Mossbauer-derived equilibrium iron isotope fractionation factors for troilite (FeS) and aegirine (NaFeSi2O6)

Troilite was synthesized and its Mossbauer spectra in the temperature range 90 divided by 295 K were obtained. The equilibrium iron isotope fractionation factors (beta-factors) for troilite were estimated from the temperature shift (TS) in the Mossbauer spectra. The TS was described by the Debye model, and the Mossbauer temperature (theta(M)) was calculated. It is shown that the quantum component of TS, at temperatures above similar to 0.6 theta(M), does not exceed the statistical error of the measurements. The use of experimental results at these temperatures leads to significant errors in the estimation of theta(M). Based on Mossbauer data at temperatures below 0.6 theta(M) (from 90 to 190 K), theta(M) = 319 K was found. The temperature dependence of the iron beta-factor for troilite, calculated from this value of theta(M), is as follows: Fe-57/Fe-54 10(3)ln beta = 0.42388x - 0.51351 x 10(-3)x(2) + 0.96769 x 10(-6)x(3); x = 10(6)/T-2 where T is the absolute temperature. The Mossbauer temperature dependence of the iron beta-factor for troilite agrees well with the results of its estimation by nuclear resonance inelastic X-ray scattering on Fe-57 nuclei. The same approach was applied to assess the iron beta-factors for aegirine. Previously obtained theta(M) = 540 K for aegirine was corrected down to theta(M) = 479 K using Mossbauer data at temperatures below 0.6 theta(M). The temperature dependence of the iron beta-factor for aegirine: Fe-57/Fe-54 10(3)ln beta = 0.95573x - 2.6105 x 10(-3)x(2) 11.09185 x 10(-6)x(3) matches with that from the first principal calculations. This resolves the contradiction between Mossbauer-derived and first principle calculated iron beta-factors for aegirine.