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Title: Guide to Rocks and Minerals of Illinois - Educational Series 5
Author: Anonymous
Language: English
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ILLINOIS ***
_Educational Series 5_
_Guide to_
ROCKS AND MINERALS
OF ILLINOIS
_Illinois State Geological Survey_
[Illustration: uncaptioned]
STATE of ILLINOIS
William G. Stratton, Governor
DEPARTMENT of
REGISTRATION and EDUCATION
Vera M. Binks, Director
First printing 1959
Second printing 1960
ILLINOIS STATE GEOLOGICAL SURVEY
John C. Frye, Chief
URBANA, ILLINOIS
Printed by Authority of the State of Illinois
_Guide to_ ROCKS AND MINERALS OF ILLINOIS
Illinois has so long been known as the Prairie State that at first
glance it seems a most unlikely place in which to collect rock and
mineral specimens.
But Illinois has a surprising wealth of rock and mineral resources, not
only to be collected as interesting specimens but to be put to practical
and profitable use.
The rich prairies that gave the state its nickname are themselves
derived from ancient rocks, worn and changed by millions of years of
action by weather, water, wind, plants, and animals. Unmeasured depths
of rock underlie the prairies, hills, and valleys, and in some parts of
the state are exposed in outcrops, canyons, and river valleys. Boulders
and gravel brought in by the glaciers thousands of years ago are strewn
over many parts of the state.
These resources are of great value. Besides the rich agriculture based
on the rock-derived soil, much of our industry, manufacturing, and
transportation is dependent on rock and mineral materials. Every county
in Illinois possesses some rocks and minerals that either are being used
or have potential future value.
The Illinois State Geological Survey several years ago began to prepare
sets of typical rocks and minerals of Illinois for use by the schools
and other educational groups in Illinois. This booklet is designed to
furnish a brief geological background and explanation of these common
Illinois rocks and minerals. It also should be useful to the student or
amateur interested in making his own collection.
Even though Illinois has no mountain ranges or deep canyons, the geology
of the state has many complexities. In fact, the very flatness of our
topography is a complicating factor because in order to study the
geology at many places in the state it is necessary to use information
from mines and descriptions (logs) and samples (cores) of the rock
penetrated during drilling of deep wells. There are also geophysical
methods of learning something about the rocks beneath the surface.
[Illustration: Fig. 1.—Geologic map of Illinois.]
CAMBRIAN
ORDOVICIAN
ORDOVICIAN-SILURIAN
ORDOVICIAN-SILURIAN UNDER DRIFT
DEVONIAN-MISSISSIPPIAN
PENNSYLVANIAN UNDER DRIFT
CRETACEOUS-TERTIARY
KEY
Wisconsin glacial drift
Illinoian glacial drift
Pennsylvanian boundary
sharp folding of rock layers such as can be seen in the Rocky Mountains,
but rather by the changes in composition, thickness, and character of
the rock layers that are only gently warped or relatively flat. At
several places in the state, especially in the southern part, faults, or
breaks, in the rock layers do occur, but over much of our area this is
not common.
The presence of usable minerals at considerable depth is known at many
places; coal is mined from depths greater than 800 feet, and oil is
produced from saturated rock layers, called pay zones, several thousand
feet below the surface. Lead and zinc ores, fluorspar, silica sand,
limestone, sand, gravel, clay, and shale are all produced at shallower
depths. However, the student can see only those rocks and minerals that
are to be found at or near the surface. For that reason the following
paragraphs describing their geologic occurrence deal only with surface
geology.
The youngest of the major geologic divisions of our rocks is called the
Pleistocene, which is the scientific name for the “Ice Age” deposits.
During this relatively recent period of geologic time, which began about
a million years ago, glaciers flowed southward from Canada and spread a
layer of “glacial drift” over all of the state except the northwest
corner, the southwest edge of the state along the Mississippi River, and
extreme southern Illinois (fig. 1).
Most of the glacial deposits that we see were formed by the last two of
the four major periods of glacial advance, the Illinoian and the
Wisconsin. The Illinoian was the most extensive, reaching as far south
as Carbondale and Harrisburg. The Wisconsin, so called because its
deposits are so widely spread in that state, reached only to Mattoon and
Peoria.
The glacial drift is the youngest and uppermost of the divisions of the
rock column (fig. 2). Within the drift can be found the widest diversity
of rock and mineral types—quartzite, schist, and other metamorphic
rocks; granite, gabbro, and other igneous rocks; and of course the
sedimentary rocks, limestone, dolomite, sandstone, shale, and even
pieces of coal, which occur in bedded layers of the older rocks in
Illinois.
Sand and gravel were carried and deposited by flowing streams before,
during, and after glaciation, but the major deposits were made while the
glaciers were melting. They contain a wide variety of rock and mineral
types.
[Illustration: Figure 2—Diagram of layers of rocks in Illinois.]
Era General Types of Rocks
Period or System and Thickness
Epoch
CENOZOIC “Recent Life”
Age of Mammals
Quaternary
0-500′
Pleistocene or Glacial Age
Recent—alluvium in river valleys
Glacial till, glacial outwash, gravel, sand,
silt lake deposits of clay and silt, loess and
sand dunes; covers nearly all of state except
northwest corner and southern tip
Tertiary
0-500′
Pliocene Chert gravel; present in northern, southern, and
western Illinois
Eocene Mostly micaceous sand with some silt and clay;
present only in southern Illinois
Paleocene Mostly clay, little sand; present only in
southern Illinois
MESOZOIC “Middle Life”
Age of Reptiles
Cretaceous Mostly sand, some thin beds of clay and,
0-300′ locally, gravel; present only in southern
Illinois
PALEOZOIC “Ancient Life”
Age of Amphibians and Early Plants
Pennsylvanian Largely shale and sandstone with beds of coal,
0-3,000′ limestone, and clay
(“Coal Measures”)
Mississippian Black and gray shale at base; middle zone of
0-3,500′ thick limestone that grades to siltstone, chert,
and shale; upper zone of interbedded sandstone,
shale and limestone
Age of Fishes
Devonian Thick limestone, minor sandstones and shales;
0-1,500′ largely chert and cherty limestone in southern
Illinois
Age of Invertebrates
Silurian Principally dolomite and limestone
0-1,000′
Ordovician Largely dolomite and limestone but contains
500-2,000′ sandstone, shale, and siltstone formations
Cambrian Chiefly sandstones with some dolomite and shale;
1,500-3,000′ exposed only in small areas in north-central
Illinois
ARCHEOZOIC and PROTEROZOIC
Igneous and metamorphic rocks; known in Illinois
only from deep wells
As shown by the diagrammatic rock column (fig. 2), rocks placed in the
divisions called early Cenozoic and Mesozoic are next in age to the
Pleistocene. The map (fig. 1) shows that the Cenozoic and Mesozoic rocks
occur only in the extreme southern tip of Illinois because only that
part of Illinois was covered by a northward extension of the forerunner
of the Gulf of Mexico in which the deposits of sand, gravel, and clay
were laid down.
The next older division of Illinois rocks is called Pennsylvanian—or
“Coal Measures”—because during the last century they were first
extensively described by geologists working in Pennsylvania.
The Pennsylvanian is one of our most important groups of rock strata
because it contains all of our minable coal beds, as well as important
deposits of limestone, shale, clay, sandstone, and some oil and gas. The
Pennsylvanian rocks are very widespread in Illinois, occurring under the
glacial drift from depths of a few feet to several hundred feet
throughout about two-thirds of the glaciated area (fig. 1).
Next below the Pennsylvanian are the Mississippian rocks. We in Illinois
are particularly interested in this division of rocks because they take
their name from the excellent exposures along the Mississippi River
valley in western Illinois, southeastern Iowa, and eastern Missouri.
They are composed of extensive beds of limestone and cherty limestone,
sandstone, and shale.
Mississippian rocks are of great economic importance in the structural
area known as the Illinois Basin, where they are the most important oil
producing rocks. They also contain our fluorspar deposits and along the
valley bluffs are an excellent source of limestone for quarrying.
Rocks older than the Mississippian—except for small areas along the
Mississippi and Illinois River valleys—are found at the surface only in
the northern quarter of the state and locally in Hardin County near the
southern tip of the state. They are nonetheless economically important
because from these older rocks are produced lead and zinc, some oil and
gas, silica sand, limestone, dolomite, and shale.
