Chao, Edward C. T. U.S. Geological Survey, Department of the Interior, Reston, Virginia.
Last reviewed:January 2021
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A mineral with the chemical composition CaSO4. Anhydrite (anhydrous calcium sulfate) occurs commonly in white and grayish granular masses, rarely in large, orthorhombic crystals (see illustration). Fracture is uneven and luster is pearly to vitreous. Hardness is 3–3.5 on Mohs scale and specific gravity is 2.98. It fuses readily to a white enamel. It is soluble in acids and slightly soluble in water. Anhydrite is an important rock-forming mineral and occurs in association with gypsum, limestone, dolomite, and salt beds. It is deposited directly by evaporation of seawater of high salinity at or above 42°C (108°F). Anhydrite can be produced artificially by dehydration of gypsum at about 200°C (390°F). Under natural conditions anhydrite hydrates slowly, but readily, to gypsum. It is not used as widely as gypsum. Anhydrite is of worldwide distribution. Large deposits occur in the Carlsbad district, Eddy County, New Mexico, and in salt-dome areas in Texas and Louisiana. See also: Gypsum; Saline evaporites
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