Halotrichite, also known as feather alum, is a highly hydrated sulfate of aluminium. 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; Dresden ; San Juan County, Utah and Mont Saint-Hilaire.
The name is from Latin : halotrichum for salt hair which accurately describes the precipitate Gallery * Halotrichite from California * Halotrichite from the abandoned Golden Queen mine on Soledad Mountain south of Mojave, California