Gypsum

Real mineral · common · CaSO₄·2H₂O

Hydrated calcium sulfate deposited when shallow seas evaporate. The transparent selenite variety can grow into crystals several metres long in cave environments with stable humidity. Dehydrates to plaster of Paris at 150°C.

Identity

Formula

CaSO₄·2H₂O

Chemical class

sulfate

Also known as

Selenite (transparent variety), Alabaster (fine-grained), Satin spar (fibrous)

Crystallography

Crystal system

monoclinic

Habits

tabular, prismatic, fibrous, massive

Fracture

fibrous

Physical properties

Mohs hardness

2

Density

2.32 g/cm³

Luster

vitreous, silky, pearly

Colors

colorless, white, yellow, pink, gray

Streak

white

Transparency

transparent, translucent

Geology

Origins

sedimentary

Host rocks

limestone, shale, evaporite

Associated minerals

Halite, Calcite, Anhydrite, Sulfur

Geography

worldwide

Classification

Rarity

common

Gem quality

No

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