Iron oxide and hydroxides can be precipitated from solution with both Fe (super 2+) and Fe (super 3+) states by a microbial consortium enriched from surface water draining a granitic batholith. The Fe (super 2+) /Fe (super 3+) ratio of the microbial precipitate is determined by both the initial environment and subsequent diagenesis. To evaluate the thermal aspects of diagenesis, biological precipitates, either largely Fe (super 2+) or equally divided between Fe (super 2+) and Fe (super 3+) 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 spectroscopy showed the iron oxide and hydroxides precipitated as Fe (super 2+) to be more stable than that as Fe (super 3+) . Only under air at 80 degrees C are the ferrous 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 contrast, chemically precipitated Fe compounds, when incubated with the consortium, only form Fe (super 3+) compounds, mainly fine-grained hematite. When no microbes are present, goethite is formed during diagenesis. Fe speciation in sediments may reflect a combination of microbial mediation that causes the initial precipitation of iron oxides and hydroxides and the subsequent conditions of the diagenetic processes characteristic of that particular depositional environment.