What is Jarosite?
Jarosite is a yellow to ochre iron sulfate of oxidised sulfide deposits, famous for its weathering story and for its occurrence in terrestrial and Martian environments.
Mineralogy and crystal structure
Reference formula: KFe3(SO4)2(OH)6. Jarosite has a trigonal alunite-group structure with potassium in large structural sites. Sodium, hydronium and other substitutions produce related jarosite-group compositions, so locality and chemistry help define the specimen. Jarosite is classified with sulfates and has a trigonal crystal system. Typical hardness is 2.5–3.5 on the Mohs scale and specific gravity is approximately 2.9–3.3.
Colour, lustre and habit
Typical colours include yellow, ochre, brown, greenish-yellow. The usual rhombohedral, crusty, earthy habit and vitreous, resinous, dull lustre help describe a specimen, but colour alone is not diagnostic.
Formation and notable localities
Jarosite forms in or is associated with acid sulfate weathering zones, mine dumps, hot springs and evaporitic settings. Important documented localities include Spain, Portugal, the United States, Mexico, Australia and South Africa.
A golden surface with a planetary story
Jarosite's ochre colour and earthy crusts make it a strong teaching specimen for oxidation, acid-sulfate weathering and mineral formation in extreme environments. Its association with Mars research gives even a small terrestrial sample an unusually broad scientific story.
Ochre, time and landscape
Ochre minerals have long been used as pigments and are often assigned earth-centred meanings in modern symbolism. For jarosite, the strongest story remains geological: iron, sulfate, water and acidity working together at the surface.
Collecting and care
Soft and acid-sensitive; keep dry, avoid dust and do not use acidic cleaners. Treat unknown mine material as potentially contaminated.
