COAL, an organic stratified rock, is formed from accumulated plant
material and partially decayed plants that were buried during the “Coal
Measures” period in Illinois more than 200 million years ago.
Sediments deposited over the peat-like organic material compacted it.
Chemical changes gradually took place and resulted in the loss of water
and gases, leaving a higher percentage of carbon than the original
material contained.
The amount of such change that has taken place determines the rank of
the coal. The lowest ranks are called lignite, the intermediate group is
called bituminous (soft) coal, and coals of the highest rank, with the
highest carbon content, are called anthracite (hard coal). Mineral
matter, such as shale, clay, or pyrite, generally is present in the coal
and becomes ash when the coal is burned.
Most coal mined in the United States is bituminous coal. It is black,
brittle, breaks into angular blocks, has a shiny luster, and generally
shows a banded structure.
Coal mining is an important industry in Illinois, and the state contains
the largest known reserves of bituminous coal in the United States.
Minable coal beds underlie about two-thirds of the state. As many as 20
different coal beds have been mined in Illinois, the most important
being the Herrin (No. 6) and the Springfield (No. 5) or Harrisburg (No.
5). The coal in most mining areas averages 5 to 7 feet thick and in
places attains a thickness of 15 feet.
In underground mines the coal is approached by vertical or inclined
shafts. In open cut, or strip, mines all of the overlying material
(overburden) is removed, leaving the coal exposed. Coal as much as 100
feet deep is now being strip mined in Illinois.
Illinois coal is now used mainly for generating electric power, for
industrial purposes, and for heating. In industry, coal is used
extensively for power, heating, burning cement, firing clay products
such as brick, tile, pottery, porcelain, and china, and making coke.
Certain Illinois coals when mixed with coal from the eastern part of the
United States, produce metallurgical coke for making steel. Gases, oils,
and tars derived in processing coal for coke have been used for making
many chemical products, including dyes, perfumes, explosives, medicines,
insecticides, plastics, and road tar.