Guide to Rocks and Minerals of Illinois

Sedimentary Rocks

Conglomerate

Conglomerate is a sedimentary rock made up of pebbles or other rock fragments (sand and gravel) cemented together by a 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 or by waves along beaches.

If the pebbles embedded in the matrix are sharp and angular, freshly broken, and not worn, the rock is called breccia; breccia is generally found close to the place where the fragments originated. Conglomerate or breccia may be made up of any type of rock or mineral, but most commonly a 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 Pennsylvanian aged rocks. They are also found in some gravel deposits.

Sandstone

Sandstone is a clastic sedimentary rock consisting of sand-sized grains (0.0025 to 0.08 inch or 0.064 to 2.032 mm in diameter) held together by a cementing material. In general, the individual grains in sandstone are visible to the unaided eye. As sandstones become more finely grained, they grade into siltstones; as they become more coarsely grained, they grade into conglomerates. 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, 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 largely depends on the type of cement, the amount of organic material present, and the amount and degree of oxidation of iron in the rock.

The 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. In contrast to their metamorphic equivalents, quartzites, which break across their individual grains, sandstones break around the grains, giving the broken surface a granular appearance.

Sandstone crops out in many places throughout the state. In Ogle and La Salle Counties, for example, many tons of almost pure quartz sand are mined from the St. Peter Sandstone and are 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.

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.

Siltstone

Siltstone is a clastic sedimentary rock consisting of silt-sized grains (0.00016 to 0.0025 inch or 0.0041 to 0.064 mm) held together by a cementing material; some siltstones are well cemented, and some are weakly cemented and disintegrate readily. This consolidated rock is intermediate in grain size between sandstone and shale. The composition is predominantly quartz and clay minerals; mica flakes may be present. In general, the individual grains in a siltstone are visible using a 10× magnification. If the individual grains are not distinguishable, then the fine-grained sedimentary rock is classified as a shale. Siltstones are most common in Illinois among rocks of the Pennsylvanian and in some rocks of the Mississippian Period. Siltstones generally do not have commercial value, although some occasionally have been used locally for building stone walls and similar projects.

Shale

Shale is a common sedimentary rock composed primarily of lithified clay or mud. It is so fine-grained that the minerals forming it generally cannot be identified without the aid of x-ray diffraction or scanning electron microscopy. The particles of most clay minerals are thin and flat and overlap each other. These flat, overlapping clay minerals permit the shale to weather or be split into very thin layers that are approximately parallel to bedding. Shales that exhibit this characteristic are said to be fissile. This property of splitting into layers is called fissility and is a characteristic of shales and slates.

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 some silt or sand particles; if silt or sand is present in large quantities, the rock is called silty shale or sandy shale. If mica minerals are present in quantity in a shale, it is called micaceous shale. Mudstone is a general term used to describe rocks with the composition of shale but lacking the fissility.

Shales have a wide range of colors, but most Illinois shales are gray or black. The black color is caused by organic matter in the shale; red, brown, yellow, or green colors are caused by iron compounds.

Crushed shale is used as a source for the clay used in the manufacture of bricks, drain tile, and building tile and can also be used as a lightweight aggregate.

Shale is widely distributed in Illinois, especially in Pennsylvanian age rocks.

Clay

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. If clay is well consolidated, the result is called claystone. If clay is lithified, it is called shale or mudstone.

Clays are very fine-grained, and their minerals have tiny, flat crystals that can be distinguished from one another only by laboratory methods. Although clays may appear to be similar, their compositions and properties 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, drain tile, paint, rubber, and drilling mud.

Fuller's Earth

Fuller's earth is a very fine-grained earthy material composed of clay-sized or silty clay-sized material that is chiefly made up of the clay mineral montmorillonite. Fuller's earth is soft, nonplastic, and 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. Fuller's earth 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 that still afford a clay with absorbent properties that make it useful as cat litter and as an absorbent cleaning compound for oil spills.

Limestone and Dolomite

Limestone is a sedimentary rock composed of particles of calcite (calcium carbonate) or calcitic fragments of shelly organisms. The crystals may range from fine to coarse. Many limestones contain other minerals, such as chert, clay, or sand, and, in some places, limestones grade into dolomite (calcium-magnesium carbonate).

Many limestones are very light gray or gray. Yellow or brown shades are caused by iron oxide impurities, and dark gray to black colors are from 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 rounded, sand-size grains is called oolite or oolitic limestone.

Limestone effervesces freely in dilute hydrochloric acid or full-strength vinegar, but dolomite must be powdered before it effervesces. In nature, limestones may be dissolved by percolating groundwater containing weak acid (such as carbonic acid, composed of water and carbon dioxide). In many places, such solution of limestones has produced sinkholes, caves, and caverns, especially in the well-known Illinois karst terrain of southwestern Illinois in St. Clair, Monroe, and Randolph Counties.

Limestone has many uses: building stone, road surfacing, railroad ballast, portland cement manufacture, and, if of high purity, for making lime and chemicals and as a flux in smelting metals. Limestone also is added at coal-burning plants to curtail acid emissions—an important consideration when using Illinois coal. Agricultural limestone is used to add calcium to the soil, which reduces the soil acidity.

Limestone outcrops are abundant in Illinois, especially along the bluffs of the Mississippi, Ohio, and Illinois Rivers. (View limestone palisades, Madison County, Illinois) Quarries that mine limestone or dolomite are scattered throughout Illinois. (Blacktop Quarry, Carroll County)

Tripoli

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.

Tripoli 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.

Rocks and Minerals of Illinois Table of Contents

Updated 11/29/2011 SLD