mineral

Selenite (satin spar gypsum)

Gypsum, CaSO₄·2H₂O; fibrous variety commonly called satin spar

AI-generated identification · may be incorrect

Typically white to translucent, silky or pearly luster, fibrous structure, and excellent cleavage; Mohs hardness 2, specific gravity about 2.3. The elongated, parallel fibers and soft, easily scratched surface fit satin spar rather than glass or calcite.

Identified September 16, 2026More mineral →

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Physical properties

Formation & geological history

Forms when evaporating saline waters precipitate gypsum, commonly in sedimentary evaporite beds; deposits range from recent to hundreds of millions of years old. This specimen appears shaped or polished from a natural fibrous piece.

Uses & applications

Used decoratively, for mineral collecting, and occasionally in jewelry; gypsum is also used in plaster, drywall, cement, and soil amendment. Satin spar has modest ornamental value.

Geological facts

“Selenite” technically refers to transparent crystalline gypsum, while opaque fibrous material is more precisely satin spar. It can be scratched with a fingernail and should be kept dry because gypsum is water-soluble over long periods.

Field identification & locations

Common localities include Morocco, Mexico, the United States, Spain, and Australia; look for softness, parallel fibers, white translucency, and silky chatoyancy. Avoid soaking, abrasion, and prolonged humidity.