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Anhydrite, calcium sulfate, is a mineral that occurs throughout the world, sometimes in thick, massive beds. It acquires its name from a lack of water in its structure, versus the equally common mineral, gypsum, which is calcium sulfate with water in its formula. Although generally white, anhydrite can sometimes acquire a bright hue as a result of impurities. This material from Naica, Mexico, a famous gypsum locality, is attractive and unusual.