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Alunite - USGS Mineral Specimens 015.jpg
Alunite from Utah - USGS
Category Sulfate minerals
(repeating unit)
Strunz classification 7.BC.10
Crystal system Trigonal
Crystal class Hexagonal scalenohedral (3m)
Space group: (3 2/m)
Space group R3m
Unit cell a = 6.98, c = 17.32 [Å]; Z = 3
Color Yellow, red, to reddish brown, colorless if pure; may be white, pale shades of gray
Crystal habit fibrous to columnar, porcelaneous, commonly granular to dense massive
Cleavage On {0001}, perfect
Fracture Conchoidal
Tenacity Brittle
Mohs scale hardness 3.5 – 4
Luster Vitreous, somewhat pearly on {0001} for crystals, earthy if massive
Streak White
Diaphaneity Transparent to translucent
Specific gravity 2.6 – 2.9
Optical properties Uniaxial (+)
Refractive index nω = 1.572 nε = 1.592
Birefringence δ = 0.020
Other characteristics Strongly pyroelectric
References [1][2][3][4][5]

Alunite is a hydrated aluminium potassium sulfate mineral, formula KAl3(SO4)2(OH)6. It was first observed in the 15th century at Tolfa, near Rome, where it is mined for the manufacture of alum. First called aluminilite by J.C. Delamétherie in 1797, this name was contracted by François Beudant three decades later to alunite.[6]

Alunite crystals morphologically are rhombohedra with interfacial angles of 90° 50', causing them to resemble cubes. Crystal symmetry is trigonal. 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. It is insoluble in water or weak acids, but soluble in sulfuric acid.[6]

Sodium can substitute for potassium in the mineral, and when the sodium content is high, is called natroalunite.

Alunite is an analog of Jarosite, where aluminium replaces Fe3+. Alunite occurs as a secondary mineral on iron sulfate ores.

Alunite from Slovakia
Alunite Marysvale, Utah

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.[6]

Historically extensive deposits were mined in Tuscany and Hungary, and at Bulahdelah, New South Wales, Australia.[6] It is currently mined at Tolfa, Italy. In the United States it is found in the San Juan district of Colorado; Goldfield, Nevada; the ghost town of Alunite, Utah near Marysvale; and Red Mountain near 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.[4] Some of the ore deposits were located by airborne and satellite multispectral imaging.

Crystal structure of alunite

See also[edit]


  1. ^ Handbook of Mineralogy
  2. ^
  3. ^ Webmineral data
  4. ^ a b Hurlbut, Cornelius S.; Cornelis Klein (1985). Manual of Mineralogy (20th ed.). ISBN 0-471-80580-7. 
  5. ^ Mineralienatlas
  6. ^ a b c d  One or more of the preceding sentences incorporates text from a publication now in the public domainSpencer, Leonard James (1911). "Alunite". In Chisholm, Hugh. Encyclopædia Britannica. 1 (11th ed.). Cambridge University Press. p. 773. 
  • Hurlbut, Cornelius S. (1966). Dana's Manual of Mineralogy (17th ed.). ISBN 0-471-03288-3. 

External links[edit]