mineral
Selenite (satin spar gypsum)
Gypsum, CaSO₄·2H₂O; fibrous variety commonly sold as satin spar
AI-generated identification · may be incorrectMohs hardness 2, white to translucent, pearly or silky luster, monoclinic crystals, and perfect cleavage in one direction. The parallel fibers and easily scratched surface are characteristic.
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Physical properties
Mohs hardness 2, white to translucent, pearly or silky luster, monoclinic crystals, and perfect cleavage in one direction. The parallel fibers and easily scratched surface are characteristic.
Formation & geological history
Forms by evaporation of sulfate-rich water in sedimentary evaporite basins, often during the Permian through modern periods. This elongated piece is likely a natural or lightly shaped gypsum crystal.
Uses & applications
Used for plaster, drywall, cement retarder, soil amendment, and decorative carvings; attractive satin spar is sold as a low-cost collectible or ornamental object.
Geological facts
Gypsum is soft enough to scratch with a fingernail and can be flexed slightly in thin fibers. “Selenite” technically refers to clear, nonfibrous gypsum, while this fibrous material is more precisely satin spar.
Field identification & locations
Identify it by its white silky fibers, very low hardness, light weight, and lack of fizzing in dilute acid. Common localities include Morocco, Mexico, Spain, the United States, and Tunisia.
