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anhydrite Mineral sulphate, CaSO4, found in evaporite deposits; sp. gr. 2.9–3.0; hardness 3.0–3.5; orthorhombic; normally white to colourless, sometimes with a bluish tinge, but occasionally grey and reddish; white streak; vitreous to pearly lustre; crystals rare, the mineral being usually massive, granular, and fibrous; three cleavages at right angles, perfect {010}, good {100}, {001}; occurs with gypsum and halite, deposited directly from sea water at temperatures in excess of 42°C, can form from the dehydration of gypsum; may occur as a cap rock above salt domes, and as a minor gangue mineral in hydrothermal veins. It is used as a raw material for cements.