Effect of Coordination Structure of Iron Ions on Iron Oxide Activities in Na2O-SiO2-FeO-Fe2O3 Melts

Activities of FeO and FeO1.5 in Na2O-SiO2-FeO-Fe2O3 melts have been investigated in terms of the coordination structure of iron ions. The melts were placed in Pt containers at 1 573 K and equilibrated at partial pressures of oxygen in the range between 10(-9) atm and 10(-6) atm, and the activities were derived from Fe concentrations in the Pt containers using the activity coefficient of Fe in Pt-Fe alloys reported as a function of molar fraction of Fe. At the same time, the percentages of Fe2+, Fe3+ in octahedral symmetry (Fe3+(oct)) and Fe3+ in tetrahedral symmetry (Fe3+(tetr)) were also measured by Mossbauer spectroscopy. It has been found that the activity coefficients of FeO (gamma(FeO)) are larger than those of FeO1.5 (gamma(FeO1.5)), suggesting that FeO is prone to liberate from the silicate network more than FeO1.5. It has also been found that the values of gamma(FeO) monotonically increase with increasing Fe2+/Fe-total ratio; in contrast, the values of gamma FeO1.5 seem relevant to neither Fe3+(oct)/Fe-total nor Fe3+(tetr)/Fe-total ratio. The activity coefficients have been discussed from the perspective of the coordination structure via the effective ionic radii of Fe2+ Fe3+(oct) and Fe3+(tetr). The magnitude of effective ionic radii is in the hierarchy of Fe2+ > Fe3+ (oct) > Fe3+(tetr), and thereby the bond strength between iron ion and oxide ion is in the hierarchy of Fe3+(tetr) > Fe3+(oct) > Fe2+. This suggests that Fe2+ ions are more loosely bound to the silicate skeleton than Fe3+(tetr) and Fe3+(oct) ions, which situation would be reflected in the magnitude of the activity coefficients and their dependencies on Fe2+/Fe-total, Fe3+(oct)/Fe-total and Fe3+(tetr)/Fe-total ratios.