FORMATION OF MACKINAWITE BY MICROBIAL REDUCTION OF JAROSITE AND ITS APPLICATION TO TIDAL SEDIMENTS

A study was made of the microbial reduction of jarosite (KFe3(SO4)2(OH)6), a mineral formed by the aerobic iron-oxidizing bacterium Thiobacillus ferrooxidans, and frequently found in acid sulfate soils developed on marine sediments containing pyrite (FeS2). Under anaerobic conditions and in the presence of organic matter (lactate) and the sulfate-reducing bacterium Desulfovibrio desulfuricans, jarosite was soon reduced to a sulfide, a poorly crystalline form of mackinawite (FeS). Due to the presence of phosphate ions in the cultural medium, vivianite or its polymorph metavivianite (Fe3(PO4)2.cntdot.8H2O) was formed also. Because aging transforms mackinawite into pyrite, it is suggested that in marshy areas where pyritic sediments and acid sulfate soils (cat clays) occur, and where aerobic and anaerobic conditions exist, there is a generic relationship between pyrite and jarosite, and the 2 microbes help to maintain this relationship by cycling S and Fe between the 2 minerals. At the same time they perhaps aid in cycling P.