Gypsum
Sulfate · CaSO₄·2H₂O · also: Selenite, Satin spar, Alabaster
Gypsum is a soft, hydrated calcium sulfate that you can scratch with a fingernail; its clear variety, selenite, forms giant glassy crystals.
What is gypsum?
Gypsum is hydrated calcium sulfate and one of the softest common minerals. It has several well-known varieties: clear, glassy selenite; fibrous, silky satin spar; and fine-grained alabaster used for carving. It forms in evaporite deposits and, in the Naica cave of Mexico, produced some of the largest crystals ever found.
Properties
- Chemical formula
- CaSO₄·2H₂O
- Category
- Sulfate
- Hardness (Mohs)
- 2
- Crystal system
- Monoclinic
- Lustre
- Vitreous, silky or pearly
- Streak
- White
- Colour
- Colourless, white, grey
- Cleavage / fracture
- Perfect in one direction
How to identify gypsum
- →So soft it can be scratched with a fingernail (hardness 2).
- →Clear selenite crystals show one perfect cleavage into flexible sheets.
- →Silky fibrous varieties (satin spar) show a cat’s-eye sheen.
- →Light weight and white streak.
Where gypsum is found
Gypsum is worldwide. Spectacular selenite comes from Naica (Mexico); satin spar and alabaster from England and Italy.
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