mineral
Satin spar gypsum (selenite)
Gypsum, CaSO₄·2H₂O
AI-generated identification · may be incorrectWhite, translucent, silky or pearly luster with fibrous striations; Mohs hardness 2, perfect cleavage, monoclinic crystals, and low specific gravity (~2.3). The rectangular form appears cut or polished.
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Physical properties
White, translucent, silky or pearly luster with fibrous striations; Mohs hardness 2, perfect cleavage, monoclinic crystals, and low specific gravity (~2.3). The rectangular form appears cut or polished.
Formation & geological history
Forms by evaporation in sedimentary evaporite deposits, commonly during arid-basin or restricted-seawater conditions; deposits range from ancient Paleozoic to modern. This specimen is likely a processed mineral sample.
Uses & applications
Used to manufacture plaster, drywall, cement additives, and fertilizer; attractive satin-spar pieces are sold as décor or inexpensive mineral specimens, but it is generally unsuitable for jewelry because it scratches easily.
Geological facts
Satin spar is a fibrous variety of gypsum, while transparent, well-cleaved gypsum is commonly called selenite. It can be scratched by a fingernail and may dissolve slowly in water.
Field identification & locations
Identify by its very low hardness, white fibrous sheen, and easy cleavage; common localities include Morocco, Mexico, the United States, Spain, and the United Kingdom. Avoid prolonged moisture and abrasion.
