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 9o° 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)5 . It is insoluble in water, but soluble in sulphuric acid . First called aluminilite by J . C .

Delametherie in 1797, this name was contracted by F . S .

Beudant in 1824 to alunite . (L . J .