On the generalized rock column (fig. 2) these older rocks are grouped
into two units. The uppermost contains the Devonian and Silurian and the
lower contains the Ordovician and Cambrian. In general they include
dolomite, limestone, and shale, with sandstone at several places,
especially in the lower unit.
DESCRIPTION OF ROCKS AND MINERALS
The terms mineral and rock are often confused. They are frequently used
together and the materials they describe are closely related. In
general, a mineral is a naturally occurring chemical element or compound
formed by inorganic processes, whereas a rock is a mixture of particles
or grains of several minerals.
However, when we refer to mineral resources or industrial minerals, we
generally include materials that are technically rocks—such as
limestone, dolomite, shale—and also coal and oil that are in fact
organic substances. On the other hand, we include in the rock category
high purity sandstone that is composed almost entirely of one mineral
(quartz) and high purity limestone that is composed largely of the
mineral calcite.
MINERALS
A few minerals are composed of only one element, such as diamond
(carbon) and native copper, but most minerals are chemical compounds
that contain several elements.
Most minerals grow into distinctive shapes if they are free to grow. A
familiar example is the formation of salt crystals that grow on a saucer
of evaporating salt water. The distinctive shapes of crystals are called
their habits, and the flat surfaces that develop are called crystal
faces, the angles of which may be used to identify the mineral.
The individual atoms of a crystal always arrange themselves in the same
way, so that each mineral breaks characteristically. Some minerals break
more easily in particular directions and present a flat, smooth surface.
This characteristic is called cleavage and the cleavage surfaces,
although sometimes confused with crystal faces, may be useful for
identifying a particular mineral. The manner in which a mineral breaks
when the broken surface does not include cleavage surfaces is called its
fracture, and this too may give a clue to the identification of the
mineral.
There are other physical features useful in identifying minerals. Some
of them, such as color, are quite obvious. The color of the powder left
when a mineral is scraped on a rough, white surface, such as unglazed
porcelain or tile, is called streak. Luster refers to the brightness of
light reflected from the mineral’s surface. Transparency and
translucency refer to the mineral’s ability to transmit light, and
tenacity is a measure of its toughness.
Two special physical characteristics of minerals are important to their
identification—specific gravity and hardness. Specific gravity simply
means the ratio of the weight of the mineral to an equivalent volume of
water. For example, if a mineral has a specific gravity of 4, then a
cubic inch of the mineral weighs as much as 4 cubic inches of water.
Hardness is measured by the ability of one mineral to scratch another,
and a set of ten standard minerals has been selected for determining
this characteristic. The listing below, from soft to hard, is known as
Mohs scale.
1—Talc
2—Gypsum
3—Calcite
4—Fluorite
5—Apatite
6—Orthoclase
7—Quartz
8—Topaz
9—Corundum
10—Diamond
A rough measure of hardness can be made by using handy objects. Your
fingernail has a hardness ranging from 2 to 3, a penny is a little
harder than 3, window glass ranges from less than 5 to approximately 6
in hardness, and a knife blade is generally in the range of 5 to 6.
ROCKS
Rocks, being mixtures of minerals, are more complex than minerals and
are therefore classified in a more complicated way. The broadest
grouping of rocks is based on the origin of the rock rather than on the
minerals that compose it. In this scheme all rocks are divided into
three general groups, igneous, sedimentary, and metamorphic.
The igneous rocks are mentioned first because they are produced directly
from hot liquids that come from deep within the earth. These hot liquids
are essentially molten rock and are called magmas. When they cool, the
elements of the individual minerals come together and crystallize, as
water crystallizes into ice on a winter day.
Different minerals crystallize, or “freeze”, at different temperatures
so that if the magma cools slowly some individual grains have
opportunity to grow larger than others. If the magma cools quickly, as
does lava or basalt, the separate mineral grains will be small. Igneous
rocks are classified on the basis of the size and arrangement of the
individual crystals and the kinds of minerals present.
The glacial drift in Illinois contains many pieces of igneous and
metamorphic rocks, but most of the rocks native to Illinois are
sedimentary.
Some sedimentary rocks are made up of weathered fragments of other rocks
that have been moved by rivers, waves, winds, or glaciers. These
sediments have been deposited and later compacted or cemented by the
mineral matter carried in water moving through them. Such sedimentary
rocks are called clastic (meaning broken pieces) rocks, as opposed to
those formed by chemical precipitation from water and those that consist
of fossil remains.
Clastic sedimentary rocks are classified first on the basis of the size
of the grains of gravel, sand, silt, and clay of which they are
composed; on the type of deposition, such as glacial drift and
wind-blown silt or loess, that produced them; and then on the basis of
their mineral composition. Rocks such as gypsum and some of our
limestones were formed by chemical precipitation from sea water.
Metamorphic rock literally means rock that has changed form. The change
of form has been caused by heat and pressures that occur below the
surface of the earth or by heat from upward moving hot magmas or melted
rocks. Examples of metamorphic rocks are found in Illinois as boulders
and pebbles in the glacial drift.
Descriptions of the rocks and minerals represented by specimens in the
Geological Survey’s rock and mineral sets follow. The numbers preceding
the descriptions correspond to the numbers on the specimens in the sets.
Although the set includes the rocks and minerals that are most commonly
found in Illinois, many others can be collected. For that reason,
following the descriptions is a key for identifying other Illinois
minerals and rocks.
As a further aid to the beginner and the amateur, a list of equipment
useful in making a rock and mineral collection is given at the end of
the book.
GRANITE (1)
GRANITE is one of the most widespread intrusive (originating deep within
the earth) igneous rocks. It consists chiefly of feldspar and quartz
with small amounts of biotite, muscovite, or hornblende. Most granite is
light colored, but it can be white, gray, yellow, pink, or deep red. The
texture ranges from medium grained to coarse grained.
Granite pebbles or boulders are the most common igneous rocks found in
glacial deposits in Illinois. They are not native to the state but were
brought here by the great ice sheets or glaciers that advanced from
southern Canada to cover much of northern United States during
Pleistocene time.
Native granitic rock probably lies very deep beneath the entire state.
It has been found in deep oil-test drillings along the western and
northern margins of Illinois.
GABBRO (2)
GABBRO is another intrusive igneous rock, but it is heavier and darker
than granite. It is composed mainly of feldspar and dark iron-bearing
minerals that give the rock a dark color. It is coarse grained and
contains little or no quartz.
Mineral crystals of gabbro are especially tightly interlocked, making
the rock very difficult to break. Weathered gabbro is a rusty color on
the surface, because the iron in gabbro changes color just as a piece of
metallic iron becomes coated with rust when left out of doors.
Like other igneous rocks found near the surface in Illinois, gabbro was
carried into Illinois by the glaciers and deposited as glacial debris.
PORPHYRY (3)
PORPHYRY is an igneous rock identified by its texture rather than its
mineral content, which is variable. Distinct crystals (phenocrysts) of
minerals are embedded in a matrix of fine-grained rock. The phenocrysts
formed before the main mass of the rock hardened.
Any igneous rock may have a porphyritic variety, such as granite
porphyry and rhyolite porphyry, although porphyries are most likely to
form in association with fine-grained igneous rocks.
Porphyry is found in Illinois only in glacial drift.
BASALT (4)
BASALT is the most widely distributed volcanic rock. Pyroxene, feldspar,
magnetite (an iron ore), and in some instances olivine, biotite, and
hornblende, compose this rock. The dark green, gray, or black color is
due to the dark-colored minerals that make up much of the rock. The
minerals in basalt are fine grained and are packed closely together.
Phenocrysts of olivine, pyroxene, and hornblende may be present. Basalt
is easily identified by its color and fine-grained texture.
The glaciers brought basalt into Illinois along with other igneous
rocks.
PERIDOTITE (5)
PERIDOTITE is the only igneous rock native to Illinois that crops out at
the surface. It is found as dikes (irregular veins) or sills (thin
sheets) that were formed when molten rock from deeper in the earth
intruded into cracks and fissures in the bedrock of southeastern
Illinois (Hardin, Pope, Gallatin, and Saline Counties). Peridotite
pebbles and boulders also may be found in the glacial drift.
Peridotite ranges from very fine grained to medium grained and has an
even texture. It is dark gray to greenish gray, depending on the
minerals present. In general it is composed of olivine, hornblende,
pyroxene, and mica, with little or no feldspar or quartz.
