mineral
Selenite (gypsum)
Gypsum, CaSO₄·2H₂O; variety: selenite
AI-generated identification · may be incorrectHardness 2 on the Mohs scale; colorless to white with a pearly, glassy, or silky luster and transparent-to-translucent appearance. It has excellent one-directional cleavage, commonly forms tabular crystals, and is easily scratched with a fingernail; the squared piece appears cut or polished.
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Physical properties
Hardness 2 on the Mohs scale; colorless to white with a pearly, glassy, or silky luster and transparent-to-translucent appearance. It has excellent one-directional cleavage, commonly forms tabular crystals, and is easily scratched with a fingernail; the squared piece appears cut or polished.
Formation & geological history
Forms when evaporating saline or hypersaline water precipitates calcium sulfate, often in sedimentary evaporite beds, caves, and hot-spring deposits. Gypsum deposits range from recent to hundreds of millions of years old.
Uses & applications
Used to make plaster, drywall, cement additives, and soil amendments; clear selenite is popular for specimens, carving, and decorative objects. It is generally inexpensive and unsuitable for durable jewelry because it is very soft and water-sensitive.
Geological facts
The name selenite derives from Selene, the Greek Moon goddess, referring to its moonlike sheen. “Satin spar” is fibrous gypsum, while alabaster is fine-grained massive gypsum.
Field identification & locations
Identify it by extreme softness, perfect cleavage, light weight, and white translucent appearance; it should not fizz in dilute acid. Common localities include Mexico, Morocco, Spain, the United States, and Canada.
