Local heterogeneity in the distribution of iron ions in CR-spinel from Ural ultramafic massifs: Evidence from Mössbauer spectroscopy data

The crystal chemistry of Fe ions in Cr-spinel from the largest Ural ultramafic massifs has been studied by Mossbauer spectroscopy at room temperature and the boiling temperature of liquid nitrogen. The spectra substantially depend on the mineral composition (stoichiometry) and measurement temperature; Fe2+ and Fe3+ doublets significantly overlap; the Fe2+ doublet lines are markedly broadened. According to the Mossbauer data, the degree of iron oxidation is 7–35% and appreciably differs from that in the stoichiometric approximation. The disturbance of integral stoichiometry by di- and trivalent cations (deviation of the Me3+/Me2+ value from 2.0) may be caused not only by partial inversion of the mineral structure but also by local micro- and nanoscale heterogeneity of the mineral, clustering of Fe2+ and Al (Cr, Fe3+) cations, and the appearance of associates. Possible application of the QS-distribution method for analyzing nonequivalent nuclear iron states and the thermal dynamics of Mossbauer spectra for studying local clustering effects of iron cations is discussed. It is shown that these approaches give new information on local heterogeneity of structural sites occupied by iron ions.