GNEISS AND SCHIST (6)
GNEISS is a metamorphic rock composed of roughly parallel bands of
minerals. It is medium grained to coarse grained and is generally light
in color. The names given to gneiss emphasize a distinctive texture or
mineral or indicate composition. For example, biotite gneiss emphasizes
a mineral, and granite gneiss indicates the composition of the rock.
SCHIST is much like gneiss but is fine grained and has a thinly layered
structure that makes the rock break with a wavy surface. Some common
types of schistose rocks are talc schist, chlorite schist, and
hornblende schist. As the names indicate, they are characterized by
their dominant mineral. Mica schist may be formed by the metamorphism of
either sedimentary or igneous rocks.
Gneiss and schist are not native to Illinois but are found in the
glacial drift.
CONGLOMERATE (7)
CONGLOMERATE is a sedimentary rock made up of pebbles or other rock
fragments cemented in a background or matrix of finer material,
generally silica, calcium carbonate, clay, iron oxide, or a mixture of
these substances. The rounded rock fragments have been worn by being
rolled in streams and along beaches.
[Illustration: uncaptioned]
If the pebbles embedded in the matrix are sharp and angular, freshly
broken and not worn, the rock is called breccia and is generally found
near the place where the fragments originated. Conglomerate or breccia
may be made up of any type of rock or mineral, most commonly durable
material such as chert, quartz, quartzite, granite, and gneiss.
In Illinois, conglomerates commonly are found at the base of sandstone
formations and as beds in the lower “Coal Measures.” They also are found
in some gravel deposits.
SANDSTONE (8)
SANDSTONE is a clastic sedimentary rock consisting of sand-sized grains
(one sixty-fourth to one-sixteenth inch in diameter) held together by a
cementing material. As sandstones become finer grained they grade into
siltstones; as they become coarser grained they grade into conglomerate.
The shape of sand grains in sandstones ranges from rounded to angular.
Quartz is the dominant mineral in sandstone, but other rock grains and
mineral grains (especially chalcedony, feldspar, muscovite, hornblende,
magnetite, or garnet) generally are present.
Sandstones are commonly cemented by carbonates, silica, iron oxides, or
clays. Most sandstones are a shade of gray or brown, but the color may
vary from gray or white to yellow, brown, or red. The color probably
depends on the type of cement, the amount of organic material present,
and the amount and degree of oxidation of iron in the rock.
Durability of sandstones depends largely on the character of the cement.
Some sandstones crumble easily, but others, especially those cemented by
iron oxides or silica, are tough and durable. Sandstones break around
the grains, giving the broken surface a granular appearance.
Sandstone crops out in many places throughout much of the state. In
LaSalle and Ogle Counties, large tonnages of sand are mined from the St.
Peter Sandstone and sold for a variety of uses, including abrasive sand,
molding sand, and sand for making glass. In extreme southern Illinois
attractively colored sandstones have been quarried for building stone.
QUARTZITE (9)
QUARTZITE is a metamorphic rock that originally was quartz sandstone.
Quartzites are produced by intense heat and/or pressure, probably aided
by hot silica-bearing solutions. The quartz grains may be so closely
interlocked that individual grains are no longer recognizable. The rock
fractures conchoidally through both the grains and cement, so the broken
surface, unlike that of sandstone, is smooth and may even be glassy like
quartz.
Color depends upon the amount and kind of impurities present. A
quartzite that is all quartz is white or gray, but iron or other
elements may change the color to shades of purple, yellow, brown, or
red. Quartzite is a very resistant, hard rock and cannot be scratched by
a knife.
Quartzite is abundant as boulders and pebbles in glacial drift of
Illinois, having been brought into the state during the “Ice Age.”
SHALE (10)
SHALE is a common and important sedimentary rock composed of compacted
clay or mud. It is so fine grained that the minerals forming it
generally cannot be identified without the aid of X-ray.
Shales are composed mainly of clay minerals but, like other sedimentary
rocks, generally include other minerals. Shales containing calcium
carbonate are called calcareous shales. Most shales contain silt or sand
particles; if silt or sand is present in large quantity, the rock is
called silty shale or sandy shale. If mica minerals are present in
quantity in a shale it is called micaceous shale.
The particles of most clay minerals are thin and flat and overlap each
other.
Shales have a wide range of colors but most of those in Illinois are
gray. A gray, black, or blue-gray color is caused by organic matter in
the shale; shades of red, brown, yellow, or green are caused by iron
compounds.
Shale is widely distributed in Illinois, especially in “Coal Measures”
rocks, and is used in manufacturing bricks, drain tile, building tile,
and lightweight aggregate.
CLAY (11)
CLAY is an unconsolidated rock made up of a group of hydrous aluminum
silicate minerals, of which chlorite, montmorillonite, kaolinite, and
illite are the most abundant. These minerals are formed by the
weathering or alteration of other rocks and minerals.
Clays are very fine grained and their minerals have tiny, flat crystals
that can be distinguished from each other only by laboratory methods.
Although clays may appear to be similar, their compositions vary
greatly.
Some clays are white, but most are colored by iron compounds and organic
matter. Wet clays have an earthy odor and generally are slick and
plastic, but dry clays are relatively hard and are greasy to the touch.
Clays are abundant in Illinois, especially in soils, in shales, and as
clay deposits. In Illinois the underclays that occur beneath coal beds
are particularly well suited to the manufacture of bricks, pottery,
stoneware, and drain tile.
LIMESTONE (12)
LIMESTONE is a sedimentary rock composed of particles of calcite
(calcium carbonate). The crystals may range from fine to coarse. Many
limestones contain other minerals, such as chert, clay, or sand, and in
some places they grade into dolomite (calcium magnesium carbonate).
Many limestones are white or gray. Yellow or brown shades are caused by
iron oxide impurities and dark gray to black colors by organic matter.
Limestones form in various ways. Some are deposited when calcium
carbonate precipitates from solution; others are formed when the shells
or skeletons of organisms such as brachiopods, clams, and corals
accumulate on a sea floor. If such fossils are very abundant, the rock
is called fossiliferous limestone. Limestone composed of tiny, rounded
concretions is called oolite or oolitic limestone.
Limestone effervesces freely in dilute hydrochloric acid, but dolomite
must be powdered before it will effervesce. In nature, limestones may be
dissolved by percolating water containing weak acid (such as carbonic
acid, composed of water and carbon dioxide). At many places in southern
and southwestern Illinois such solution of limestones has produced caves
and caverns.
Limestone outcrops are abundant in Illinois, especially along the bluffs
of the Mississippi, Ohio, and Illinois Rivers.
Limestone has many uses. It is used for building stone, road surfacing,
railroad ballast, in the manufacture of portland cement, and, if of high
purity, for making lime and chemicals and as a flux in smelting metals.
It also is used, as agricultural limestone, to add calcium to the soil.
PEAT (13)
PEAT is produced by the partial decomposition of plants that accumulate,
with varying amounts of mineral matter, in old ponds, swamps, and lakes,
and in abandoned channels in valley bottoms along many rivers and
streams. Peat may be an early stage in the formation of coal.
Peat ranges from light to dark brown, the color, decomposition, and
compaction increasing with depth in the deposit. Some of the plant
remains are clearly distinguishable and appear as fibrous fragments held
together by the fine peat particles. The type of peat common in Illinois
has a high water content. Before it is dried it is soft and spongy to
the touch; upon drying, it loses much water and becomes harder.
Peat is used as a fuel in some parts of the world, but its heating value
is low compared to that of coal. It burns with a long flame and leaves a
great deal of ash because of the silt and sand that were buried with the
vegetation. Peat and peat moss are used chiefly as an absorbent, as
stable litter, as insulating and packing material, and by gardeners to
increase the water-holding capacity of soils.
Peat is found in many places in Illinois but the largest deposits are in
northeastern Illinois.
COAL (14)
COAL, an organic stratified rock, is formed from accumulated plant
material and partially decayed plants that were buried during the “Coal
Measures” period in Illinois more than 200 million years ago.
Sediments deposited over the peat-like organic material compacted it.
Chemical changes gradually took place and resulted in the loss of water
and gases, leaving a higher percentage of carbon than the original
material contained.
