Alunite
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Alunite is a hydroxylated aluminium potassium sulfate mineral, formula KAl₃(SO₄)₂(OH)₆. It was first observed in the 15th century at Tolfa, near Rome, where it was mined for the manufacture of alum. Wikipedia
Cleavage: On, perfect
Color: Yellow, red, to reddish brown, colorless if pure; may be white, pale shades of gray
Crystal class: Hexagonal scalenohedral (3m); Space group: (3 2/m)
Crystal habit: fibrous to columnar, porcelaneous, commonly granular to dense massive
Crystal system: Trigonal
Formula (repeating unit): KAl3(SO4)2(OH)6
IMA symbol: Alu