Alunite

Formula:  KAl₃(SO₄)₂(OH)₆

Origin of name:  From Latin almen - alum

Crystal system:  Hexagonal

Crystal class:  -3 2/m

Twinning:

Unit cell: a = 6.97 Å, c = 17.38 Å

Colour:  White, often discoloured grayish, yellowish, reddish, brownish

Diaphaneity: Transparent to nearly opaque

Luster:  Vitreous, pearly on base

Habit:  Crystals rhombohedral, often pseudocubic; as druses or aggregates; usually massive; fibrous

Hardness:  3.5 to 4

Specific gravity:  2.6 to 2.7

Cleavage:  2; {0001} distinct, {01-12} in traces

Tenacity:

Streak:  White

Synonyms/varieties:

Comments: Rock-forming mineral where acid, often ore-bearing, have altered orthoclase feldspar-rich rocks.