alunite

Alunite is a potassium aluminum sulfate mineral with the formula KAl3(SO4)2(OH)6. It is an alum mineral and forms in the oxidation and hydrothermal alteration zones of alumina-bearing rocks, especially felsic volcanic rocks and intrusive bodies. It commonly crystallizes from acidic, sulfate-rich fluids that alter feldspars and other aluminous minerals. Alunite is found in veins, disseminated crusts, and as tabular or platy crystals, and may occur as radiating aggregates or earthy masses. Colors range from white and colorless to yellow, pink, or red due to trace impurities; transparent crystals are rare.

In mineral associations, alunite often occurs with jarosite, natroalunite, goethite, quartz, and other sulfate or oxide minerals. It can be an indicator of advanced argillic alteration in hydrothermal systems and is studied in exploration for porphyry and epithermal deposits. Alunite is also used in geochronology; it contains potassium and can be dated using the Argon-Argon dating method to determine ages of hydrothermal events.

Historically, alunite has been the main source of alum, potassium aluminum sulfate, used for tanning, dyeing, and water purification, though today alum production more commonly uses other sources. The name derives from alumen, the Latin word for alum, reflecting its relation to alum and alumina-bearing minerals.