Bassanite

Not to be confused with basanite or Bassanitae.

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
White radial-acicular bassanite crystals from Kimba, Eyre Peninsula, South Australia
General
Category	Sulfate mineral
Formula	CaSO4·⁠1/2⁠H2O
IMA symbol	Bss[1]
Strunz classification	7.CD.45
Crystal system	Monoclinic
Space group	C2 (No. 5)
Unit cell	a = 12.0317 Å, b = 6.9269 Å, c = 12.6712 Å, β = 90.27°; Z = 12
Identification
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	[2][3][4]

Bassanite was first described in 1910 for an occurrence on Mount Vesuvius. It was named for Italian paleontologist Francesco Bassani (1853–1916).^[2]

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.^[3]

H. Schmidt and coinvestigators reported in 2011 that under dry conditions, the structure is monoclinic with space group C2, but at 75% humidity, the structure is trigonal with space group P3₂21. This reflects the incorporation of additional water of hydration, such that the trigonal form has the formula CaSO₄·0.625H₂O.^[5]