Part 4

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.