MOESSBAUER SPECTROSCOPY IN STUDYING THE THERMALLY INDUCED OXIDATION OF Fe~(2+) CATIONS IN IRON-BEARING SILICATE MINERALS: Examples of applications with almandine, pyrope and olivine

摘要

Mossbauer spectroscopy was used for the identification of the primary oxidation phase during thermal treatment of almandine at 750 °C and for the study of the secondary polymorphic transformations of Fe_2O_3. Thus, superparamagnetic nanoparticles of maghemite were found as the direct product of oxidation of Fe~(2+) ions in the almandine structure, and ε-Fe_2O_3 was identified as the intermediate product during structural change of maghemite to hematite. The application of Mossbauer spectroscopy for the study of the oxidation mechanism in the defined site of the mineral structure was demonstrated with the example of thermal study of pyrope at 1000 °C. The possibility of the air-oxidation of Fe~(2+) ions in the dodecahedral 24c position accompanied by the formation of hematite was proved. The significant contribution of the Mossbauer spectroscopy for the investigation of the mechanism of thermally induced oxidative decompositions was illustrated with olivine at 1200 °C. Two-step formation mechanism of the spinel structure magnesioferrite was suggested on the basis of combined Mossbauer and XRD analysis.