Ammonioalunite

Formula:  (NH₄)Al₃(SO₄)₂(OH)₆

Origin of name:  From composition

Crystal system:  Hexagonal

Crystal class:  3/m or -3 2/m

Twinning:

Unit cell: a = 7.013 Å, c = 17.885 Å

Colour:  Grayish-white

Diaphaneity: Transparent to translucent

Luster:  Vitreous

Habit: crystals covering matrix, massif, granular

Hardness: 2-3

Specific gravity:  2.4

Cleavage:  None

Tenacity:

Streak: White

Synonyms/varieties:

Comments: Formed in acidic hot springs with temperatures below 100°C and low potassium content of the waters.