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. <Gallery> File:Halotrichite Hydrous iron aluminum sulfate Corral, California 3009.jpg|Halotrichite from California File:Halotrichite-179634.jpg|Halotrichite from the abandoned Golden Queen mine on Soledad Mountain south of Mojave, California </Gallery>