bassanite

{{Short description|Calcium sulfate mineral}}

{{distinguish|basanite|Bassanitae}}

{{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·{{sfrac|1|2}}H₂O

|IMAsymbol=Bss{{Cite journal|last=Warr|first=L.N.|date=2021|title=IMA–CNMNC approved mineral symbols|journal=Mineralogical Magazine|volume=85|issue=3|pages=291–320|doi=10.1180/mgm.2021.43|bibcode=2021MinM...85..291W|s2cid=235729616|doi-access=free}}

| 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 = n_α = 1.550–1.559, n_β = 1.560,

n_γ = 1.577–1.584

| birefringence =

| pleochroism =

| 2V = 10–15°

| dispersion =

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| length fast/slow =

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| melt =

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| impurities =

| alteration = Dehydrates on heating to anhydrite

| other =

| references = [http://www.mindat.org/min-557.html Bassanite on Mindat.org][http://rruff.info/doclib/hom/bassanite.pdf Bassanite in the Handbook of Mineralogy][http://www.webmineral.com/data/Bassanite.shtml#.UsTP6tLjW1U Bassanite data on Webmineral]

}}

Bassanite is a calcium sulfate mineral with formula CaSO₄·water of crystallization 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 Mount Vesuvius. It was named for Italian paleontologist Francesco Bassani (1853–1916).

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.

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.{{cite journal |last1=Schmidt |first1=Horst |last2=Paschke |first2=Iris |last3=Freyer |first3=Daniela |last4=Voigt |first4=Wolfgang |title=Water channel structure of bassanite at high air humidity: crystal structure of CaSO 4 ·0.625H 2 O |journal=Acta Crystallographica Section B: Structural Science |date=1 December 2011 |volume=67 |issue=6 |pages=467–475 |doi=10.1107/S0108768111041759|pmid=22101536 }}