Alunite

sulphate

hydrothermal

Formula: KAl3(SO4)2(OH)6
Anhydrous sulphate containing hydroxyl
Forms a series with jarosite
Specific gravity: 2.6 to 2.9
Hardness: 3½ to 4
Streak: White
Colour: White, pale shades of gray, yellow, red, to reddish brown
Solubility: Insoluble in water.
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).

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) related to lead-zinc mineralisation (AM72.178-187).

Common impurities: Na,Fe

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