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Formula: KAl3(SO4)2(OH)6
Anhydrous sulphate containing hydroxyl, alunite group,
forms a series with jarosite
Crystal System: Trigonal
Specific gravity: 2.6 to 2.9 measured, 2.82 calculated
Hardness: 3½ to 4
Streak: White
Colour: White, pale shades of gray, yellow, red, to reddish brown
Solubility: Insoluble in water.
Common impurities: Na,Fe
Environments:
Sedimentary environments
Hydrothermal environments
Alunite is formed between 15oC and 400oC by the action of sulphate, which may be generated from
pyrite or volcanic activity characterised by the emission gases and vapors, on
aluminous rocks. Associated minerals include kaolinite,
halloysite, diaspore,
pyrite, gypsum and
quartz
(HOM Mindat).
Localities
In the Mount Isa region of northwest Queensland, Australia, minerals of the
alunite-jarosite family occur in
gossans (iron-bearing weathered products overlying sulphide deposits, formed by the oxidation of sulphides and the leaching-out
of the sulphur and most metals, leaving hydrated iron oxides and rarely sulphates) (Mindat) related to
lead-zinc mineralisation
(AM72.178-187).
At the Mount Deverell variscite deposit, Milgun Station, Western Australia,
alunite
occurs in thin veins and coatings on fracture surfaces; It also forms pseudomorphs
of pyrite in
variscite. The variscite deposits are hosted by
marine sedimentary rocks
(AJM 20.2.24).
At Grainsgill, Caldbeck Fells, Cumbria, England, UK, alunite occurs as powdery coatings with
jarosite on quartz
(C&S).
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