What is Natroalunite?
Natroalunite is the sodium-rich alunite-group sulfate of acidic volcanic and hydrothermal alteration systems.
Mineralogy and crystal structure
Reference formula: NaAl3(SO4)2(OH)6. Natroalunite has a trigonal alunite-group structure and forms a compositional series with alunite through sodium-potassium substitution. Fine-grained material may require diffraction or chemical analysis. Natroalunite is classified with sulfates and has a trigonal crystal system. Typical hardness is 3.5–4 on the Mohs scale and specific gravity is approximately 2.7–2.9.
Colour, lustre and habit
Typical colours include white, yellow, grey, pink. The usual rhombohedral, granular, crusty habit and vitreous, pearly, dull lustre help describe a specimen, but colour alone is not diagnostic.
Formation and notable localities
Natroalunite forms in or is associated with advanced argillic alteration, acidic volcanic rocks, hot-spring deposits and weathered sulfide systems. Important documented localities include Japan, the United States, Chile, Peru, Australia and Italy.
A pale sulfate from an acid-alteration zone
Natroalunite is a quiet but powerful field indicator. Its pale grains encode acidic fluids that have transformed volcanic or ore-bearing rock near the surface.
Collecting and care
Protect from acids and abrasion; keep dry and labelled with the alteration assemblage.
