AbstractDuring the oxidation of ferrous iron in solution by the iron‐oxidizing bacterium, Thiobacillus ferrooxidans, formation of jarosite [KFe3(SO4)2(OH)6] occurred in the presence of glauconite, illite and microcline whereas natrojarosite [NaFe3(SO4)2(OH)6] was formed in the presence of albite. The required K and Na were supplied by these minerals and considerable amounts (up to 12%) of jarosite were formed during four months of reaction. When NH4 ions were present in solution, K release from the minerals appeared to be blocked and ammoniojarosite [NH4Fe3(SO4)2(OH)6] was formed. In the absence of NH4 ions or K‐ and Na‐bearing minerals an amorphous precipitate formed. X‐ray diffraction and chemical data indicate that interlayer K was preferentially removed from glauconite which resulted in an expanding phase having a d(001) spacing of 18Å with Mg and glycerol saturation. There was little preferential removal of interlayer K from illite as most of the K was removed by approximately stoichiometric dissolution. The K from microcline and the Na from albite were also removed stoichiometrically during jarosite and natrojarosite formation. The K released for jarosite formation followed the order glauconite>illite>microcline. The availability of Na from albite was somewhat greater than the availability of K from microcline.The results show that in acid sulfate soils where ferrous iron, Thiobaccillus ferrooxidans and basic ferric sulfates are abundant, clay minerals can supply the alkali cations for the formation of these sulfates.