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Published August 2001 | Submitted
Journal Article Open

Theoretical estimates of equilibrium Fe-isotope fractionations from vibrational spectroscopy


The magnitude and direction of equilibrium iron-isotope (^(54)Fe–^(56)Fe) fractionations among simple iron-bearing complexes and α-Fe metal are calculated using a combination of force-field modeling and existing infrared, Raman, and inelastic neutron scattering measurements of vibrational frequencies. Fractionations of up to several per mil are predicted between complexes in which iron is bonded to different ligands (i.e. 4 per mil for [Fe(H_(2)O)_6]^3+ vs. [FeCl_4]^− at 25°C). Similar fractionations are predicted between the different oxidation states of iron. The heavy iron isotopes will be concentrated in complexes with high-frequency metal-ligand stretching vibrations, which means that ^(56)Fe/^(54)Fe will be higher in complexes with strongly bonding ligands such as CN− and H2O relative to complexes with weakly bonding ligands like Cl^− and Br^−. 56Fe/54Fe will also usually be higher in Fe(III) compounds than in Fe(II)-bearing species; the Fe(II) and Fe(III) hexacyano complexes are exceptions to this rule of thumb. Heavy iron isotopes will be concentrated in sites of 4-fold coordination relative to 6-fold coordination. Model results for a ferrous hexacyanide complex, [Fe(CN)_6]^4−, are in agreement with predictions based on Mössbauer spectra (Polyakov, 1997), suggesting that both approaches give reasonable estimates of iron-isotope partitioning behavior.

Additional Information

© 2001 Elsevier Science Ltd. Received June 15, 2000; accepted in revised form February 22, 2001. We thank Ariel Anbar, Robert Housley, and Alan Matthews for encouragement and timely advice, and Elizabeth Johnson, John Eiler, Beth Holman, Ronit Kessel, Clark Johnson, Ariel Anbar, and an anonymous referee for reviewing the manuscript. K. W. Hipps kindly helped estimate the errors in measured vibrational frequencies for K_4Fe(CN)_6 • 3H_2O. This research was funded by the National Science Foundation, grant nos. EAR-9725811 to EAS and HPT, and EAR-9804871 to GRR. Associate editor: D. R. Cole

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Submitted - Theoretical_estimates_of_equilibrium_Fe-isotope_fractionations_from_vibrational_spectroscopy.pdf



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