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Bassanite (3.0 by 2.6 cm white mass with some gypsum crystals)
Category Sulfate mineral
(repeating unit)
Strunz classification 7.CD.45
Crystal system Monoclinic
Space group Spheroidal (2)
(same H-M symbol)
Unit cell a = 12.0317 Å,
b = 6.9269 Å,
c = 12.6712 Å, β = 90.27°; Z = 12
Color White
Crystal habit Microscopic acicular crystals in parallel aggregates, pseudohexagonal
Twinning Twin plane {101}
Luster Earthy
Streak White
Diaphaneity Semitransparent
Specific gravity 2.69–2.76
Optical properties Biaxial (+)
Refractive index

nα = 1.550–1.559, nβ = 1.560,

nγ = 1.577–1.584
2V angle 10-15°
Alters to Dehydrates on heating to anhydrite
References [1][2][3]

Bassanite is a calcium sulfate mineral with formula CaSO4·0.5(H2O) or 2CaSO4·H2O. In other words it has half a water per CaSO4 unit hence its synonym hemihydrate.

Bassanite was first described in 1910 for an occurrence on Mount Vesuvius. It was named for Italian paleontologist Francesco Bassani (1853–1916).[1]

At Vesuvius it occurs as alterations from gypsum within leucite tephrite and as fumarole deposits. It occurs in dry lake beds in California and Australia. It also occurs interlayered with gypsum in caves.[2]