Equilibrium iron isotope fractionation factors of minerals: Reevaluation from the data of nuclear inelastic resonant X-ray scattering and Mossbauer spectroscopy

We have critically reevaluated equilibrium iron isotope fractionation factors for oxide and sulfide minerals using recently acquired data obtained by Mossbauer spectroscopy and inelastic nuclear resonant X-ray scattering (INRXS) synchrotron radiation. Good agreement was observed in the iron beta-factors of metallic iron (alpha-Fe) and hematite calculated using both Mossbauer- and INRXS-derived data, which supports the validity and reliability of the calculations. Based on this excellent agreement, we suggest the use of the present data on the iron beta-factors of hematite as a reference. The previous Mossbauer-derived iron beta-factor for magnetite has been modified significantly based on the Fe-sublattice density of states obtained from the INRXS experiments. This resolves the disagreement between naturally observed iron isotope fractionation factors for mineral pairs involving magnetite and those obtained from the calculated -factors. The correctness of iron beta-factor for pyrite has been corroborated by the good agreement with experimental data of sulfur isotope geothermometers of pyrite-galena and pyrite-sphalerite. A good correlation between the potential energy of the cation site, the oxidation state of iron and the iron beta-factor value has been established. Specifically, ferric compounds, which have a higher potential energy of iron than ferrous compounds, have higher beta-factors. A similar dependence of beta-factors on the oxidation state and potential energy could be extended to other transition metals. Extremely low values of INRXS-derived iron beta-factors for troilite and Fe3S significantly widen the range of iron beta-factors for covalently bonded compounds. (c) 2007 Elsevier Ltd. All rights reserved.