bassanite

{{infobox mineral | name = Bassanite | image = Bassanite-192766.jpg | imagesize = 260px | alt = | caption = White radial-acicular bassanite crystals from Kimba, Eyre Peninsula, South Australia | category = Sulfate mineral | formula = CaSO4·H2O |IMAsymbol=Bss | strunz = 7.CD.45 | dana = | system = Monoclinic | symmetry = C2 (No. 5) | unit cell = a = 12.0317 Å, b = 6.9269 Å, c = 12.6712 Å, β = 90.27°; Z = 12 | color = White | habit = Microscopic acicular crystals in parallel aggregates, pseudohexagonal | twinning = Twin plane {101} | cleavage = | fracture = | tenacity = | mohs = | luster = Earthy | streak = White | diaphaneity = Semitransparent | gravity = 2.69–2.76 | density = | polish = | opticalprop = Biaxial (+) | refractive = nα = 1.550–1.559, nβ = 1.560, nγ = 1.577–1.584 | birefringence = | pleochroism = | 2V = 10–15° | dispersion = | extinction = | length fast/slow = | fluorescence = | absorption = | melt = | fusibility = | diagnostic = | solubility = | impurities = | alteration = Dehydrates on heating to anhydrite | other = | references = }} Bassanite is a sulfate mineral composed of calcium sulfate hemihydrate, with the chemical formula or . It has half a water molecule per CaSO4 unit.

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