The amount of such change that has taken place determines the rank of
the coal. The lowest ranks are called lignite, the intermediate group is
called bituminous (soft) coal, and coals of the highest rank, with the
highest carbon content, are called anthracite (hard coal). Mineral
matter, such as shale, clay, or pyrite, generally is present in the coal
and becomes ash when the coal is burned.
Most coal mined in the United States is bituminous coal. It is black,
brittle, breaks into angular blocks, has a shiny luster, and generally
shows a banded structure.
Coal mining is an important industry in Illinois, and the state contains
the largest known reserves of bituminous coal in the United States.
Minable coal beds underlie about two-thirds of the state. As many as 20
different coal beds have been mined in Illinois, the most important
being the Herrin (No. 6) and the Springfield (No. 5) or Harrisburg (No.
5). The coal in most mining areas averages 5 to 7 feet thick and in
places attains a thickness of 15 feet.
In underground mines the coal is approached by vertical or inclined
shafts. In open cut, or strip, mines all of the overlying material
(overburden) is removed, leaving the coal exposed. Coal as much as 100
feet deep is now being strip mined in Illinois.
Illinois coal is now used mainly for generating electric power, for
industrial purposes, and for heating. In industry, coal is used
extensively for power, heating, burning cement, firing clay products
such as brick, tile, pottery, porcelain, and china, and making coke.
Certain Illinois coals when mixed with coal from the eastern part of the
United States, produce metallurgical coke for making steel. Gases, oils,
and tars derived in processing coal for coke have been used for making
many chemical products, including dyes, perfumes, explosives, medicines,
insecticides, plastics, and road tar.
PETROLEUM (15)
PETROLEUM (crude oil) is classed as a mineral resource although it is a
liquid hydrocarbon and not technically a mineral or rock. It is,
however, found in the pores and fractures of rocks. The color of crude
oil ranges from yellow through green and brown to black.
[Illustration: uncaptioned]
Arch (anticline)
Gas
Porous rock
Nonporous rock
Oil saturated rock
Water
Break (fault)
Nonporous rock
Oil saturated porous rock
Water saturated porous rock
Trap formed by change in character of rock
Water
Nonporous rock
Porous rock
Petroleum had its origin in the plants and animals buried in ancient
sediments. The organic matter changed slowly into the complex mixture of
hydrogen and carbon compounds that is petroleum.
Because gas is lighter than oil, and oil in turn lighter than water, gas
and oil move upward in a porous rock containing all three. Gas moves to
the highest position with oil next below and water in the lowest part of
the rock. Oil pools exist where geologic barriers have stopped the
movement of gas and oil.
Arches (upward folds or anticlines), breaks (faults), and lateral
changes from porous to nonporous rock are geologic features that serve
to localize oil pools within the reservoir rock.
Petroleum is obtained by drilling wells into the reservoir rock. Gas
that is free or dissolved in the petroleum expands as pressure is
released when the well is drilled and drives the oil to the well. Water
in the reservoir rock also acts as a driving force. When this natural
pressure is no longer effective, other methods (secondary recovery) are
used to recover the oil remaining in the reservoir.
Porous sandstones and limestones are the oil-bearing rocks. Illinois
also has deposits of oil shale from which petroleum may be produced in
the future.
Thousands of products are derived from petroleum, including gasoline,
kerosene, naphtha, lubricating oils and waxes, medicinal oils, salves,
heavy fuels, road oils, tar, and asphalt.
GLACIATED PEBBLES (16)
GLACIATED PEBBLES are small stones whose shapes have been altered by the
grinding action of a glacier. Such pebbles commonly have at least one
flattened side that shows scratches (called striae). The striae were
produced when the pebbles were pushed over bedrock or ground against
other pieces of rock.
Glaciers tore fragments from the bedrock over which they moved and the
fragments accumulated in, on, and under the mass of ice. The rock
fragments were transported, some of them far from their source, and were
deposited as the glacier moved along or when the ice melted.
Soft rocks like limestone and dolomite are easily scratched, but soft
rocks cannot make grooves in hard igneous and metamorphic rocks.
Therefore soft rocks have more striae.
Glaciated pebbles can be found in deposits in many parts of the state,
especially in northeastern and east-central Illinois. However, many
deposits near the surface have been weathered and striae have been
destroyed. An especially good place to look for striated pebbles is in
quarries and strip mines where glacial drift overburden has been
removed. Pebbles found in such deposits show good striae for they are
but little weathered.
SILICA SAND (17)
SILICA SAND is the commercial name for sand composed almost entirely of
grains of quartz. Sand of this kind is mined in Illinois from the St.
Peter Sandstone in LaSalle and Ogle Counties. The coarser grains of the
sand are characteristically rounded and frosted. The frosting causes the
sand to look white.
Illinois silica sand has many uses. It is used just as it is mined for
molding sand in which metal castings are made, for lining industrial
furnaces, and for many other purposes. Some of the sand is washed to
remove the small amount of impurities present. The washed sand is used
for such purposes as making glass, for grinding plate glass smooth, for
sand blasting, for molding metal, as fracturing sand to increase the
production of oil wells. Some silica sand is ground to a fine powder and
used as a fine abrasive, as a filler in paint, and as an ingredient in
pottery, glazes, and enamel.
A specially prepared St. Peter sand, known throughout the world as
Standard Ottawa Testing Sand, is used to test the strength of cements
and as a laboratory standard in physical tests of other sands.
MOLDING SAND (18)
MOLDING SAND is a mixture of sand and clay or other bonding material and
is used to make molds in which metal is cast into various useful shapes.
There are two kinds of molding sands, synthetic and natural bonded. The
first is an artificial mixture of silica sand and clay; the second is a
naturally occurring mixture of sand and a bonding material.
For casting, molding sand is first moistened with water and shaped into
a mold of the metal part to be reproduced. The mold is then allowed to
dry and the molten metal poured into it. The bonding material in the
sand must be strong enough to keep the mold in shape during these
operations.
Natural bonded molding sand is produced in Fayette, Bond, Bureau,
Carroll, Kankakee, and Rock Island Counties, Illinois.
TRIPOLI (19)
TRIPOLI, called amorphous silica in southern Illinois, is a white or
light brown, powdery substance that rubs off on the hands like chalk. It
consists mostly of very small particles of quartz that result from the
weathering of calcareous chert or highly siliceous limestone.
It is finely ground and used as “white rouge” for polishing optical
lenses, as a filler in paints, in making ceramic products, as a
component of buffing compounds, and as a fine abrasive.
Tripoli occurs in Alexander and Union Counties and is milled at Elco and
Tamms in Alexander County.
FULLER’S EARTH (20)
FULLER’S EARTH is clay or silty clay material that contains very fine
silica. It is soft, nonplastic, opaque, has a greasy feel when wet, and
does not readily break up in water. Its color varies from blue-gray to
yellow or buff.
Fuller’s earth is valuable for its unique property of absorbing and
decolorizing substances. The material was first used to “full” or remove
grease from woolen cloth, hence its name. It also has been used to
filter and bleach mineral and vegetable oils by absorbing dark organic
matter.
In Pulaski County in extreme southern Illinois the Porter’s Creek
Formation contains deposits of clay that were at one time the source of
fuller’s earth and still afford clay whose absorbent properties make it
useful as litter and as sweeping and cleaning compound.
QUARTZ (21)
QUARTZ is the most common of all minerals, making up about 12 percent of
the earth’s crust. There are two main types of quartz—crystalline quartz
and dense, crypto-crystalline (submicroscopic) quartz. Many dense
varieties occur in Illinois, probably the most common are chert or
flint.
[Illustration: uncaptioned]
Crystals of quartz are typically six-sided, elongated, have sharply
pointed pyramid-like ends, and are apt to grow together forming twins.
Good crystals are rare in Illinois, and the crystal structure is not
apparent in the commonly occurring grains and masses.
Quartz is brittle and hard. It may be colorless or tinted, transparent
or translucent, but more commonly it is white and nearly opaque.
Transparent quartz looks much like ordinary glass, but it scratches
glass easily. It has a glassy to brilliant luster and breaks irregularly
or with a good conchoidal fracture.
Some varieties of quartz that are used for semiprecious gems are
chalcedony, agate, onyx, and jasper. Chalcedony is waxy, smooth,
generally translucent, white to gray, blue, brown, or black. Agate is a
form of chalcedony that has a mottled or variegated banded appearance
and may be yellow, green, red, brown, blue, gray, or black. Onyx is
agate with parallel bands that as a rule are brown and white or black
and white. Jasper, an impure opaque quartz, generally is red.
