Redox equilibria of iron in Ti-bearing calcium silicate quenched glasses

Abstract

Mössbauer spectroscopy has been employed to evaluate relations between redox equilibria of iron, Ti content and degree of polymerization of quenched glasses in the system CaOTiO 2SiO 2FeO. The data show that Fe 3+/ εFe is positively correlated with Ti/(Ti+Si), with iron content and with NBO/T (non-bridging oxygen per tetrahedrally coordinated cation) of the quenched glasses. In general, isomer shifts of ferric and ferrous iron are consistent with tetrahedrally coordinated Fe 3+ (isomer shift near 0.3 mm/s relative to metallic Fe at 298 K) and octahedrally coordinated Fe 2+ (isomer shifts between 1 and 1.1 mm/s). Neither isomer shift nor quadrupole splitting is sensitive to the bulk compositional variables (Ti and Fe content) with the exception of the quadrupole splitting of Fe 2+. The latter parameter shows positive correlation with the Ti/(Ti+Si) of the quenched glasses, suggesting increasing distortion of the Fe 2+-O polyhedra. The ln(Fe 3+/Fe 2+) is linearly correlated with 1/ T ( T is absolute temperature) and with ln( f O2) (oxygen fugacity) resulting in an enthalpy of reduction of ferric to ferrous iron in the range of 100–200 kJ/mol. This enthalpy increases with increasing Ti and with increasing total iron content. From the log(Fe 3+/Fe 2+) vs. oxygen fugacity data, the activity coefficient ratio of Fe 3+/Fe 2+ is negatively correlated with NBO/T and with Ti content. As the oxygen fugacity decreases below that of air (and, therefore, the Fe 3+/ εFe also decreases), the isomer shift of ferric iron begins to increase (from ∼ 0.3 mm/s to > 0.4 mm/s) so that, when Fe 3+/ εFe < 0.3, the isomer shift values are consistent with ferric iron being in octahedral coordination, whereas with Fe 3+/ εFe > 0.5, Fe 3+ is in tetrahedral coordination. In the Fe 3+/ εFe range 0.3–0.5, Fe 3+ probably exists both in tetrahedral and octahedral coordination in the quenched glasses. The data are interpreted to suggest that solution of Ti 4+ in iron-bearing silicate quenched glasses enhances the stability of Fe 3+ relative to Fe 2+. This enhancement is the result of complex solution mechanisms involving both ferrous iron titanate complexing and formation of highly polymerized Ti-complexes in the melts.