Iron and sulfur isotope factors for pyrite from experimental gamma-resonant studies and heat capacity

Pyrite Moessbauer spectra (FeS2) is measured in the temperature range from 90 to 295 K. The temperature dependence of the isomer shift is described by the Debye model with Moessbauer temperature θM=551.4 K. Using these results, we calculated the kinetic energy of thermal vibrations of the iron sublattice of pyrite and the iron β-factor for pyrite: 103lnβ57Fe/54Fe=(1.2665±0.0391)x–(0.4584±0.0283) × 10-2x2+(0.2581±0.0239) × 10-4x3; x=106/T 2 (K-2) The Moessbauer-derived iron β-factor for pyrite agrees well with results of ab initio calculations, 57Fe nuclear inelastic γ-resonant scattering synchrotron experiments and direct isotope exchange experiments between pyrite and Fe2+ dissolved in water. Heat capacity of pyrite is measured at temperatures from 79 to 300 K. Its temperature dependence are described using the Thirring expansion. Based on this expansion, the kinetic energy of thermal vibrations of total crystalline lattice of pyrite is calculated. The kinetic energy of the thermal vibrations of the sulfur sublattice in pyrite is found by subtracting the iron sublattice kinetic energy from the total kinetic energy of pyrite crystalline lattice. Temperature dependence of 34S/32S β-factor for pyrite calculated from the kinetic energy of the sulfur sublattice is following: 103lnβ34Fe/32Fe=(1.7532±0.0623) x–(1.0470±0.0752) × 10-2 x2+(1.0424±0.1126) × 10-4 x3; x=106/T 2 (K-2) This 34S/32S β-factor values exhibit a good agreement with of ab initio calculations and isotope-exchange experimental results in the pyrite-sphalerite-galenite system.