jarosite

noun

1. a yellowish or brownish mineral, a hydrous sulfate of potassium and iron, KFe3(SO4)2(OH)6, occurring in small crystals or large masses.

noun

1. a yellow to brown secondary mineral consisting of basic hydrated sulphate of iron and potassium in masses or hexagonal crystals. Formula: KFe 3 (SO 4) 2 (OH) 6