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The oxidation state of iron in silicate melts

A.J. Berry, H.StC. O'Neill, J.M.G. Shelley, G.J. Foran¹, K.D. Jayasuriya² and S. Campbell^{2 1} ANSTO
² School of Physics, Australian Defence Force Academy

Although iron oxidation states can be accurately determined by Mössbauer spectroscopy, the technique does not have the spatial resolution important for many geological problems. X-ray absorption near edge structure (XANES) spectroscopy can be used to determine and quantify iron oxidation states with micron resolution. We have recorded Mössbauer and XANES spectra for a glass of anorthite-diopside eutectic composition containing 0-7 wt% ⁵⁷Fe prepared over a range of oxygen fugacities. The Mössbauer results confirm that the iron oxidation state ratio is related to the oxygen fugacity by a factor of 0.25 (log[Fe³⁺/Fe²⁺] µ 0.25logfO₂) as expected from the reaction FeO + 1/4O₂ = 1/2Fe₂O₃. This result is in agreement with theory but differs from a literature value of 0.2185 determined empirically for natural compositions. The discrepancy is attributed to non-Henry's Law behaviour at high Fe₂O₃ concentrations. The energy centroid of the iron 1s-3d transition in the XANES spectrum was found to be linearly correlated with the oxidation state determined by Mössbauer spectroscopy. This provides a calibration for extracting oxidation state ratios for a silicate glass. Further work will determine the generality of this calibration for a range of glass compositions.