FLUORITE (25)
FLUORITE, or fluorspar, is made up of the elements calcium and fluorine. The mineral is easily identified by its perfect cleavage, color, and hardness.
It occurs in cubic crystals that may be twinned but is more often found as irregular masses. It can be split into diamond-shaped, eight-sided forms (octahedrons). Fluorite is commonly gray, white, or colorless, but it may be green, blue, purple, pink, or yellow. The streak is colorless and the luster glassy. It can be scratched by a knife or a piece of window glass, is fairly light weight, and is transparent to translucent.
Extensive deposits of fluorite, one of Illinois’ important commercial minerals, occur in Hardin and Pope Counties in extreme southern Illinois, where it is associated with galena, sphalerite, calcite, barite, and other less abundant minerals.
Fluorite is used to make hydrofluoric acid, to form a fluid slag in the production of iron and steel, in the manufacture of aluminum, to make many chemical products, and in the ceramic industry, in which it is used to make colored glass, enamels, and glazes.
GYPSUM (26)
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.
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.