
static metamorphism: 13
Stonewall County: 52
streak or powder of minerals: 17
streak plate: 17, 22
Streeter: 48, 94
striations: 81, 95
strontianite: 49
strontium: 49
sulfur: 29, 30, 31, 32, 34, 36, 81, 90
Sutton County: 18, 19
system: 2
T
talc: 28, 29, 34, 87, 93
schist: 87, 93
Talc Rock: 93
Taylor County: 45, 49, 60
Terlingua area: 46, 47, 49, 50, 65
Tertiary: 2, 3, 9, 21, 45, 49, 51, 52, 56, 57, 60, 70, 77, 78, 79,
83, 84, 85, 86, 90, 97, 98
terrazzo—
chips: 55, 76
floors: 88
tests, chemical (_see_ chemical tests)
thorium: 80
Thrall oil field: 88
time, geologic: 2, 3
tin: 47
topaz: 36, 38, 94
tourmaline: 33, 82, 94
Town Mountain: 95
Trans-Pecos area: 9, 10, 45, 60, 62, 69, 78, 82, 83, 84, 85, 87,
89, 90, 98
trap rock: 45
travertine: 46
Travis County: 20, 45, 46, 49, 68, 69, 70, 88
tremolite: 44, 93
Triassic: 3, 45, 87
Trinity County: 98
tufa, calcareous: 46
twinned crystals: 46, 63
tyuyamunite: 97
U
Upshur County: 70
uranium: 80, 95, 96
minerals: 95
uraninite: 97
uranophane: 31, 95, 97
Uvalde County: 45, 60, 84, 88
V
Valley Spring Gneiss: 59
Val Verde County: 45, 74, 75
Van Horn area: 42, 52, 53, 58, 59, 60, 75, 76, 79, 85, 87, 90
Van Horn Mountains: 45, 55, 76, 79, 95
Van Zandt County: 66
verde antique: 88
vesicles: 45
vitrophyre: 40, 77
volcanic ash: 41, 97
volcanic igneous rocks, definition of: 9
volcanoes: 9, 97, 98
W
Waco: 88
wad: 73, 74
Walker County: 52
Ward County: 66
Washington County: 78, 84
weathering: 11
Webb County: 11, 84
Weches strata: 70
Wharton County: 91
white mica (_see_ muscovite)
Wilbarger County: 98
Willacy County: 66
Williamson County: 20, 60, 69, 70, 88
Willow City: 94
wood opal: 78
wood, petrified: 20, 52, 78, 84
Wylie Mountains: 95
Y
yellow copper ore: 53
Yoakum County: 66
Young County: 58
yttrium: 80
Z
Zapata County: 83, 84
Zavala County: 86
zirconium: 80
Transcriber’s Notes
--Retained publication information from the printed edition: this eBook
is public-domain in the country of publication.
--Corrected a few palpable typos.
--Included a transcription of the text within some images.
--In the text versions only, text in italics is delimited by
_underscores_.
--Added a label to one figure (to match the Table of Illustrations).
*** End of this LibraryBlog Digital Book "Texas Rocks and Minerals - An Amateur's Guide" ***
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