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Mineraly.sk - halotrichit.jpg
A sample of Halotrichite
Category Sulfate minerals
(repeating unit)
Strunz classification 07.CB.85
Crystal system Monoclinic
Unit cell a = 20.51 Å, b = 24.29 Å, c = 6.18 Å; β = 100.99°; Z=4
Color Colorless to white, yellowish, greenish
Crystal habit Acicular to asbestiform clusters, incrustations and efflorescences
Crystal symmetry Monoclinic sphenoidal
H-M symbol: (2)
Space group: P 2
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 [1][2][3]

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.[3]

Halotrichite from California
Halotrichite from the abandoned Golden Queen mine on Soledad Mountain south of Mojave, California