Alunite(redirected from Alum stone)
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a mineral of the complex sulfate group, with extra (OH)- anions. The chemical composition KA13[SO4]2(OH)6 sometimes includes admixtures of Na+ and Fe3+. Alunite crystallizes very rarely in a trigonal system. The mineral is mostly found in the form of fine-grained or friable earthy, sometimes fibrous, aggregates. The color is white, with grayish, yellowish, and reddish tints. The hardness of alunite on the mineralogical scale is 3.7–4.5; its density is 2,600–2,800 kg/m3. Alunite is formed by reworking acid and neutral volcanic rocks that contain orthoclase with hydrothermal sulfuric acid solutions and also by the action of sulfate surface water on aluminiferous rocks. Rocks containing alunite are a source of alum, alumina, potassium salts, and other substances.
A. B. PAVLOVSKII