mineral
Satin spar gypsum (selenite)
Gypsum, CaSO₄·2H₂O; fibrous variety satin spar
AI-generated identification · may be incorrectTypically white to pale gray with silky, pearly luster and parallel fibrous crystal structure; Mohs hardness 2, perfect one-direction cleavage, and specific gravity about 2.3. The specimen appears as a naturally cleaved or lightly shaped fibrous piece.
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Physical properties
Typically white to pale gray with silky, pearly luster and parallel fibrous crystal structure; Mohs hardness 2, perfect one-direction cleavage, and specific gravity about 2.3. The specimen appears as a naturally cleaved or lightly shaped fibrous piece.
Formation & geological history
Forms by evaporation of saline water in sedimentary evaporite basins, often during Permian to recent deposits. It commonly occurs with halite, anhydrite, and other evaporite minerals.
Uses & applications
Used to manufacture plaster, drywall, cement additives, and fertilizer; attractive pieces are sold as display minerals or carved objects. It is generally inexpensive and not suitable for durable jewelry.
Geological facts
Gypsum is soft enough to scratch with a fingernail and can be dehydrated to make plaster of Paris. Satin spar is commonly marketed as “selenite,” although true selenite usually has transparent, glassier crystals.
Field identification & locations
Identify it by its very low hardness, white fibrous sheen, light weight, and cleavage; avoid prolonged water exposure because it can dissolve or weaken. Common localities include Mexico, Morocco, Spain, the United States, and Australia.
