mineral
Gypsum (selenite/alabaster variety)
Gypsum, CaSO₄·2H₂O
AI-generated identification · may be incorrectWhite to translucent, vitreous-to-silky luster, with a fibrous or layered appearance and perfect cleavage. Mohs hardness 2; monoclinic crystal system; specific gravity about 2.3. The rusty-yellow bands are likely iron-oxide staining.
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Physical properties
White to translucent, vitreous-to-silky luster, with a fibrous or layered appearance and perfect cleavage. Mohs hardness 2; monoclinic crystal system; specific gravity about 2.3. The rusty-yellow bands are likely iron-oxide staining.
Formation & geological history
Forms when sulfate-rich water evaporates in sedimentary basins, or through hydration of anhydrite. Gypsum deposits range from recent to hundreds of millions of years old.
Uses & applications
Used to make plaster, drywall, cement, and soil amendments; clear selenite is also collected and used decoratively.
Geological facts
Gypsum is soft enough to scratch with a fingernail and can be scratched by a copper coin.
Field identification & locations
Look for softness, pale color, and cleavage; gypsum commonly occurs in evaporite deposits worldwide, including Spain, Mexico, and the United States. The small rough specimen is difficult to identify conclusively from the photo alone.
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