Bassanite

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Bassanite
Bassanite-192766.jpg
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
ColorWhite
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 propertiesBiaxial (+)
Refractive index nα = 1.550–1.559, nβ = 1.560, nγ = 1.577–1.584
2V angle 10–15°
Alters toDehydrates on heating to anhydrite
References [2] [3] [4]

Bassanite is a sulfate mineral composed of calcium sulfate hemihydrate, with the chemical formula CaSO4·0.5H2O or 2CaSO4·H2O. 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). [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

  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. 1 2 Bassanite on Mindat.org
  3. 1 2 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.