mineral
Selenite (gypsum crystal)
Gypsum, CaSO₄·2H₂O; transparent-to-translucent variety commonly called selenite
AI-generated identification · may be incorrectUsually colorless to white with a vitreous-to-pearly luster and prominent lengthwise striations; monoclinic crystals with perfect cleavage in one direction. Mohs hardness 2, specific gravity about 2.3; the specimen appears to be a cleaved or partly polished block.
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Physical properties
Usually colorless to white with a vitreous-to-pearly luster and prominent lengthwise striations; monoclinic crystals with perfect cleavage in one direction. Mohs hardness 2, specific gravity about 2.3; the specimen appears to be a cleaved or partly polished block.
Formation & geological history
Forms when evaporating saline or groundwater precipitates calcium sulfate, commonly in sedimentary evaporite beds, caves, and clay-rich deposits. Gypsum deposits range from modern to hundreds of millions of years old.
Uses & applications
Used to manufacture plaster, drywall, cement additives, and soil conditioners; clear attractive pieces are sold as ornamental or metaphysical specimens. It is generally inexpensive unless exceptionally transparent, large, or well formed.
Geological facts
Selenite is soft enough to scratch with a fingernail and can be scratched or dulled easily. Its fibrous, silky variety is satin spar, while clear bladed crystals are often marketed as selenite.
Field identification & locations
Identify it by its low hardness, light weight, perfect cleavage, white streak, and striated or fibrous appearance; it should not fizz in dilute acid. Common sources include Mexico, Morocco, Spain, the United States, and Australia.
