Selenite (mineral)

Selenite
General
Category	Sulfate mineral
Formula (repeating unit)	CaSO4·2H2O
Crystal system	Monoclinic (2/m) Space group: A2/a
Identification
Formula mass	172.17
Color	Brown green, brownish yellow, greenish, gray green, gray white
Crystal habit	Earthy – dull, clay-like texture with no visible crystalline affinities, (e.g. howlite)
Cleavage	[010] Perfect, [100] Distinct, [011] Distinct
Fracture	Fibrous – thin, elongated fractures produced by crystal forms or intersecting cleavages (e.g. asbestos).
Mohs scale hardness	2
Luster	Pearly
Streak	White
Specific gravity	2.9
Optical properties	Biaxial (+) 2V=58
Refractive index	nα = 1.519–1.521, nβ = 1.522–1.523, nγ = 1.529–1.53
Birefringence	δ = 0.0090–0.0100
Other characteristics	Non-radioactive, non-magnetic, fluorescent
References	[1]

Selenite, satin spar, desert rose, and gypsum flower are crystal habit varieties of the mineral gypsum.

All varieties of gypsum, including selenite and alabaster, are composed of calcium sulfate dihydrate (meaning that it has two molecules of water), with the chemical formula CaSO₄·2H₂O. Selenite contains no significant selenium – The similar names both derive from Greek selḗnē (σελήνη 'Moon').

Some of the largest crystals ever found are of selenite, the largest specimen found in the Naica Mine's Cave of the Crystals being 12 metres long and weighing 55 tons.