Halotrichite

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
A sample of halotrichite
General
Category	Sulfate minerals
Formula	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; Idrija, Slovenia; and Mont Saint-Hilaire, Canada.

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

Gallery

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