mineral
Selenite (gypsum crystal)
Gypsum — calcium sulfate dihydrate, CaSO₄·2H₂O
AI-generated identification · may be incorrectWhite, translucent-to-opaque, silky or pearly luster with prominent lengthwise striations; Mohs hardness 2, monoclinic crystal structure, perfect cleavage in one direction, and specific gravity about 2.3. The elongated form is consistent with a small satin-spar/selenite fragment.
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Physical properties
White, translucent-to-opaque, silky or pearly luster with prominent lengthwise striations; Mohs hardness 2, monoclinic crystal structure, perfect cleavage in one direction, and specific gravity about 2.3. The elongated form is consistent with a small satin-spar/selenite fragment.
Formation & geological history
Forms when evaporating saline water precipitates calcium sulfate, commonly in sedimentary evaporite beds, caves, and hot-spring deposits; specimens range from modern to geologically ancient. The photograph alone cannot establish its precise age or locality.
Uses & applications
Used to make plaster, drywall, cement additives, soil conditioners, and casts; clear crystals are sold as decorative or metaphysical specimens. This small piece has modest commercial value.
Geological facts
Gypsum is extremely soft and can be scratched easily with a fingernail; its name derives from Greek gypsos, meaning plaster. “Satin spar” is fibrous gypsum, while transparent blade-like crystals are typically called selenite.
Field identification & locations
Identify it by its very low hardness, white color, fibrous/striated appearance, and ability to be scratched with a fingernail; it is commonly found in evaporite regions such as Mexico, Morocco, Spain, and the United States. Avoid prolonged water exposure and abrasion.
