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a mineral of the sulfate class, with chemical composition KFe3(SO4)2(OH)6. Jarosite, which usually contains admixtures of sodium, crystallizes in the trigonal system. It usually occurs in the form of ocher yellow or yellowish brown earthy and fine-grained masses; sometimes it is found in the form of small orthorhombic and pseudoisometric crystals, with a strong vitreous or adamantine luster. It exhibits pyroelectric properties and very high double refraction. It has a hardness of 2.5–3.5 on Mohs’ scale and a density of 3,150–3,260 kg/m3.
Jarosite is formed in the oxidation zone of ore deposits owing to the presence of iron sulfides, such as pyrite and pyrrhotite, mainly under dry climatic conditions. When there is substantial humidity, it decomposes with the formation of iron hydroxides. It is also precipitated from acidic sulfate waters in regions of active volcanoes. Jarosite occurs in association with gypsum, native sulfur, goethite, and native gold. Jarosite deposits in the USSR are located in the southern Ural Mountains, in Kazakhstan, and in the Altai Mountains; other deposits are found in Spain, Czechoslovakia, and France. Pure varieties of jarosite are roasted to yield polishing abrasives with the composition Fe2O3 (called jeweler’s rouge).
A. M. PORTNOV