mineral

Satin Spar

Fibrous Gypsum (CaSO4·2H2O)

Hardness: 2 on Mohs scale (can be scratched by a fingernail); Color: Pearly white to translucent; Luster: Silky/Satin; Crystal Structure: Monoclinic; Cleavage: Perfect in one direction, yielding fibrous shards; Specific Gravity: 2.3

Hardness: 2 on Mohs scale (can be scratched by a fingernail)
Color: Pearly white to translucent
Luster: Silky/Satin

Identified May 6, 2026More mineral →

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Physical properties

Formation & geological history

Forms primarily through the evaporation of saline water in sedimentary environments or as a secondary mineral in oxidized zones of sulfide deposits. It commonly occurs in veins where gypsum has crystallized in a fibrous habit.

Uses & applications

Used in producing plaster of Paris, drywall (as gypsum), as a soil conditioner, and commonly sold as decorative specimens or carved into metaphysical items (often mislabeled as Selenite).

Geological facts

Though often called 'Selenite' in trade, true Selenite is the clear, platy variety of gypsum; Satin Spar is specifically the fibrous variety. It is so soft it can be easily damaged by water or handled roughly.

Field identification & locations

Identified in the field by its intense silky luster, fibrous structure that looks like bundled threads, and its extreme softness (it is easily scratched by a fingernail). Found in desert regions, evaporite basins, and clay beds.