mineral

Fibrous gypsum (selenite), likely satin spar

Gypsum — CaSO₄·2H₂O; monoclinic sulfate mineral

AI-generated identification · may be incorrect

Typically white to cream, pearly or silky, with prominent fibrous cleavage and a vitreous-to-satiny luster; Mohs hardness 2, specific gravity about 2.3. The specimen appears to be an elongated, weathered fibrous aggregate rather than a well-formed crystal.

Identified September 9, 2026More mineral →

Explore Fibrous gypsum (selenite), likely satin spar in the encyclopedia →

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Physical properties

Formation & geological history

Forms by evaporation of saline water in sedimentary basins, especially evaporite deposits, and also by hydration of anhydrite. Gypsum deposits range from Paleozoic to modern in age; the exact age cannot be determined from the image.

Uses & applications

Used extensively for drywall, plaster, cement additive, and fertilizer; clear selenite is collected and occasionally used decoratively. This rough specimen has modest commercial value.

Geological facts

Gypsum is soft enough to scratch with a fingernail and can be split into thin sheets. “Satin spar” describes fibrous gypsum, while “selenite” more commonly refers to transparent or large-cleaved varieties.

Field identification & locations

Common in evaporite regions worldwide, including Spain, Mexico, the United States, Morocco, and Australia. Confirm by fingernail scratching, very low weight, and lack of fizz in dilute acid; avoid confusing it with fibrous calcite or weathered barite.