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

Troilite was synthesized and its Mössbauer spectra in the temperature range 90 ÷ 295 K were obtained. The equilibrium iron isotope fractionation factors (β-factors) for troilite were estimated from the temperature shift (TS) in the Mössbauer spectra. The TS was described by the Debye model, and the Mössbauer temperature (θ_M) was calculated. It is shown that the quantum component of TS, at temperatures above ~ 0.6θ_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 θ_M. Based on Mössbauer data at temperatures below 0.6θ_M (from 90 to 190 K), θ_M = 319 K was found. The temperature dependence of the iron β-factor for troilite, calculated from this value of θ_M, is as follows: ⁵⁷Fe/⁵⁴Fe 10³lnβ = 0.42388x − 0.51351 × 10⁻³x² + 0.96769 × 10⁻⁶x³; x = 10⁶/T² where T is the absolute temperature. The Mössbauer temperature dependence of the iron β-factor for troilite agrees well with the results of its estimation by nuclear resonance inelastic X-ray scattering on ⁵⁷Fe nuclei. The same approach was applied to assess the iron β-factors for aegirine. Previously obtained θ_M = 540 K for aegirine was corrected down to θ_M = 479 K using Mössbauer data at temperatures below 0.6θ_M. The temperature dependence of the iron β-factor for aegirine: ⁵⁷Fe/⁵⁴Fe 10³lnβ = 0.95573x − 2.6105 × 10⁻³x² 11.09185 × 10⁻⁶x³ matches with that from the first principal calculations. This resolves the contradiction between Mössbauer-derived and first principle calculated iron β-factors for aegirine.