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Mineraly.sk - halotrichit.jpg
A sample of Halotrichite
Category Sulfate minerals
(repeating unit)
Strunz classification 07.CB.85
Crystal symmetry Monoclinic sphenoidal
H-M symbol: (2)
Space group: P 2
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 system Monoclinic
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. Occurrences include the Atacama Desert, Chile, Dresden, Saxony, Germany, San Juan County, Utah, Iceland, and Mont Saint-Hilaire, Canada.

The name is from Latin: halotrichum for salt hair.[3]

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