Quartz occurs as rock crystal (colorless, transparent), milky quartz
(white, nearly opaque), and smoky quartz (smoky yellow to gray or brown)
in geodes from the Warsaw and Keokuk Limestones of the
Nauvoo-Hamilton-Warsaw area and as vein and cavity fillings associated
locally with fluorite, sphalerite, and galena in extreme southern
Illinois. It also occurs as vug (cavity) fillings in limestones and
sandstones.
FELDSPAR (22)
FELDSPAR is the name applied to a group of minerals that are the second
most common of all the earth’s minerals. All feldspars are composed of
aluminum, silicon, and oxygen, combined with varying amounts of one or
more metals, particularly potassium, sodium, calcium, and lithium.
The minerals are hard, have a smooth glassy or pearly luster, and cleave
along two planes nearly at right angles to each other. Feldspars are
fairly light weight. The streak is white, but the color of the mineral
is highly variable, although potassium and sodium-bearing feldspar are
commonly white or pink and most plagioclase feldspar is gray.
Feldspars are essential parts of the crystalline igneous rocks. Their
decomposition products are present in most soils. In Illinois relatively
small feldspar crystals can be found associated with quartz and other
minerals in granite and gneiss boulders and pebbles in glacial drift.
MICA (23)
MICA is the name of a family of complex aluminum silicate minerals that
can be split easily into paper-thin, flexible sheets. If broken across
the grain at right angles to the flat, smooth surface they fracture
raggedly. In a single mica crystal the sheets range from more or less
transparent to translucent and are arranged one on top of another like a
deck of cards.
Micas are tough and somewhat elastic, soft enough to be split and
scratched by a fingernail, and are light weight. They have a
nonmetallic, glassy or pearly luster, although yellow mica may appear to
be metallic. Color and streak depend upon the chemical composition of
the mineral. Muscovite, or white mica, contains potassium and makes a
colorless or white streak. Biotite, or black mica, contains iron and
magnesium and is commonly dark green or black, although it may be shades
of yellow or brown; its streak is uncolored.
Mica is abundant as tiny, shimmering flakes in Illinois sands,
sandstones, and shales (which are then said to be micaceous). It also is
common in many varieties of igneous and metamorphic rocks. White or
yellow flakes may show a brilliant luster and may be mistaken for
silver, platinum, or gold, but those minerals are heavy and malleable
whereas mica is not.
CALCITE (24)
CALCITE, a common rock-forming mineral, consists of calcium carbonate.
The mineral is white or colorless, but impurities may tint it shades of
yellow or gray. Transparent calcite is more rare than the tinted
varieties.
Transparent calcite possesses the property of double refraction; an
image appears double when viewed through a calcite cleavage block.
Calcite has a glassy luster, its streak is white or colorless. The
mineral is of medium hardness and can be scratched by a penny or a piece
of window glass but not by the fingernail. It is fairly light weight and
effervesces freely in cold dilute hydrochloric acid.
[Illustration: uncaptioned]
Calcite has a variety of crystal forms but in Illinois flattened
block-shaped crystals and elongate crystals with tapering points
(“dogtooth spar”) are the most common. When broken, calcite cleaves into
six-sided blocks called rhombs.
Crystals of calcite are found in Illinois as linings in geodes in
certain limestones and shales, especially in the Nauvoo-Hamilton-Warsaw
area, and as crystalline masses in limestone and dolomite. Small amounts
of clear crystalline calcite are associated with various ores in
northwestern and extreme southern Illinois.
Calcite is the principal mineral in limestones and occurs as a component
of many concretions.
FLUORITE (25)
FLUORITE, or fluorspar, is made up of the elements calcium and fluorine.
The mineral is easily identified by its perfect cleavage, color, and
hardness.
It occurs in cubic crystals that may be twinned but is more often found
as irregular masses. It can be split into diamond-shaped, eight-sided
forms (octahedrons). Fluorite is commonly gray, white, or colorless, but
it may be green, blue, purple, pink, or yellow. The streak is colorless
and the luster glassy. It can be scratched by a knife or a piece of
window glass, is fairly light weight, and is transparent to translucent.
Extensive deposits of fluorite, one of Illinois’ important commercial
minerals, occur in Hardin and Pope Counties in extreme southern
Illinois, where it is associated with galena, sphalerite, calcite,
barite, and other less abundant minerals.
Fluorite is used to make hydrofluoric acid, to form a fluid slag in the
production of iron and steel, in the manufacture of aluminum, to make
many chemical products, and in the ceramic industry, in which it is used
to make colored glass, enamels, and glazes.
GYPSUM (26)
GYPSUM, hydrous calcium sulfate, is a colorless, transparent to
translucent mineral when pure, but it often is stained yellow by
impurities. It has a white streak, is soft enough to be scratched by a
fingernail, and is light weight.
[Illustration: uncaptioned]
Gypsum occurs in several forms. Selenite is a coarsely crystalline,
transparent variety, composed of flat, nearly diamond-shaped crystals
that can be split easily into thin sheets, have a glassy luster, and
often grow together to form “fishtail twins.” Crystals of selenite occur
in shales of the “Coal Measures” of southern, north-central, and western
Illinois, and can be picked up at the surface.
Satin spar has crystals like silky threads closely packed together,
splits parallel to the fibers, and is found as fillings in rock cracks
and as thin layers in shales. Massive gypsum is granular.
Gypsum deposits occur deep underground in Illinois but thus far have not
been mined.
CHERT (27)
CHERT, one of the main forms of silicon dioxide, is cryptocrystalline
(submicroscopic) quartz. Most of the chert in Illinois is white or gray,
but impurities stain many deposits yellow, brown, or even pink. Chert is
so hard that it can scratch glass and ordinary steel. It is fairly light
weight, dense, opaque, and brittle; the luster is dull.
Flint, a variety of chert, is generally dark colored, more dense, may
have a glassy luster and be transparent in thin flakes. Both chert and
flint have a smooth, curved (conchoidal) fracture, but flint tends to
break with thinner, sharper edges. Indians used flint and chert to make
arrow points and spearheads.
Chert occurs as rounded masses (nodules and concretions) or as irregular
layers in limestones and dolomites in northern, western, southwestern,
and southern Illinois. Because chert is hard and more resistant to
weathering than limestone or dolomite, it often remains after the rest
of the rock has weathered away.
Chert also is abundant in many glacial deposits because it is hard and
resists solution. Streams that flow through cherty bedrock or glacial
deposits carry pebbles along and concentrate them as gravel in stream
channels. Cherty stream gravels are especially abundant in western and
southern Illinois.
Brown chert gravels in the southern part of the state are used for road
gravel. Other deposits in extreme southern Illinois, consisting of
angular fragments of chert and a small amount of clay (known locally as
novaculite gravel) also are used for road surfacing.
White and gray chert occur as massive bedrock deposits several hundred
feet thick in Union and Alexander Counties.
PYRITE AND MARCASITE (28)
PYRITE and MARCASITE are iron disulfide compounds. They look much alike
but have different crystal forms. Both are brittle, hard, brassy yellow
with metallic luster, and opaque. The best distinguishing feature is
crystal shape. The pyrite crystals are cubes, but the marcasite crystals
are blade- or needle-shaped.
Pyrite and marcasite have been mistaken for gold because they are yellow
and metallic and hence are sometimes referred to as “fool’s gold”. They,
however, are harder than gold, tarnish, and leave a dark streak, whereas
gold is soft, very heavy, does not tarnish, and leaves a yellow streak.
Gold is malleable, but pyrite or marcasite are reduced to powder if they
are pounded and give off a noticeable odor of sulfur dioxide gas if they
are heated.
Pyrite and marcasite are found in many deposits in Illinois. They occur
as grains or larger masses in some clays, shales, and limestones. They
also occur with the lead and zinc ores of northwestern Illinois and, in
small amounts, with the fluorite and associated minerals in the extreme
southern part of the state.
Both pyrite and marcasite are common as surface coatings, veins, and
concretionary structures in coal and in dark shales associated with
coal. They are referred to as “coal brasses” or “sulfur” when found as
impurities in coal.
A potential use for pyrite and marcasite is in the manufacture of
sulfuric acid for industrial use. Coal brasses recovered from Illinois
coal have been so used.
