Pale brown crystal aggregates of the rare mineral ammonioalunite from Kladno (Kladno District, Central Bohemian Region, Czech Republic). — Photo: David Hospital · CC BY-SA 4.0
Ammonioalunite
Ammonioalunite is an IMA-recognized mineral species in the sulfate class, with the chemical formula (NH₄)Al₃(SO₄)₂(OH)₆.
It crystallizes in the hexagonal crystal system (space group R-3m). It is a member of the Alunite-Alunite structural group.
Key properties: Mohs hardness 2–3; specific gravity 2.4; White streak; Vitreous luster; optically uniaxial.
IMA symbol Aalu, IMA status: approved, named in 1986.
Formed in hot springs under very acid conditions, rich in ammonium and sulfate, poor in potassium, below 100 ° C.
It is found associated with Ammoniojarosite, amorphous SiO2.
As the ammonium analog of alunite.