Iron isotope fractionation between pyrite (FeS2), hematite (Fe2O3) and siderite (FeCO3): A first-principles density functional theory study

摘要

In addition to equilibrium isotopic fractionation factors experimentally derived, theoretical predictions are needed forinterpreting isotopic compositions measured on natural samples because they allow exploring more easily a broader rangeof temperature and composition. For iron isotopes, only aqueous species were studied by first-principles methods and thecombination of these data with those obtained by different methods for minerals leads to discrepancies between theoreticaland experimental isotopic fractionation factors. In this paper, equilibrium iron isotope fractionation factors for the commonminerals pyrite, hematite, and siderite were determined as a function of temperature, using first-principles methods based onthe density functional theory (DFT). In these minerals belonging to the sulfide, oxide and carbonate class, iron is presentunder two different oxidation states and is involved in contrasted types of interatomic bonds. Equilibrium fractionation fac-tors calculated between hematite and siderite compare well with the one estimated from experimental data (In a57Fe/54Fe = 4.59 ± 0.30‰ and 5.46 ± 0.63‰ at 20 °C for theoretical and experimental data, respectively) while those forFe(III)aq-hematite and Fe(II)aq-siderite are significantly higher that experimental values. This suggests that the absolute valuesof the reduced partition functions (β-factors) of aqueous species are not accurate enough to be combined with those calculatedfor minerals. When compared to previous predictions derived from Mossbauer or INRXS data [Polyakov V. B., Clayton R.N., Horita J. and Mineev S. D. (2007) Equilibrium iron isotope fractionation factors of minerals: reevaluation from the dataof nuclear inelastic resonant X-ray scattering and Mossbauer spectroscopy. Geochim. Cosmochim. Acta 71, 3833-3846], ouriron /3-factors are in good agreement for siderite and hematite while a discrepancy is observed for pyrite. However, thedetailed investigation of the structural, electronic and vibrational properties of pyrite as well as the study of sulfur isotopefractionation between pyrite and two other sulfides (sphalerite and galena) indicate that DFT-derived β-factors of pyriteare as accurate as for hematite and siderite. We thus suggest that experimental vibrational density of states of pyrite shouldbe re-examined.