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Cation site occupancy of biogenic magnetite compared to polygenic ferrite spinels determined by X-ray magnetic circular dichroism

¹ School of Earth, Atmospheric & Environmental Sciences & Williamson Research Centre for Molecular Environmental Science, University of Manchester, Manchester, M13 9PL, UK
² Magnetic Spectroscopy Group, Daresbury Laboratory, Warrington WA4 4AD, UK
³ Max-Planck Institute for Marine Microbiology, Celsiusstr. 1, 28359 Bremen, Germany
⁴ Advanced Light Source, Lawrence Berkeley National Laboratory, Berkeley, CA 94720, USA

^* Corresponding author, e-mail: vicky.coker{at}manchester.ac.uk

Ferrite spinels, especially magnetite (Fe₃O₄), can be formed either by geological, biological or chemical processes leading to chemically similar phases that show different physical characteristics. We compare, for the first time, magnetite produced by these three different methods using X-ray magnetic circular dichroism (XMCD), a synchrotron radiation based technique able to determine the site occupancy of Fe cations in the ferrite spinels. Extracellular nanoscale magnetite produced by different Fe(III)-reducing bacteria was shown to have different degrees of stoichiometry depending on the bacteria and the method of formation, but all were oxygen deficient due to formation under anoxic conditions. Intracellular nano-magnetite synthesized in the magnetosomes of magnetotactic bacteria was found to have a Fe cation site occupancy ratio most similar to stoichiometric magnetite, possibly due to the tight physiological controls exerted by the magnetosome membrane. Chemically-synthesised nano-magnetite and bulk magnetite produced as a result of geological processes were both found to be cation deficient with a composition between magnetite and maghemite (oxidised magnetite).

Key-words: X-ray magnetic circular dichroism, XMCD, nanoparticles, metal reduction, Fe(III) reduction, Geobacter, biomineralogy, biogenic magnetite.

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