Anhydrite's name means "waterless stone," and it forms where low heat drives the water out from gypsum. Generally you won't see anhydrite except in underground mines, because at the Earth's surface it swiftly combines with water and becomes gypsum. This specimen was mined in Chihuahua, Mexico, and is in the Harvard Natural History Museum.
Anhydrite is calcium sulfate, CaSO4, similar to gypsum but without its water of hydration. (more below)
Courtesy Alcinoe through Wikimedia Commons
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