Gypsum, a soft mineral with a Mohs hardness of 2, is composed of hydrated calcium sulfate (CaSO4·2H2O). It is extensively mined for use in fertilizers, plasters, drywall, and chalks. Gypsum can form translucent selenite crystals and occurs as an evaporite or hydration product of anhydrite. Its scratch hardness is the basis for its rank on the Mohs scale of mineral hardness.
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