Halotrichite
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Halotrichite, also known as feather alum, is a highly hydrated sulfate of aluminium and iron. It is formed by the weathering and decomposition of pyrite commonly near or in volcanic vents. Its chemical formula is FeAl2(SO4)4⋅22H2O. It forms fibrous monoclinic crystals. The crystals are water soluble. Occurrences include the Atacama Desert, Chile, Dresden, Saxony, Germany, San Juan County, Utah, and Mont Saint-Hilaire, Canada.
The name is from Latin: halotrichum for salt hair.