Gypsum

CaSO4.2H2O

Monoclinic - Crystals are of simple habit, tabular, diamond shaped, twinning common often resulting in swallowtail twins.

Physical Properties

Hardness - 2 (about the same as your fingernail). Specific Gravity 2.32; Luster usually vitreous; Color - colorless, white, gray, yellow brown, green (color is caused by impurities or inclusions.

Varieties

Selenite - coarsely crystalline, transparent. It consists of flat, diamond shaped crystals. In western Kansas, selenite is common in dark shales such as the Kiowa, Carlile and Pierre shales of Cretaceous age.

Satin Spar - is white or pink, fibrous and has a silky luster. It is found as thin layers in beds of rock gypsum and in certain shales.

Rock Gypsum - is coarsely to fine granular, white to gray and contains various amounts of impurities. A very fine grained type of massive gypsum is called alabaster which can be used in making ornaments and in carvings. Large quantities of rock gypsum are mined in Barber and Marshall Counties. Gypsite or gypsum dirt is formed in the solid or in shallow lakes and consequently is a sandy or earthy deposit. Gypsite occurs in Barber and Marshall counties.

Occurrence

Gypsum is a common mineral widely distributed in sedimentary rocks, often as thick beds (in Permian age rocks of Marshall, Saline, Dickinson, Comanche and Barber counties in Kansas). It frequently occurs interstratified with limestones and shales and is usually found as a layer underlying beds of rock salt, having been deposited there as one of the first minerals to crystallize on the evaporation of salt waters. May recrystallize in veins, forming satin spar. Occurs also as lenticular bodies or scattered crystals in clays and shales. Frequently formed by the alteration of anhydrite (CaSO4) and under these circumstances may show folding because of increased volume. Found in volcanic regions, especially where limestones have been acted upon by sulfur vapors. Also common as a gangue mineral in metallic veins. Associated with many different minerals, the more common being halite, anhydrite, dolomite, calcite, sulfur, pyrite, sphalerite and quartz.

Gypsum is the most common sulfate and extensive deposits are found in many localities throughout the world. The principal producers are the United States, Canada, France, England, Australia, and republics of the former USSR In the United States commercial deposit are found in many states, but chief producers are located in New York, Michigan, Iowa, Kansas, Texas and California. Gypsum is found in large deposit in Arizona and New Mexico in the form of windblown sand.

Use

Gypsum is used chiefly for the production of plaster or Paris. In the manufacture of this material, the gypsum is ground and then heated until about 75% of the water has been driven off. This material when mixed with water, slowly absorbs the water, crystallizes and thus hardens or "sets". Plaster of Paris is used extensively for "staff" the material from which temporary exposition buildings are built, for gypsum lath, wallboard/drywall, and for molds and casts of all kinds. Serves as a soil conditioner for fertilizer. Uncalcined gypsum is used as a retarder in Portland cement. Satin spar and alabaster are cut and polished for various ornamental purposes but are restricted in there uses because of their softness.