Anhydrite - Encyclopedia

    Class : Sulphates, chromates, molybdates
    Subclass : Anhydrous sulfates
    Crystal System : Orthorhombic
    Chemistry : CaSO₄
    Rarity : Very common

Anhydrite is a common mineral in sedimentary evaporitic deposits : it precipitates directly from seawater in a lagoon environment as soon as the temperature reaches 42°C, thus constituting thick layers, associated with gypsum or halite. Still in an evaporitic context, anhydrite is common in large clusters in the sulphur cap-rocks that cap the salt domes of Texas, Louisiana and Germany. It is also a hydrothermal alteration mineral of limestone and dolomitic rock, exceptionally a matrix mineral from hydrothermal veins (Balmat, New York). By hydration, it is transformed into gypsum with an increase in volume ; it is nevertheless much less frequent than the latter. Its name comes from the Greek anhudôr (without water) as opposed to gypsum which is hydrated. Its crystals are rare, tabular or prismatic, frequently flattened. Anhydrite occurs mainly in large crystalline masses showing three perpendicular cleavages between them, simulating a cubic system. It is rarely fibrous (parallel fibers, radiating and often curved), or in concretions. Anhydrite is white or grayish, often tinged with gray-blue or purple, more rarely reddish, brownish, or a magnificent pink-mauve. Anhydrite is used for acid soil amendment and sometimes as a set retarder in some cements. It has also been used as a source of sulfur and for the production of sulfuric acid in England and Germany, but this use remains marginal. Particularly aesthetic specimens of anhydrite are sometimes cut in cabochon for jewelry or carved, this is particularly the case of the famous angelite, a massive Peruvian blue variety.

Anhydrite presents twin on {011} as contact twins and polysynthetic lamellae as well as on {120} as a contact twin (rare).