Guide to Rocks and Minerals of Illinois
Descriptions of Illinois Minerals
The most common Illinois rocks and minerals are described in this publication, but many other rocks and minerals can be collected in the state. For that reason, keys for identifying other Illinois minerals and rocks are included beginning on the page Keys for Identification of Common Illinois Rocks and Minerals. Those keys include specific hardness values and chemical composition as well as other identifying characteristics.
Quartz (SiO2) is the most common of all minerals, making up 12% of the Earth's crust. There are two main types of quartz: (1) crystalline quartz and (2) dense, microcrystalline to cryptocrystalline (microscopic to submicroscopic) quartz. Many dense varieties occur in Illinois; the most common is chert.
Well-formed, prismatic crystals of quartz are typically six-sided and elongated with sharply pointed pyramid-like ends. Quartz crystals are apt to grow together, in clusters. Good, large crystals are rare in Illinois. However, good, well-formed crystals occur within some geodes (see Subsidiary Rock Forms) and within certain openings (vugs) in some limestone layers.
Quartz is brittle and hard. It may be colorless or tinted, transparent or translucent; more commonly, it is white and nearly opaque. Transparent quartz appears similar to ordinary glass, but scratches glass easily. Transparent quartz has a glassy (vitreous) to brilliant luster and breaks irregularly or with a good curved (conchoidal) fracture. Specific gravity is 2.65.
Lake Superior agates
Some varieties of cryptocrystalline quartz that are used for semiprecious gems are chalcedony, agate, onyx, and jasper. Chalcedony is waxy, smooth, generally translucent, and 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 chert, generally is red or yellow-brown.
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. Quartz also occurs 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.
Chert, one of the main forms of silicon dioxide, is cryptocrystalline (microscopic) quartz. Most of the chert in Illinois is white, gray, or black, 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 lightweight, opaque, dull, and brittle. Specific gravity is about 2.6 to 2.64.
Flint, a variety of chert, is generally dark colored, is more dense, may have a glassy luster, and may be translucent in thin flakes. Both chert and flint have a smooth, curved (conchoidal) fracture, but flint tends to break with thinner, sharper edges. Native Americans used flint and chert to make arrow points and spearheads. Archeologists reserve the term flint for the dark varieties of chert from the Old World and use the term chert for the New World varieties.
Chert occurs as rounded masses (nodules and concretions) or as irregular layers in limestones and dolomites throughout 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 resistant to erosion. 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.
Ancient 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, but chert reacts adversely with cement and cannot be present in aggregate used for roads or foundations.
White and gray chert occurs as thick lenses within massive bedrock deposits several hundred feet thick in Union and Alexander Counties.
Feldspar is the name applied to a group of minerals that is the second most common of all the minerals. All feldspars are composed of aluminum, silicon, and oxygen combined with varying amounts of one or more metals, particularly potassium, sodium, and calcium.
Feldspars have a hardness of 6, have a smooth, glassy or pearly luster, and show good cleavages along two planes at nearly right angles to each other. Specific gravity is about 2.6. The streak is white, but the color of the mineral is highly variable. Potassium feldspars (or K-feldspars) contain potassium, and color is commonly pink to reddish, but otherwise white, gray, yellowish, or pale green (amazonite variety).
Plagioclase feldspars contain varying amounts of calcium and/or sodium, and color is commonly white or gray. A diagnostic feature is fine lines or striations seen with a hand lens on cleavage faces.
Feldspar is used as flux for glass and ceramic manufacture, abrasives, and fillers in paint and plastics.
Feldspars are essential minerals in most crystalline igneous rocks. Their decomposition products are clay minerals that 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 larger crystals occur in some pebbles in glacial drift.
Muscovite mica (left) and a transparent sheet of mica.
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 lightweight. They have a nonmetallic, glassy to silky 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 (KAl2AlSi3)O10(OH)2), contains potassium and is colorless and transparent in thin sheets. In thick sheets, muscovite varies in color from light yellow to light brown, green, and red and is translucent. It makes a colorless or white streak. Muscovite is named for a region in Russia where large muscovite sheets are used as window panes (muscovy glass). Biotite, also called black mica (K(Mg,Fe)3(AlSi3)O10(OH)2), contains iron and magnesium and is commonly dark green or brown to black, although it may be shades of yellow or brown; its streak is colorless. Thin sheets generally have a smoky color.
Mica is used in electronics, insulators, filler and extender in plasterboard, cement, paint, and drilling mud.
Mica can be 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.
Calcite (CaCO3), a common rock-forming mineral, consists of calcium carbonate. Calcite is the principal mineral in limestones, and crystals occur as a component in many concretions. Calcite is white or colorless, but impurities may tint it shades of yellow or gray. Transparent calcite is rarer than the tinted varieties. Transparent calcite (known as "Iceland Spar") 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 but not by a fingernail. Specific gravity is 2.71. Calcite effervesces freely in cold dilute hydrochloric acid or full-strength vinegar.
Calcite has a variety of crystal forms but, in Illinois, most commonly contains flattened, block-shaped (rhombohedron) crystals and elongate crystals with tapering points (scalenohedron) known as "dogtooth spar". When broken along its three cleavage planes, calcite forms six-sided blocks called rhombs.
