Авторы: KUKKADAPU R., ZACHARA J., FREDRICKSON J.

2005 г.

Amer.Miner.

  An ~1:1 mixture of ferrihydrite and nanocrystalline akaganeite (β-FeOOH; 10–15 nm) was incubated with Shewanella putrefaciens (strain CN32) under anoxic conditions with lactate as an electron donor and anthraquinone-2,6-disulfonate (AQDS) as an electron shuttle. The incubation was carried out in a 1,4-piperazinediethanesulfonic acid (PIPES)-buffered medium, without PO4 3– at circumneutral pH. Iron reduction was measured as a function of time (as determined by 0.5 N HCl extraction), and solids were characterized by X-ray diffraction (XRD), electron microscopy, and Mössbauer spectroscopy. The biogenic reduction of Fe3+was rapid; with 60% of the total Fe (FeTOT) reduced in one day. Only an additional 10% of FeTOT was reduced over the next three years. A Þ ne-grained (~10 nm), cation-excess (CE) magnetite with an Fe2+/FeTOT ratio of 0.5–0.6 was the sole biogenic product after one day of incubation. The CE magnetite was unstable and partially transformed to micrometer-sized ferrous hydroxy carbonate [FHC; Fe2(OH)2CO3(s)], a rosasite-type mineral, with time. Ferrous hydroxy carbonate dominated the mineral composition of the three year incubated sample. The Fe2+/FeTOT ratio of the residual CE magnetite after three years of incubation was lower than the day 1 sample and was close to that of the stoichiometric magnetite (0.33). To the best of our knowledge, this is the Þ rst report of biogenic FHC, and was only reported twice in literature but in a very different context. Ferrous hydroxy carbonate appeared to form by slow reaction of microbially produced carbonate with Fe2+-excess magnetite. The FHC may be an overlooked mineral phase that explains the infrequent occurrence of Þ ne-grained, biogenic magnetite in anoxic sediments.

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