Alunite, or alumstone, is a mineral that was first observed in the 15th century in Monti della Tolfa, north to Rome, where it was mined for the manufacture of alum. First called aluminilite by J.C. Delametherie in 1707, this name was contracted by François Beudant in 1824 to alunite.
Distinct crystals of alunite are rarely found in cavities in the massive material. Alunite crystallizes in the hexagonal system with crystals forming trigonal pyramids that resemble 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. Alunite varies in color from white to yellow gray. The hardness is 4 and the specific gravity is between 2.6 and 2.8. The mineral is a hydrated aluminium potassium sulfate, KAl3(SO4)2(OH)6. Sodium substitutes for potassium and when high in sodium it is called natroalunite. It is insoluble in water or weak acids, but soluble in sulfuric acid. Jarosite is an iron analogue in which Fe3+ replaces the aluminium. It occurs as a secondary mineral on iron sulfate ores.
Alunite occurs as veins and replacement masses in trachyte, rhyolite, and similar potassium rich volcanic rocks It is formed by the action of sulfuric acid bearing solutions on these rocks during the oxidation and leaching of metal sulfide deposits. Alunite also is found near volcanic fumaroles. The white, finely granular masses closely resemble finely granular limestone, dolomite, anhydrite, and magnesite in appearance. The more compact kinds from Hungary are so hard and tough that they have been used for millstones.
Historically extensive deposits were mined in Tuscany and Hungary, and at Bulladelah in New South Wales. Currently found at Tolfa, Italy. In the United States it is found in the San Juan district of Colorado; Goldfield, Nevada; Marysvale, Utah; and Red Mountain in Patagonia, Arizona. The Arizona occurrence lies appropriately above a canyon named Alum Gulch. Alunite is mined as an ore of both potassium and aluminium at Marysvale. Some of the ore deposits were located by airborne and satellite multispectral imaging.