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Gypsum Rose

Sulfate Mineral Pictures

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Gypsum is a soft mineral, hydrous calcium sulfate or CaSO4·2H2O. Gypsum is the standard for hardness degree 2 on the Mohs mineral hardness scale. (more below)
Hydrated calcium sulfate
Photo (c) 2009 Andrew Alden, licensed to About.com (fair use policy)
Your fingernail will scratch this clear, white to gold or brown mineral—that's the simplest way to identify gypsum. It's the most common sulfate mineral. Gypsum forms where seawater grows concentrated from evaporation, and it's associated with rock salt and anhydrite in evaporite rocks.

Gypsum forms bladed concretions called desert roses or sand roses, growing in sediments that are subjected to concentrated brines. The crystals grow from a central point, and the roses emerge when the matrix weathers away. They don't last long at the surface, just a few years, unless someone collects them. Besides gypsum, barite, celestine and calcite also form roses. See other common mineral shapes in the Mineral Habits Gallery

Gypsum also occurs in a massive form called alabaster, a silky mass of thin crystals called satin spar, and in clear crystals called selenite. But most gypsum occurs in massive chalky beds of rock gypsum. It's mined for the manufacture of plaster, and household wallboard is filled with gypsum. Plaster of Paris is a roasted gypsum with most of its associated water driven off, so it readily combines with water to return to gypsum.

Other Evaporitic Minerals

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