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From Wikipedia, the free encyclopedia
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.
Halotrichite | |
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![]() A sample of halotrichite | |
General | |
Category | Sulfate minerals |
Formula (repeating unit) | FeAl2(SO4)4·22H2O |
IMA symbol | Hth[1] |
Strunz classification | 7.CB.85 |
Crystal system | Monoclinic |
Crystal class | Prismatic (2m) (same H-M symbol) |
Space group | P21/c |
Unit cell | a = 20.51, b = 24.29 c = 6.18 [Å]; β = 100.99°; Z = 4 |
Identification | |
Color | Colorless to white, yellowish, greenish |
Crystal habit | Acicular to asbestiform clusters, incrustations and efflorescences |
Cleavage | Poor on {010} |
Fracture | Conchoidal |
Tenacity | Brittle |
Mohs scale hardness | 1.5–2 |
Luster | Vitreous |
Diaphaneity | Transparent, translucent |
Specific gravity | 1.89 |
Optical properties | Biaxial (−) |
Refractive index | nα = 1.480 nβ = 1.486 nγ = 1.490 |
Birefringence | δ = 0.010 |
2V angle | Measured: 35° |
Solubility | Soluble in water |
Other characteristics | Astringent taste |
References | [2][3][4] |
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]
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