Part 1

_Educational Series 5_

_Guide to_ ROCKS AND MINERALS OF ILLINOIS

_Illinois State Geological Survey_

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.

CAMBRIAN ORDOVICIAN ORDOVICIAN-SILURIAN ORDOVICIAN-SILURIAN UNDER DRIFT DEVONIAN-MISSISSIPPIAN PENNSYLVANIAN UNDER DRIFT CRETACEOUS-TERTIARY KEY Wisconsin glacial drift Illinoian glacial drift Pennsylvanian boundary

The complexity of Illinois geology is not produced by the upturning and 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.

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.

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)