mineral

Selenite (satin-spar gypsum)

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

AI-generated identification · may be incorrect

Mohs hardness 2; translucent white to pale gray with silky, pearly luster and parallel fibrous striations. Monoclinic; perfect cleavage in one direction. The pictured piece appears cut and polished.

Identified September 26, 2026More mineral →

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Physical properties

Mohs hardness 2; translucent white to pale gray with silky, pearly luster and parallel fibrous striations. Monoclinic; perfect cleavage in one direction. The pictured piece appears cut and polished.

Formation & geological history

Gypsum precipitates as evaporite in saline lakes, lagoons, and marine basins as water evaporates; deposits formed throughout geological time. Fibrous satin spar develops as gypsum crystals grow in parallel.

Uses & applications

Used for plaster, drywall, cement, and soil amendment; satin spar is sold as decorative carvings and inexpensive collectible specimens.

Geological facts

Gypsum is soft enough to scratch with a fingernail and dissolves slightly in water; selenite is a clear crystalline form of the same mineral.

Field identification & locations

Look for softness, pale color, and silky parallel fibers; common in evaporite deposits worldwide, including the United States, Mexico, and Spain. Avoid soaking or scratching polished specimens.