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epsomite (Epsom salts) Mineral, MgSO4.7H2O; sp. gr. 1.7; hardness 2.5; orthorhombic; colourless to white; vitreous to earthy lustre; crystals rare except locally, and show a fibrous structure; cleavage one direction, perfect; occurs as encrusting masses in caves and old mine workings, and also in the oxidized zones of pyrite deposits in arid regions. It is readily soluble in water and has a bitter taste. It is named after mineral springs at Epsom, England.