What is Voltaite?
Voltaite is a hydrated cubic iron-aluminium sulfate that forms dark crystals and crusts in strongly oxidised, sulfate-rich environments.
Mineralogy and structure
Reference formula: K2Fe5(Fe,Al)(SO4)12·18H2O. Voltaite belongs to a compositionally variable sulfate group; iron, aluminium and magnesium occupancy can change. COD 9009569 is used as a labelled representative model rather than a universal end member. Voltaite is classified with sulfates and has cubic crystal symmetry. Typical hardness is 2.5–3 on the Mohs scale and specific gravity is approximately 2.7–2.9.
Colour, lustre and habit
Typical colours include black, greenish-black, brown, yellow-brown. The usual cubic, granular, crusty habit and vitreous, resinous, dull lustre help describe a specimen, but colour alone is not diagnostic.
Formation and notable localities
Voltaite forms in or is associated with acid sulfate zones, fumarolic settings, evaporitic deposits and weathering of sulfide ores. Important documented localities include Italy, Greece, Spain, Chile, the United States and Russia.
Dark cubes from an acid-sulfate environment
Voltaite is a mineralogical snapshot of aggressive oxidation: dark cubic forms can crystallise where sulfate-rich fluids concentrate iron and potassium. Freshness and locality carry the commercial story.
Collecting and care
Hydrated and acid-sensitive: keep dry, avoid water and acids, and handle friable material as a protected sulfate specimen.