LIMONITE (29)
LIMONITE is an iron oxide containing water and has a complex chemical
composition. The limonite found in Illinois may be yellow, orange, red,
brown, or black, but its streak is always yellowish brown. The mineral
may have a glassy or an earthy luster. It may be too hard to be
scratched by a knife. It is of medium weight.
Limonite is common and occurs as concretions and cavity fillings in
sedimentary rocks, and as coatings on them, especially sandstone. It
also occurs as iron rust, as scum on stagnant water, and it accumulates
around rootlets in soils. Small amounts color limestone, dolomite, clay,
shale, sandstone, and gravel. Some sands are firmly cemented by brown or
black limonite and look much like iron ore. Clays containing a high
percentage of limonite are called ocher.
In some states limonite is mined as an iron ore, and in Illinois it was
so used in Hardin County in the middle 1800’s, but deposits are not
large enough for profitable use now.
SPHALERITE (30)
SPHALERITE, zinc sulfide, is a major ore of zinc. It has a resinous
luster and a white, yellow, or brown streak. Illinois sphalerite is
generally yellow, yellowish brown, reddish brown, or brownish black. It
is of medium weight, brittle, can be scratched by a piece of window
glass but not by a penny. It is commonly opaque but may be translucent
on thin edges.
Sphalerite is mined with galena in northwestern Illinois and in extreme
southern Illinois with galena and fluorite. Small crystals occasionally
are found in limestones and as crystalline masses in clay-ironstone
concretions.
GALENA (31)
GALENA, lead sulfide, is the principal ore of lead. It is steel gray,
heavy, opaque, and has a bright metallic luster, though the shiny
surface may be dulled by a coating of lead carbonate. It has a gray or
black streak, is soft enough to mark paper, and can be scratched by a
penny. The cube-shaped crystals readily break into cubic, right-angled
fragments. Probably the most obvious features of the mineral are its
bright metallic luster on fresh surfaces, high specific gravity, and
cubic cleavage.
[Illustration: uncaptioned]
At many places galena is argentiferous (silver-bearing), but Illinois
galena is relatively unimportant for its silver content. As a source of
lead, however, it is an important commercial product of the state.
Scattered pieces of galena are found at many places in Illinois. Some
occur in the glacial deposits, others occur as small pockets and as
crystals in limestones and geodes. In only two areas of the state are
deposits of commercial value. In northwestern Illinois galena occurs in
association with sphalerite; in extreme southern Illinois it occurs in
association with fluorite and sphalerite.
CONCRETIONS (32)
CONCRETIONS are concentrations of inorganic sedimentary material within
other sediments. Minerals that commonly form concretions are silica (in
the form of opal, chert, chalcedony, and quartz), calcite, siderite,
pyrite, marcasite, and limonite.
Concretions may form either as the sediment around them is forming or
after the sediment around them has hardened. They may be formed when
water containing dissolved minerals seeps through the sediment or rock
and leaves a concentration of mineral matter in a cavity or around a
central particle (nucleus) such as the remains of a plant or animal.
Portions of rock may also become firmly cemented by such mineral matter.
Concretions range in size from minute particles to objects several feet
in diameter. Shapes range from spheres to tubes. Many are globular or
lumpy-surfaced, some are smooth. Because concretions generally are
harder than the surrounding rock in which they have formed, they do not
weather away as readily and may remain after the surrounding material
has been eroded.
Concentrations of calcite are found in loess deposits. They may look
like bizarre, knobby figurines, and the Germans called them loess
kindchen (little children of the loess).
Ironstone concretions, especially common in many Illinois shales, are
formed by a local concentration of the mineral siderite (iron carbonate)
in the rock. The concretions found in weathered outcrops commonly are
partly or entirely weathered to limonite. Some ironstone concretions
grow together into odd shapes. Mazon Creek ironstone concretions of
northeastern Illinois, world famous for their fossils, are sideritic.
The concretions are commonly covered with limonite, the result of
oxidation.
Limonite concretions, generally with a high content of clay, silt, or
sand, occur in loess, shale, and sandstone.
Concretions of chert and other forms of silica are common in limestones.
In many places, because of their greater resistance to weathering,
lenses and nodules of chert protrude from the beds.
Pyrite or marcasite occur as concretions or concretion-like masses in
some coal beds and in the black shales, sometimes popularly called
“slates,” above coal beds. Some other Pennsylvanian clays and shales
also contain concretions or coarsely crystalline aggregates of these
minerals.
GEODES (33)
GEODES are roughly spherical bodies that may be filled with layers of
minerals, lined with crystals, or both. The outer layer of geodes found
in Illinois as a rule is composed of chalcedony, a form of finely
crystalline silica.
Geodes differ from concretions in that they form inward from the outer
shell, whereas concretions develop outward from a center. Even if geodes
have been completely filled by mineral matter, their inward-projecting
crystals prove that they formed within a cavity.
In a partly filled cavity, crystals generally are well formed because
they grew without being crowded. Some of the best mineral specimens
known in Illinois are found as crystal linings in geodes.
[Illustration: uncaptioned]
Quartz is the most common mineral deposited in geodes, but calcite,
aragonite, dolomite, siderite, pyrite, galena, fluorite, and sphalerite
also are found.
Geodes ranging in size from less than one inch to a foot or more in
diameter can be gathered from streams where they have accumulated as
residual boulders after the rock in which they were enclosed has been
eroded.
Hollow geodes are the most desirable because they have better crystals.
They can be distinguished from solid ones by their comparative lightness
of weight.
Geodes are commonly associated with limestone and dolomite, at some
places with shale. In Illinois they can be found most easily in the
Warsaw Formation in the area of Nauvoo, Hamilton, and Warsaw, but they
also occur in other areas and other formations.
ANIMAL FOSSILS (34)
Prehistoric animals lived in water, on land, and in the air, and left
both direct and indirect evidence of their existence, evidence we now
call fossils.
Millions of ancient animals died without leaving a trace, but some,
especially those that had hard parts such as shells, bones, or teeth,
may be found preserved in rocks much as they were when buried beneath
sediment on the floor of an ancient sea. Sometimes only imprints of the
outside or fillings of the inside of the shells remain, the original
material having been completely dissolved. Footprints of land or
amphibious animals, burrows made by clams, or holes made by worms also
are fossils.
[Illustration: uncaptioned]
The animals whose remains are fossilized lived and died while the
sediments that contained them were being deposited, and they provide
clues to the types of life and climate then existing. Fossils of animals
characteristic of a certain time are an index to the age of formations
where they occur. For example, if a certain trilobite (an ancient
relative of the crayfish and lobster) is known to have lived only during
a definite time, then all rocks in which it is found are the same age.
Fossils of animals that lived in the sea are exposed in rocks in many
parts of Illinois, especially in quarries, river bluffs, and road cuts.
The oldest fossils found in Illinois are shells of marine
animals—snails, corals, crinoids, brachiopods, trilobites, pelecypods
(clams), cephalopods, bryozoa, arthropods, and others. The youngest
fossils are teeth and bones of prehistoric bison, giant beavers, deer,
mammoths and mastodons of the “Ice Age,” and snails found in glacial
loess.
PLANT FOSSILS (35)
PLANT FOSSILS are the remains of prehistoric plants. Woody structures of
plants aid preservation just as hard parts of animals do. Leaves and
plants without much woody material generally were well preserved only if
they were buried quickly in fine, soft sediment.
The most famous Illinois plant fossils are those from the Mazon Creek
area in Grundy and Will Counties of northeastern Illinois. The plant
material acted as a nucleus around which iron minerals accumulated to
form concretions. Many good fossils—of trunks, branches, leaves, and
seeds—are found in coals and in shale directly overlying coals.
Descendants of “Coal Measures” plants, such as ferns, mosses, and
rushes, are still living today, but they no longer thrive as they did in
the warm, moist climate of the Pennsylvanian forests.
Some plants of Pennsylvanian age are petrified, and occasionally such
trees or stumps are found. Petrified trees are found also in the upper
Mesozoic deposits of southern Illinois. Fossils of “Ice Age” plants
closely related to forms living at the present time are occasionally
found in peat bogs or scattered throughout glacial deposits.
[Illustration: uncaptioned]
KEYS FOR IDENTIFICATION OF COMMON ILLINOIS ROCKS AND MINERALS
Two keys, one for minerals and one for rocks, briefly present clues that
may aid the collector in identifying rocks and minerals found in
Illinois. In outline form, the keys are a guide to some of the easily
observable properties that various rocks and minerals display.
