Anhydridit

Anhydridit is a mineral species consisting of anhydrous calcium sulfate, CaSO4. It crystallizes in the orthorhombic system and occurs as colorless to white crystals, commonly massive or compact. Individual crystals are often tabular or prismatic with a vitreous to pearly luster. The mineral has a Mohs hardness of 3 to 3.5 and a specific gravity around 3.9. It exhibits two directional cleavage and forms when gypsum dehydrates or evaporite deposits precipitate from saline waters. It is transparent to translucent.

Anhydridit forms in evaporite deposits, sedimentary rocks, and hydrothermal veins. It can originate by dehydration of gypsum CaSO4·2H2O upon heating or burial, and also by direct precipitation from sulfate-rich fluids in arid environments. It is commonly associated with gypsum, halite, sylvite, celestine, and dolomite. Because it lacks water of hydration, anhydridit can rehydrate to gypsum if exposed to sufficient moisture.

Etymology and use: the name derives from Greek anhydros, meaning "without water," in reference to the absence of water of crystallization in the mineral formula. In industry, anhydridit is mined as a source of calcium sulfate and can be processed or hydrated to gypsum for use in cement and plaster production. It also occurs as a decorative stone in some localities, where massive or compact forms are employed.