Effect of coordination structure of iron ions on iron oxide activities in Na₂O–SiO₂–FeO–Fe₂O₃ melts

Activities of FeO and FeO_1.5 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 Mössbauer spectroscopy. It has been found that the activity coefficients of FeO (γ_FeO) are larger than those of FeO_1.5 (γ_FeO1.5), suggesting that FeO is prone to liberate from the silicate network more than FeO_1.5. It has also been found that the values of γ_FeO monotonically increase with increasing Fe²⁺/Fe_total ratio; in contrast, the values of γ 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.