Alteration of microbially precipitated iron oxides and hydroxides

Iron oxide and hydroxides can be precipitated from solution with both Fe2and Fe3+ states by a microbial consortium enriched from surface water drain ing a granitic batholith. The Fe2+/Fe3+ ratio of the microbial precipitate is determined by both the initial environment and subsequent diagenesis. To evaluate the thermal aspects of diagenesis, biological precipitates, eithe r largely Fe2+ or equally divided between Fe2+ and Fe3+ states, were heated at 80 degrees C for 12 weeks, under various redox conditions and compared to samples maintained under the same conditions at 4 degrees C. Mossbauer s pectroscopy showed the iron oxide and hydroxides precipitated as Fe2+ to be more stable than that as Fe3+. Only under air at 80 degrees C are the ferr ous minerals altered to hematite, while the more labile ferric minerals are altered to Fe(OH)(2) at 4 degrees C and to hematite at 80 degrees C. In co ntrast, chemically precipitated Fe compounds, when incubated with the conso rtium, only form Fe3+ compounds, mainly fine-grained hematite. When no micr obes are present, goethite is formed during diagenesis. Fe speciation in se diments may reflect a combination of microbial mediation that causes the in itial precipitation of iron oxides and hydroxides and the subsequent condit ions of the diagenetic processes characteristic of that particular depositi onal environment.