Bassanite

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Bassanite
Bassanite-192766.jpg
Bassanite (30 by 26 mm white mass with some gypsum crystals)
General
CategorySulfate mineral
Formula
(repeating unit)
CaSO4·1/2H2O
IMA symbolBss[1]
Strunz classification7.CD.45
Crystal systemMonoclinic
Space groupC2 (No. 5)
Unit cella = 12.0317 Å,
b = 6.9269 Å,
c = 12.6712 Å, β = 90.27°; Z = 12
Identification
ColorWhite
Crystal habitMicroscopic acicular crystals in parallel aggregates, pseudohexagonal
TwinningTwin plane {101}
LusterEarthy
StreakWhite
DiaphaneitySemitransparent
Specific gravity2.69–2.76
Optical propertiesBiaxial (+)
Refractive indexnα = 1.550–1.559, nβ = 1.560, nγ = 1.577–1.584
2V angle10–15°
Alters toDehydrates on heating to anhydrite
References[2][3][4]

Bassanite is a calcium sulfate mineral with formula CaSO4·1/2H2O or 2CaSO4·H2O. In other words it has half a water per CaSO4 unit hence its synonym calcium sulfate hemihydrate.

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]

References[edit]

  1. ^ Warr, L.N. (2021). "IMA–CNMNC approved mineral symbols". Mineralogical Magazine. 85 (3): 291–320. Bibcode:2021MinM...85..291W. doi:10.1180/mgm.2021.43. S2CID 235729616.
  2. ^ a b Bassanite on Mindat.org
  3. ^ a b Bassanite in the Handbook of Mineralogy
  4. ^ Bassanite data on Webmineral
  5. ^ Schmidt, Horst; Paschke, Iris; Freyer, Daniela; Voigt, Wolfgang (1 December 2011). "Water channel structure of bassanite at high air humidity: crystal structure of CaSO 4 ·0.625H 2 O". Acta Crystallographica Section B Structural Science. 67 (6): 467–475. doi:10.1107/S0108768111041759. PMID 22101536.