Gypsum is a mineral that consists of calcium sulfate plus two molecules
of water (CaSO₄·2H₂O). By suitably heating it, the amount of water can
be reduced, and a product called calcined gypsum (plaster of paris)
results. This material changes back to gypsum if mixed with an
appropriate quantity of water. The ability of calcined gypsum to “set”
when water is added makes it important in the manufacture of a variety
of plasters and related products, especially building materials. Gypsum
also is used in cement making and in agriculture.
Anhydrite (CaSO₄) is like gypsum except that it contains no water and
hence cannot be made into plaster of paris. Its uses are limited in the
United States.
Wells that were drilled for oil, water, or coal have encountered gypsum
and/or anhydrite in some parts of south-central Illinois, but the gypsum
and anhydrite are not known to crop out at the surface. A study of
diamond drill cores and well cuttings on file at the Survey showed that
the shallowest gypsum and anhydrite reported occurred at a depth of 470
feet in Madison County. The greatest continuous thickness of gypsum
found was 2 feet; but in one well, over 6 feet of strata was penetrated
that averaged almost 75 percent gypsum. It is possible that thicker
deposits of gypsum might be found if drilling were done especially in
search of it.