Fluorite
Fluorite (CaF2), or fluorspar, is made up of the elements calcium and fluorine. The mineral is easily identified by its perfect cleavage, color, and hardness.
Fluorite occurs in cubic crystals that may be twinned. Most often, fluorite occurs as irregular masses in which individual crystals cannot be distinguished. Fluorite can be split along its four cleavage planes into diamond-shaped, eight-sided forms (octahedrons). Fluorite is commonly gray, white, or colorless, but it may also be green, blue, purple, pink, or yellow. The streak is colorless and the luster glassy. Fluorite can be scratched by a knife or a piece of window glass, is fairly lightweight (specific gravity ranges from 3 to 3.3), and is transparent to translucent.
Extensive deposits of fluorite, once one of Illinois' important commercial minerals, occur in Hardin and Pope Counties in extreme southern Illinois, where fluorite is associated with galena, sphalerite, calcite, barite, and other less abundant minerals.
Fluorite is used to make hydrofluoric acid. It is used to form a slag in the production of iron and steel. Fluorite is used also in toothpaste and in many chemical products and to make colored glass, enamels, and glazes in the ceramics industry. Fluorite—the state mineral—is no longer mined in Illinois, the last mine closing in 1997.