ANHYDRITE (from the Greek words "an" (negation particle) and hydor (water) * EN: anhydrite; DE: Anhydrit, Anhydritgips; FR: anhydrite; ES: anhidrita; RU: ангидрит) is the mineral of the class of sulfates, Ca[SO4]. It contains the impurities Ba, Sr, Mg. It crystallizes into the orthorhombic crystal systems. The tetrahedral group [SO4]2- is at the base of the structure. At 1193 degrees Celsius it transitions into the hexagonal modification, which is isostructural with the high-temperature modifications of barite and celestine. Thanks to the pseudocubic structure, it forms the isometric crystals with the perfect cleavage along the three mutually perpendicular directions. It is customarily found in the form of the granular, less often fibrous aggregates.
Anhydrite is colourless or bluish, gray, pinkish. The hardness is 3.5-3.8. The density is 2900-3000 kilograms per cubic metre. It is one of the main minerals of the salt deposits, where it is associated with gypsum, on the account of which it may be formed by the way of the dehydration at the increased pressure; in its turn, anhydrite is easily hydrated, and transitions into gypsum with the large increase of the volume (more than 30%); less often it is the product of the metasomatism and sublimation activity of the volcanoes.
Anhydrite is the veinous mineral of the certain hydrothermal ore deposits. Within the USSR, there are most known the Artyomovskoe deposit within Ukraine, Permian depositions along the Sukhona and Northern Dvina rivers. The deposits of anhydrite may be found abroad within many salt domes of Texas and Louisiana (USA), within the Staßfurt basin (East Germany), within the Wieliczka region (Poland), and other places. It is used as the raw material for cement, fertilizer, raw material for the obtainment of sulfuric acid, semi-precious (decorative) stone. It is extracted from the ores by flotation with the usage of the fatty acids and their soaps, alkyl sulfates within the solution of NaCl.
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