The rocks and minerals in the school set of “Typical Rocks and Minerals
of Illinois” are included, plus other relatively common ones you might
find in Illinois. Because of the great diversity of rocks and minerals
in this state, the keys are not conclusive. It is therefore suggested
you consult other more complete keys (such as that in Dana’s _Manual of
Mineralogy_) when identifying rocks and minerals that are either from
other states or are difficult to identify.
The minerals (p. 30-35) are arranged in two groups: 1) those with a
metallic luster, and 2) those with a nonmetallic luster. Each group is
arranged according to increasing hardness. Other characteristics such as
color, streak, cleavage, fracture, and composition are listed.
The rocks (p. 36-39) are arranged according to their reaction to dilute
hydrochloric acid applied to a scratched surface. (The acid reacts more
readily to powdered material produced by scratching the rock.) After the
reaction to acid has been determined, the texture and components of the
rock should be noted. Because rocks grade into one another, clear
distinctions are not always possible.
MINERAL IDENTIFICATION KEY
I. METALLIC LUSTER, STREAK COLORED
C—color H—hardness Remarks Name and
S—streak Cl—cleavage composition
F—fracture
A. Hardness not more than 2.5
C—lead gray H—2.5 Very heavy; occurs as Galena (31)
S—black Cl—cubic; crystals, grains, or PbS
perfect in 3 masses; easily
directions identified by color and
F—subconchoidal cleavage
or even
C—copper red H—2.5 Very heavy; apt to have Native copper
S—metallic, Cl—none green coating; distorted Cu
shiny F—jagged or wirelike forms;
malleable
B. Hardness greater than 2.5 but not greater than 6.5
C—yellow-brown H—5.5 (may be as In earthy masses; Limonite (29)
to black low as 1) coloring material in FeO(OH)·H₂O
S—yellow-brown Cl—none many sandstones,
F—uneven conglomerates, and
soils; often mixed with
and difficult to
distinguish from
goethite and other iron
minerals
C—brassy yellow H—6 As compact masses, Pyrite (28)
S—greenish black Cl—poor grains, cubes, and in 8- FeS₂
F—conchoidal to and 12-sided crystals;
uneven commonly associated with
coal, and with lead-zinc
ores of northwestern
Illinois
C—pale brassy H—6 As fibrous, radiating, Marcasite
yellow to Cl—poor tabular, and cocks-comb FeS₂ (28)
silver white F—uneven crystals or compact
S—greenish gray masses; usually lighter
colored than pyrite, but
difficult to distinguish
from pyrite; associated
with coal, and with
lead-zinc ores of
northwestern Illinois
II. NONMETALLIC LUSTER, STREAK WHITE
A. Hardness not greater than 2 (can be scratched by fingernail)
C—usually white H—2 Commonly found in Gypsum (26)
but may be Cl—perfect in Illinois as twinned or CaSO₄·2H₂O
almost any color one direction, needle-shaped crystals
less perfect in in weathered shales
two others containing pyrite and
calcium carbonate
C—white or a H—2 As needle-shaped Melanterite
shade of green crystals or powdery FeSO₄·7H₂O
coating on pyrite or
marcasite; has an
astringent taste
B. Hardness greater than 2 but not greater than 3 (Can be scratched by a
penny)
C—colorless, H—2-2.5 In scales or “books”; in Muscovite
silver white, Cl—perfect in splits into thin sheets; (white mica)
gray, brown one direction common in sandstones, (23)
shales, and in igneous (OH)₂KAl₂
and metamorphic rocks AlSi₃O₈
C—brown or black H—2.5-3 As scales or “books”; Biotite (23)
Cl—perfect in splits into thin sheets; (black mica)
one direction common in igneous and (OH)₂K(Mg,Fe)₃
metamorphic rocks but AlSi₃O₈
not in sedimentary rocks
such as sandstone or
shale
C—colorless, H—3 Common mineral; Calcite (24)
white, gray, Cl—perfect in effervesces vigorously CaCO₃
and various three in cold acid; occurs in
tints directions, not many crystal forms and
at right angles as fibrous, banded, and
(rhombohedral) compact masses; chief
mineral in limestones
C—white, gray, H—3 Very heavy; commonly in Barite
red, or almost Cl—perfect in tabular crystals united BaSO₄
any color one direction, in diverging groups, as
less perfect in laminated or granular
two other masses; associated with
directions fluorite in southern
Illinois
C. Hardness greater than 3 but not greater than 5
(Cannot be scratched by penny; can be scratched by knife)
C—white, gray, H—3.5 Relatively heavy; Witherite
light yellow Cl—in one effervesces in acid; BaCO₃
direction associated with fluorite
F—uneven and barite in southern
Illinois but is not
abundant
C—white, pink, H—3.5 In grains, rhombohedral Dolomite
gray, or light Cl—perfect in crystals and cleavable CaMg(CO₃)₂
brown three or granular masses;
directions, not effervesces slowly in
at right angles cold acid when powdered,
(rhombohedral) more vigorously in warm
acid; principal mineral
in rock called dolomite
C—colorless, H—3.5 In fibrous or compact Cerussite
white, gray, masses or may be in PbCO₃
grayish black orthorhombic crystals as
a coating on galena;
very heavy; effervesces
in acid; formed by
alteration of galena
C—brown to gray H—3.5 In fibrous or botryoidal Siderite
S—usually white Cl—in three masses or rhombohedral FeCO₃
but may tend directions not crystals; effervesces in
toward brown at right angles hot acid
when weathered (rhombohedral)
slightly curved
surfaces
C—yellow, H—3.5 In crystals, in fibrous Sphalerite
yellow-brown to Cl—parallel to or layered masses; ZnS (30)
almost black dodecahedral associated with galena
S—light yellow faces; in six in northwestern
to brown directions Illinois, with fluorite
and galena in southern
Illinois
C—colorless, H—4 In cubes and cleavable Fluorite (25)
white, yellow, Cl—perfect, masses; many colors; (Fluorspar)
purple, green, parallel to mined in Hardin and Pope CaF₂
blue octahedral counties
faces; in four
directions
C—white, tinted H—5 As crystalline Smithsonite
yellow, blue, incrustations or in ZnCO₃
or green earthy or compact
masses; associated with
fluorite-sphalerite ores
in southern Illinois,
with galena and
sphalerite in
northwestern Illinois
D. Hardness greater than 5 but not greater than 7
C—white, green, H—5-6 In long, slender 6-sided Amphibole Group
brown, black Cl—in two crystals; cleavage angle (Mg,Fe,Ca)₇
directions important in (Si₈O₂₂)(OH)₂
intersecting at differentiating from (may also
about 60° and pyroxenes; common in contain Na or
120° metamorphic and some Al)
igneous rocks
C—gray, dark H—5-6 Crystals short, stout, Pyroxene Group
green, black, Cl—in two and 8-sided; cleavage (Mg,Ca,Fe)₂
dark brown, directions angle important in (Si₂O₆)
bronze intersecting at differentiating from
about 90° amphiboles; common in
igneous and some
metamorphic rocks
C—white, gray, H—6 As crystals, cleavable Feldspar Group
pink, light Cl—in two masses and grains; (22)
blue, green directions common in igneous and K, Na, Ca, Ba
nearly at right metamorphic rocks, also (Al, Si)₄O₈
angles in stream gravel and
sand; many varieties
C—white when H—7 Finely crystalline Chalcedony
pure; may be Cl—none variety of quartz; SiO₂
colored by F—conchoidal botryoidal or
impurities concretionary masses;
lining in geodes
C—colorless, H—7 Most abundant mineral; Quartz (21)
white, or F—conchoidal occurs in 6-sided SiO₂
almost any color crystals capped by
pyramids, in grains or
masses; principal
mineral in sandstone,
also abundant in igneous
and metamorphic rocks;
is a variety of silica
C—red H—7 A variety of quartz Jasper
F—conchoidal usually colored red by SiO₂
hematite inclusions;
common in glacial and
river sand and gravel
found along Lake
Michigan shores and in
the Mississippi River
C—many; H—7 Cloudy banded variety of Agate
arranged in F—conchoidal silica; widely used as SiO₂
bands semi-precious stones.
