GYPSUM, hydrous calcium sulfate, is a colorless, transparent to
translucent mineral when pure, but it often is stained yellow by
impurities. It has a white streak, is soft enough to be scratched by a
fingernail, and is light weight.
[Illustration: uncaptioned]
Gypsum occurs in several forms. Selenite is a coarsely crystalline,
transparent variety, composed of flat, nearly diamond-shaped crystals
that can be split easily into thin sheets, have a glassy luster, and
often grow together to form “fishtail twins.” Crystals of selenite occur
in shales of the “Coal Measures” of southern, north-central, and western
Illinois, and can be picked up at the surface.
Satin spar has crystals like silky threads closely packed together,
splits parallel to the fibers, and is found as fillings in rock cracks
and as thin layers in shales. Massive gypsum is granular.
Gypsum deposits occur deep underground in Illinois but thus far have not
been mined.