Bassanite
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Bassanite is a calcium sulfate mineral with formula CaSO₄·1/2H₂O or 2CaSO₄·H₂O. In other words it has half a water molecule per CaSO₄ unit, hence its synonym calcium sulfate hemihydrate.
Bassanite was first described in 1910 for an occurrence on... Wikipedia
Formula (repeating unit): CaSO4·1/2H2O
Optical properties: Biaxial (+)
Refractive index: nα = 1.550–1.559, nβ = 1.560, nγ = 1.577–1.584
Space group: C2 (No. 5)
Unit cell: a = 12.0317 Å, b = 6.9269 Å, c = 12.6712 Å, β = 90.27°; Z = 12