Jarosite is a potassium iron sulfate mineral of composition KFe3(SO4)2(OH)6 usually derived from the oxidation of pyrite and other Fe-bearing sulfides. Sulfide oxidation produces sulfate plus hydronium ions during the hydrolysis of water.

Jarosite typically occurs as a yellowish mass, usually in combination with other sulfates, notably alunite [KAl3(SO4)2(OH)6] and natrojarosite [NaFe3(SO4)2(OH)6]

Importance in Soil Environments

Jarosite is a typical indicator of oxidation of sulfides, and is diagnostic in the identification of sulfuric horizons, which have “jarosite mottles” and pH < 3.5.

Menchetti S., and C. Sabelli. 1976. Neues Jb. Miner. Mh. H.9:406-417