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Gypsum is an evaporite, CaSO4•2(H2O). When dewatered at temperatures above 200°C, it becomes anhydrite.

Selenite, a variety of gypsum. This bladed form with an hourglass-shaped inclusion is only found at Great Salt Plains Lake, Oklahoma. The crystals form around grains of sand and clay to give the distinctive shape. Collected by Sara and Andrew Allen; photo by Skip Whitehurst.

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