mineral
Gypsum (Selenite crystals in Halite/Clay matrix)
Calcium Sulfate Dihydrate (CaSO4·2H2O)
Hardness: 2 on Mohs scale (can be scratched by a fingernail). Color: Colorless to white crystals with reddish-brown clay inclusions. Luster: Vitreous to pearly. Crystal structure: Monoclinic. Cleavage: Perfect in one direction. Specific gravity: 2.3.
- Hardness
- 2 on Mohs scale (can be scratched by a fingernail)
- Luster
- Vitreous to pearly
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Physical properties
Hardness: 2 on Mohs scale (can be scratched by a fingernail). Color: Colorless to white crystals with reddish-brown clay inclusions. Luster: Vitreous to pearly. Crystal structure: Monoclinic. Cleavage: Perfect in one direction. Specific gravity: 2.3.
Formation & geological history
Formed as an evaporite mineral in sedimentary environments, specifically dry lake beds (playas) or shallow marine basins where saline water evaporates, concentrating calcium and sulfate ions. This specimen likely dates from the Quaternary period.
Uses & applications
Primary use is in the manufacture of wallboard (drywall), plaster of Paris, and as a soil conditioner in agriculture. Pure selenite varieties are used in carvings and as ornamental collector specimens.
Geological facts
Gypsum is the most common sulfate mineral. The variety 'Selenite' is named after the Greek word for the moon due to its pearly luster. Large selenite crystals up to 12 meters long have been found in the Cave of the Crystals in Mexico.
Field identification & locations
Identify by its extreme softness (scratched by nail) and vitreous crystals. It is commonly found in desert regions like the Mojave Desert (as seen in the background). Look for crystalline structures in dry lake beds or areas with sedimentary mudstone.
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