Halotrichite

Halotrichite
A sample of halotrichite
General
CategorySulfate minerals
Formula
(repeating unit)		FeAl2(SO4)4·22H2O
IMA symbolHth[1]
Strunz classification7.CB.85
Crystal systemMonoclinic
Crystal classPrismatic (2m)
(same H-M symbol)
Space groupP21/c
Unit cella = 20.51, b = 24.29
c = 6.18 [Å]; β = 100.99°; Z = 4
Identification
ColorColorless to white, yellowish, greenish
Crystal habitAcicular to asbestiform clusters, incrustations and efflorescences
CleavagePoor on {010}
FractureConchoidal
TenacityBrittle
Mohs scale hardness1.5–2
LusterVitreous
DiaphaneityTransparent, translucent
Specific gravity1.89
Optical propertiesBiaxial (−)
Refractive indexnα = 1.480 nβ = 1.486 nγ = 1.490
Birefringenceδ = 0.010
2V angleMeasured: 35°
SolubilitySoluble in water
Other characteristicsAstringent taste
References[2][3][4]

Halotrichite, also known as feather alum, is a highly hydrated sulfate of aluminium and iron. Its chemical formula is FeAl2(SO4)4·22H2O. It forms fibrous monoclinic crystals. The crystals are water-soluble.

It is formed by the weathering and decomposition of pyrite commonly near or in volcanic vents. The locations of natural occurrences include: the Atacama Desert, Chile; Dresden in Saxony, Germany; San Juan County, Utah; Iceland and Mont Saint-Hilaire, Canada.

The name is from Latin: halotrichum for salt hair which accurately describes the precipitate/evaporite mineral.[4]

Gallery

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. ^ Handbook of Mineralogy
  3. ^ Halotrichite on Mindat.org
  4. ^ a b Halotrichite data on Webmineral


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