Part 3

Around the edge of the coal basin where the coal lies at shallow depths, it is commonly mined in open pits (strip mines). However, in the past, most coal has been mined underground, and more than half of Illinois coal is still produced from such mines.

Limestone and Dolomite

There are two types of carbonate rocks in Illinois, limestone and dolomite. In appearance they are almost indistinguishable, although they are different chemically. Limestone is composed mainly of calcium carbonate (the mineral calcite), whereas dolomite is calcium magnesium carbonate (the mineral dolomite).

Limestone deposits are found at many places in the western, southwestern, and southern marginal portions of the state. They are largely of Mississippian and Ordovician age. Lesser deposits of Pennsylvanian limestone are found in the central portion of the state. In the northern fifth of the state, dolomites of Silurian and Ordovician age are exposed at many places.

Limestone and dolomite are quarried for a variety of uses, including aggregate for concrete, agricultural limestone, railroad ballast, chips for bituminous roads, and for use in metallurgical and chemical processes. They also are quarried for building stone at some places, and marble is produced in southern Illinois.

KEY Quarry, Limestone or Dolomite Cement or Lime Fluorspar Zinc and Lead

Limestone and shale or clay are burned together in kilns to produce portland cement. Portland cement is manufactured from a Pennsylvanian limestone in LaSalle County and from an Ordovician limestone in Lee County.

Limestone or dolomite also is burned in kilns to make lime. Mississippian limestone is used in Adams County and Silurian dolomite is used in Cook County for lime making.

Clay and Clay Products

Illinois commercial clays are found in glacial, Cretaceous, Tertiary, and Pennsylvanian deposits. Pennsylvanian shales also are sources of clays. Glacial clays, which occur as loess (wind-blown materials), alluvial (water-deposited) sediments, and glacial till, are widespread in the state. Cretaceous and Tertiary clays, at the extreme southern tip of the state, are valuable for the manufacture of heat-resistant bricks and coatings and for floor-sweeping compounds.

Pennsylvanian clays and shales are the most important source of clay for manufacturing such products as pottery, stoneware, drain tile, sewer pipe, flue tile, building tile, brick, and special heat-resistant fire-brick.

Sand and Gravel

Deposits of sand and gravel, found in many parts of Illinois, provide large quantities of material for concrete, railroad ballast, road gravel, building sand, molding sand, and other uses. Glacial deposits are the principal sources of sand and gravel, but in extreme southern Illinois where there is no glacial drift the main source of supply is Tertiary gravel and Devonian chert gravel.

KEY Pit or Plant

Almost every county has some sand and gravel, but the major areas of production are northeastern Illinois and some of the major rivers. Considerable tonnages of sand and gravel are dredged from bars in the channels of the Mississippi, Ohio, and Wabash Rivers.

Silica Sand

Silica sand is used in making glass, for sand blasting, as grinding sand, as molding sand for casting steel, and in the fracture treatment of oil wells to increase oil production. Some of the sand is finely pulverized into a powder that is used as a filler, a fine abrasive, as a ceramic material, and for many other purposes.

This special sand, composed of grains of the mineral quartz (silica), comes from a sandstone of Ordovician age that is mined in LaSalle and Ogle Counties.

Tripoli and Ganister

Tripoli, or “amorphous” silica, is mined from Devonian rocks in Alexander County. The silica is finely ground for use as a polishing agent, as a filler, in buffing compounds, and for other purposes.

Ganister, a granular material with a high silica content, also is mined in extreme southern Illinois. It is used in making products that can withstand high temperatures.

Fluorspar

For many years Illinois has led the nation in the production of fluorspar, accounting annually for about half the total United States output. The industry is centered in Hardin and Pope Counties where the deposits occur in veins and beds in rocks of the Mississippian System. Reserves of fluorspar are adequate to supply the industry for many years.

KEY Sand and Gravel Silica Sand Molding Sand Tripoli

Fluorspar is used in the manufacture of hydrofluoric acid, as a flux in the manufacture of steel, in the refining of aluminum, for the manufacture of fluorine compounds used in a variety of products, in ceramic glazes, and for other special purposes.

Lead and Zinc

Lead and zinc are the only metallic minerals mined in Illinois. Deposits are found in Jo Daviess County in northwestern Illinois and in the fluorspar area of Hardin and Pope Counties in extreme southern Illinois. In northwestern Illinois the ore occurs in rocks of Ordovician age, but in southeastern Illinois it is associated with fluorspar in rocks of Mississippian age.

Between 1820 and 1865, the mining area of which northwestern Illinois is a part was the nation’s principal producer of lead ore. In recent years production of zinc has increased and that of lead has decreased.