Sulfates: Alunite

Composition: KAl₃(SO₄)₂(OH)₆; sodium commonly substitutes for potassium

Crystal system: Trigonal; usually massive

6.1.1
alunite

Properties:

• Vitreous to silky lustre
• Variable color; white cream, grey, reddish
• White streak
• Knife scratches very easily; fingernail does not scratch (Mohs Hardness 3.5-4)
• Average density
• Cleavage rarely apparent
• Smooth to uneven fracture
• Not ferromagnetic
• Does not react with hydrochloric acid (HCl); soluble in sulfuric acid

Occurrence:

Alunite forms by alteration of volcanic rocks around some hydrothermal ore deposits.

6.1.2 alunite	6.1.3 alunite

Alunite is a local source of aluminium.