Onyx and silicified wood
are forms of agate;
found in glacial gravels
and upper Mesozoic
sediments in southern
Illinois
E. Hardness greater than 7 (cannot be scratched by quartz)
C—red, brown, H—7.5 Irregular grains or Garnet Group
yellow, green, Cl—poor masses; sometimes as (Ca,Mn,Fe,Mg)₃
black, white F—even 12-, 24-, and 36-sided (Al,Cr)₂(SiO₄)₃
crystals; abundant in
glacial sands and Lake
Michigan beach sands;
common in metamorphic
rocks
ROCK IDENTIFICATION KEY
SAMPLE
Scratch with a knife and apply dilute acid (HCl)
If rock does not scratch, go directly to I, II, or III
No effervescence or very slight effervescence
I Coarse-grained (p. 37)
II Fine-grained (p. 38)
III Organic (p. 39)
Slight effervescence
gray, light gray, white, or brown: Dolomite
Vigorous effervescence
Composed of pebbles that effervesce
Rounded pebbles: Limestone conglomerate
Angular pebbles: Limestone breccia
Composed of crystals of calcite, fossil shells, or oolites:
Limestone
Composed of banded layers of crystalline calcite; commonly found in
caves, forming stalactites and stalagmites: Travertine
Large amount of insoluble residue left on acid-treated surface
Individual grains seen with unaided eye: Calcareous sandstone
Individual grains not seen with unaided eye: Calcareous shale
Composed of porous or cellular mass of calcite; commonly found near
springs and waterfalls: Tufa
I COARSE-GRAINED ROCKS
A. Rock consists of interlocking grains or crystals, easily seen; too
hard to scratch with a knife
1. Crystals aligned in one direction
a) Crystals in parallel bands with layers of quartz and feldspar
separated by mica and other minerals
Gneiss (6)
b) Crystals in thin parallel bands; tends to split into thin
sheets parallel to banding; some varieties may be scratched
with a knife
Schist (6)
2. Crystals not aligned in any particular direction
a) Light gray, pink, red, or tan with only a few dark minerals;
feldspar and quartz principal minerals
Granite (1)
b) Dark to medium gray; composed of feldspar and dark minerals
with little quartz
Gabbro (2)
c) Dark green to black; essentially dark minerals, may have some
feldspar; quartz generally lacking
Peridotite (5)
d) Light color; similar to granite in texture but lacks quartz;
composed of feldspar and some dark minerals
Syenite
e) Large, easily seen crystals set in a fine- to extremely
fine-grained background; any color
Porphyry (3)
f) Essentially quartz; grains may be identifiable; specimens break
through rather than around grains
Quartzite (9)
B. Rock composed of individual rock particles or fragments,
non-interlocking crystals, cemented or not cemented together; may
or may not be scratched with a knife
1. Particles or fragments not uniform in size; a mixture of pebbles,
sand, and smaller materials
a) Solid rock consisting of particles or fragments generally
rounded and cemented together
Conglomerate (7)
b) Solid rock consisting of particles or fragments, generally
angular and cemented together
Breccia
c) Fragments ranging in size from clay to large boulders; may be
compacted, but not cemented; much clay generally present; may
effervesce
Glacial till
d) Loose particles of many sizes, not cemented together; some
particles may effervesce
Gravel
2. Rock particles or fragments, about the size of grains of sugar (2
to .05 mm)
a) Loose particles consisting largely of quartz
Sand
b) Solid rock consisting largely of quartz; can be separated
easily into individual particles; granular; breaks around
rather than through grains
Sandstone (8)
II FINE-GRAINED ROCKS
A. Cannot be scratched easily with a knife; crystals or particles not
easily seen with the unaided eye; very hard, difficult to
break; may contain a few crystals or particles large enough to
see; granular
1) Dense; brittle; splintery or conchoidal fracture; sharp edges and
corners when broken; often associated with limestone; usually
white or gray; very dense, dull varieties called flint
Chert (27)
2) Light gray, pink, red, or tan varieties common; boulders or
fragments in the glacial drift
Felsite
3) Dark gray, greenish, black, or maroon varieties common; may have
small mineral-filled cavities; occurs as boulders or fragments
in the glacial drift
Basalt (4)
4) Essentially quartz; grains may be identifiable; specimens break
through rather than around grains
Quartzite (9)
B. May or may not be scratched with a knife; fairly uniformly fine
grained
1) Soft; feels slippery or soapy when wet; may disintegrate in
water; gives off an earthy odor when breathed upon
Clay
2) Loose; gritty; particles smaller than table salt
Silt
3) Solid rock; often in thin beds or sheets; separates into silt;
mica flakes may be present; may contain fossils; may effervesce
slightly
Siltstone
4) Solid rock; breaks into thin platy sheets; may feel slippery when
wet; black to gray; may contain fossils; shows thin laminations;
may effervesce
Shale (10)
5) Solid rock: does not break into thin platy fragments; may
effervesce slightly
Mudstone
6) Solid rock; usually gray or black; splits into platy sheets or
slabs; harder than shale
Slate
7) Powdery; white or light brown; commonly associated with chert and
quartz from which it forms
Tripoli (19)
III ORGANIC ROCKS (DARK COLORED)
A. Soft; spongy when wet; very lightweight when dry; forms in swampy
places
1) Fine mass with coarse plant fragments; dark gray to black
Peat (13)
2) Plant fragments small and not easily recognized; fine-grained;
black to dark gray; earthy
Muck
B. Hard but can be scratched with a knife
1) Black; contains bands of shiny and dull material; burns well
Coal (14)
2) Dark gray to black; does not contain shiny bands; splits into
thin sheets; burns poorly or not at all
Bituminous shale
EQUIPMENT FOR COLLECTING
1. Hammer (bricklayer’s) with one chisel or pick head.
2. Cold chisel about 6 inches long with an edge about ½-inch wide.
3. Dilute hydrochloric (muriatic) acid (10 percent solution) in a
dropper bottle for testing the presence of carbonate minerals.
Mark the bottle POISON. If acid is spilled on skin or clothing,
wipe immediately and, if possible, rinse with water.
4. Magnifying glass or hand lens—10 power is probably most useful.
5. Hardness testers—penny, square of window glass, pocket knife, or
nail.
6. Streak plate—piece of unglazed white porcelain (such as the back of a
tile) for testing the color of the streak of minerals.
7. Notebook and pencil for keeping records of the locality and bed from
which specimens are collected.
8. Collecting bag—a musette bag, a knapsack, or similar bag of strong
material.
9. Heavy gloves and goggles to protect hands and eyes.
10. Labels and wrappings. Field identification of specimens may be
written on adhesive tape and attached to the specimen or on a slip
of paper enclosed in the wrapping. Newspaper, brown paper, or
paper bags can be used for wrapping specimens. Label the outside
of the wrapped specimen too. Take only the best specimens home
with you. Trim specimens to hand size (about 2 by 3 inches).
All specimens should be labeled with the following information: name of
mineral or rock type, where found, collector’s name, and date. As
your collection grows, you may want to set up a system of
cataloging. List specimens and assign a number to each one. Place
a small amount of white enamel on a corner of each specimen; when
the enamel dries, number the sample with India ink; coat number
with lacquer. Corresponding numbers should be entered on your list
of specimens.
EDUCATIONAL EXTENSION PROGRAM
This book was prepared by the Educational Extension Section of the
Illinois State Geological Survey, principally Betty Jean Hanagan, I.
Edgar Odom, and Shirley J. Trueblood, under the direction of George M.
Wilson. They were assisted by other members of the Survey staff,
especially J. E. Lamar and J. C. Bradbury of the Industrial Minerals
Section.
Educational Extension also serves the public by assembling and
distributing rock and mineral collections for Illinois educational
groups, giving lectures, preparing exhibits, answering queries about
identification of rocks and minerals, reporting Survey news, and
conducting earth science field trips.
During each year six field trips are conducted in widely separated parts
of the state for teachers, students, and laymen. The general program is
especially designed to assist in teaching the earth sciences and to help
make Illinois citizens aware of the state’s great mineral wealth.
Illinois State Geological Survey
Urbana, Illinois
[Illustration: ILLINOIS _Land of Lincoln_]
Transcriber’s Notes
—Silently corrected a few typos.
—Retained publication information from the printed edition: this eBook
is public-domain in the country of publication.
—In the text versions only, text in italics is delimited by
_underscores_.
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