gypsum

Gypsum
General
Category	Sulfate minerals
Formula; (repeating unit)	CaSO4·2H2O
Strunz classification	7.CD.40
Crystal system	Monoclinic
Crystal class	Prismatic (2/m) ; H-M symbol: (2/m)
Space group	Monoclinic ; Space group: I2/a
Unit cell	a = 5.679(5), b = 15.202(14) ; c = 6.522(6) [Å]; β = 118.43°; Z = 4
Identification
Color	Colorless to white; may be yellow, tan, blue, pink, brown, reddish brown or gray due to impurities
Crystal habit	Massive, flat. Elongated and generally prismatic crystals
Twinning	Very common on {110}
Cleavage	Perfect on {010}, distinct on {100}
Fracture	Conchoidal on {100}, splintery parallel to [001]
Tenacity	Flexible, inelastic
Mohs scale hardness	1.5–2 (defining mineral for 2)
Luster	Vitreous to silky, pearly, or waxy
Streak	White
Diaphaneity	Transparent to translucent
Specific gravity	2.31–2.33
Optical properties	Biaxial (+)
Refractive index	nα = 1.519–1.521; nβ = 1.522–1.523; nγ = 1.529–1.530
Birefringence	δ = 0.010
Pleochroism	None
2V angle	58°
Fusibility	5
Solubility	Hot, dilute HCl
References
Major varieties
Satin spar	Pearly, fibrous masses
Selenite	Transparent and bladed crystals
Alabaster	Fine-grained, slightly colored

Gypsum is a soft sulfate mineral composed of calcium sulfate dihydrate, with the chemical formula CaSO₄·2H₂O. It is widely mined and is used as a fertilizer and as the main constituent in many forms of plaster, blackboard/sidewalk chalk, and drywall. A massive fine-grained white or lightly tinted variety of gypsum, called alabaster, has been used for sculpture by many cultures including Ancient Egypt, Mesopotamia, Ancient Rome, the Byzantine Empire, and the Nottingham alabasters of Medieval England. Gypsum also crystallizes as translucent crystals of selenite. It also forms as an evaporite mineral and as a hydration product of anhydrite.