1911 Encyclopædia Britannica/Alunite
ALUNITE, or Alumstone, a mineral first observed in the 15th century at Tolfa, near Rome, where it is mined for the manufacture of alum. Extensive deposits are also worked in Tuscany and Hungary, and at Bulladelah in New South Wales. By repeatedly roasting and lixiviating the mineral, alum is obtained in solution, and this is crystallized out by evaporation.
Alunite occurs as seams in trachytic and allied volcanic rocks, having been formed by the action of sulphureous vapours on these rocks. The white, finely granular masses somewhat resemble limestone in appearance, and the more compact kinds from Hungary are so hard and tough that they are used for millstones. Distinct crystals of alunite are rarely met with in cavities in the massive material; these are rhombohedra with interfacial angles of 90° 50′, so that they resemble cubes in appearance. Minute glistening crystals have also been found loose in cavities in altered rhyolite. The hardness is 4 and the specific gravity 2·6. The mineral is a hydrated basic aluminium and potassium sulphate, KAl3(SO4)2(OH)6. It is insoluble in water, but soluble in sulphuric acid. First called aluminilite by J. C. Delamétherie in 1797, this name was contracted by F. S. Beudant in 1824 to alunite. (L. J. S.)