Crystals of calcite are found in Illinois as linings in geodes, especially in the Nauvoo-Hamilton-Warsaw area. Thin calcite veins occur along joints in coal beds and as crystalline masses in vugs within limestone and dolomite. Small amounts of clear crystalline calcite are associated with various ores in northwestern and extreme southern Illinois.
Fluorite (CaF2), or fluorspar, is made up of the elements calcium and fluorine. The mineral is easily identified by its perfect cleavage, color, and hardness.
Fluorite occurs in cubic crystals that may be twinned. Most often, fluorite occurs as irregular masses in which individual crystals cannot be distinguished. Fluorite can be split along its four cleavage planes into diamond-shaped, eight-sided forms (octahedrons). Fluorite is commonly gray, white, or colorless, but it may also be green, blue, purple, pink, or yellow. The streak is colorless and the luster glassy. Fluorite can be scratched by a knife or a piece of window glass, is fairly lightweight (specific gravity ranges from 3 to 3.3), and is transparent to translucent.
Extensive deposits of fluorite, once one of Illinois' important commercial minerals, occur in Hardin and Pope Counties in extreme southern Illinois, where fluorite is associated with galena, sphalerite, calcite, barite, and other less abundant minerals.
Fluorite is used to make hydrofluoric acid. It is used to form a slag in the production of iron and steel. Fluorite is used also in toothpaste and in many chemical products and to make colored glass, enamels, and glazes in the ceramics industry. Fluorite—the state mineral—is no longer mined in Illinois, the last mine closing in 1997.
Gypsum (CaSO4•2H2O), hydrous calcium sulfate, is a colorless, transparent to translucent mineral when pure, but it is often stained yellow by impurities. It has a white streak, is soft enough to be scratched by a fingernail, and is lightweight.
Gypsum occurs in several forms. Selenite is a coarsely crystalline, transparent to translucent 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 Pennsylvanian Period of southern, north-central, and western Illinois and can be picked up at the surface. Satin spar gypsum has crystals resembling silky threads closely packed together, splits parallel to the fibers, and is found as fillings in cracks within rocks and as thin layers in shales. Massive microcrystalline gypsum is granular.
Gypsum deposits occur deep underground in Illinois, but thus far have not been mined.
Pyrite and Marcasite
Pyrite and marcasite (FeS2) are iron disulfide compounds. They look similar but have different crystal forms. Both are brittle, hard, brassy yellow with metallic luster, and opaque. Their crystal shape is the most distinguishing feature. The pyrite crystals are cubes, but the marcasite crystals are blade- or needle-shaped.
Pyrite and pyrite cube
Pyrite and marcasite have been mistaken for gold because they are yellow and metallic. They are commonly known as "fool's gold." These rocks are much harder than gold, tarnish, and leave a dark streak; gold is soft, very heavy, does not tarnish, and leaves a yellow streak. Gold is malleable, but pyrite or marcasite is reduced to powder if pounded and gives off a noticeable odor of sulfur dioxide gas if heated or cooked.
Both pyrite and marcasite are common as surface coatings, veins, and concretions in coal and in dark shales associated with coal. They are referred to as "coal brasses" or "sulfur" when found as impurities in coal. Coal brasses recovered from Illinois coal have been used in the manufacture of sulfuric acid for industrial use.
Limonite (FeO(OH)•H2O) is an iron oxide that may contain adsorbed water. Limonite 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. This medium-weight mineral may have a glassy or an earthy luster. It may be too hard to be scratched by a knife.
Limonite is common and occurs within concretions and cavity fillings in sedimentary rocks and as coatings on the rocks, especially sandstone. It also occurs as iron rust and accumulates around rootlets in soils. Small amounts of limonite discolor 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. In the middle 1800s, limonite was mined in Hardin County in Illinois, but deposits are not large enough to be mined economically.
Sphalerite (ZnS), zinc sulfide, is a major zinc ore. Sphalerite has a resinous luster and a white to yellow and brown streak. Illinois sphalerite is generally yellow, yellowish brown, reddish brown, or brownish black. It is of medium weight (specific gravity is 3.9 to 4.1) and is brittle; it can scratch a penny but not a piece of window glass. Sphalerite is commonly opaque but may be translucent on thin edges.
Sphalerite was mined with galena in northwestern Illinois and in extreme southern Illinois with galena and fluorite. Small crystals occasionally are found in limestones, within geodes, and as crystalline masses in clay-ironstone concretions.
Galena (PbS) is lead sulfide, the principal ore of lead. It is steel gray, heavy (specific gravity is 7.4 to 7.6), and opaque and has a bright metallic silver-colored luster, although the shiny surface may be dulled by a coating of lead carbonate. Galena has a gray or black streak; it is soft enough to mark paper and can be scratched by a penny. The cube-shaped crystals readily break into cubic, right-angled cleavage fragments. Probably the most obvious features of the mineral are its bright metallic luster on fresh surfaces, high specific gravity, and cubic cleavage.
Galena was an important mineral resource in Illinois during the nineteenth century and during the first half of the twentieth century. It was mined in northwestern and extreme southern Illinois. Galena can be found in small voids within limestones and dolomites and as crystals in geodes. Galena is often associated with fluorite and sphalerite deposits.
Rocks and Minerals of Illinois Table of Contents
The printed version of Guide to Rocks and Minerals in Illinois can be purchased from the Shop ISGS Web site.
Updated 11/29/2011 SLD