Natroalunite as a vein in quartzite. Castillo hill, Nogueras, Teruel, España — Photo: Bergminer · CC BY-SA 4.0
Natroalunite
The sodium analogue of alunite — a hydrated sodium-aluminium sulfate of the alunite group.
Intro adapted from Wikipedia (CC BY-SA).
Natroalunite is an IMA-recognized mineral species in the sulfate class, with the chemical formula NaAl₃(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 3.5–4; specific gravity 2.75–2.83; White streak; Vitreous to pearly on {0001} luster; optically uniaxial.
IMA symbol Nalu, IMA status: approved, named in 1902.
Less common than alunite, formed by solfataric or hydrothermal sulfate-bearing solutions reacting with clays, rarely with sillimanite; may be in laterites and as an authigenic sedimentary mineral.
It is found associated with Alunite, kaolinite, halloysite, sillimanite, quartz.
As the sodium, natrium, analog of alunite.