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Iron oxide and hydroxides can be precipitated from solution with both Fe 2+  and Fe 3+  states by a microbial consortium enriched from surface water draining a granitic batholith. The Fe 2+ /Fe 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 2+  or equally divided between Fe 2+  and Fe 3+  states, were heated at 80 °C for 12 weeks, under various redox conditions and compared to samples maintained under the same conditions at 4 °C . Mössbauer spectroscopy showed the iron oxide and hydroxides precipitated as Fe 2+  to be more stable than that as Fe 3+ . Only under air at 80 °C are the ferrous minerals altered to hematite, while the more labile ferric minerals are altered to Fe(OH) 2  at 4 °C and to hematite at 80 °C . In contrast, chemically precipitated Fe compounds, when incubated with the consortium, only form Fe 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.

Alteration of microbially precipitated iron oxides and hydroxides