REDOX EQUILIBRIA OF IRON IN TI-BEARING CALCIUM SILICATE QUENCHED GLASSES

Mossbauer spectroscopy has been employed to evaluate relations between redox equilibria of iron, Ti3 content and degree of polymerization of quenched glasses in the system CaO-TiO2-SiO2-Fe-O. The data show that Fe3+/SIGMAFe 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 Fe3+ (isomer shift near 0.3 mm/s relative to metallic Fe at 298 K) and octahedrally coordinated Fe2+ (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 Fe2+. The latter parameter shows positive correlation with the Ti/(Ti+Si) of the quenched glasses, suggesting increasing distortion of the Fe2+-O polyhedra. The ln(Fe3+/Fe2+) 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(Fe3+/Fe2+) vs. oxygen fugacity data, the activity coefficient ratio of Fe3+/Fe2+ is negatively correlated with NBO/T and with Ti content. As the oxygen fugacity decreases below that of air (and, therefore, the Fe3+/SIGMAFe also decreases), the isomer shift of ferric iron begins to increase (from approximately 0.3 mm/s to > 0.4 mm/s) so that, when Fe3+/SIGMAFe < 0.3, the isomer shift values are consistent with ferric iron being in octahedral coordination, whereas with Fe3+/SIGMAFe > 0.5, Fe3+ is in tetrahedral coordination. In the Fe3+/SIGMAFe range 0.3-0.5, Fe3+ probably exists both in tetrahedral and octahedral coordination in the quenched glasses. The data are interpreted to suggest that solution of Ti4+ in iron-bearing silicate quenched glasses enhances the stability of Fe3+ relative to Fe2+. 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.