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In page Gypsum:

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Gypsum is a soft sulfate mineral composed of calcium sulfate dihydrate, with the chemical formula CaSO4·2H2O.[4] It is widely mined and is used as a fertilizer and as the main constituent in many forms of plaster, drywall and blackboard or sidewalk chalk.[5][6][7][8] Gypsum also crystallizes as translucent crystals of selenite.[9] It forms as an evaporite mineral and as a hydration product of anhydrite.[citation needed] The Mohs scale of mineral hardness defines gypsum as hardness value 2 based on scratch hardness comparison.[9]