The effect of MgO on the diffusivity of iron and the incorporation of iron in soda-magnesia-aluminosilicate melts

Iron diffusion coefficients were measured in glass melts with the basic compositions 10Na2O · 10MgO · xAl2O3(80 − x)SiO2 (x = 0–20), 10Na2O · xMgO · 10Al2O3(80 − x)SiO2 (x = 5–20) and xNa2O · 10MgO · 10Al2O3(80 − x)SiO2 (x = 5–20), doped with 0.25 mol% Fe2O3 in the temperature range from 1000 to 1600 °C. The square-wave voltammograms recorded showed peaks attributed to the reduction of Fe3+ to Fe2+, from which peak currents diffusion coefficients were calculated. In a wide temperature range, the iron diffusion coefficients could be fitted to Arrhenius equation. Diffusion coefficients related to the same viscosity (i.e. not to the same temperature) decreased with increasing Na2O-concentration and increased with increasing Al2O3-concentration. This can be explained by a structural model. Fe3+ is incorporated as FeO 4 - -tetrahedra which need alkali for charge compensation. This results in a stronger incorporation of iron and hence in smaller diffusion coefficients with increasing alkali concentrations. Alumina also incorporated in fourfold coordination as AlO 4 - also needs alkali for charge compensation and, therefore, causes the opposite effect as Na2O. The effect of MgO is less pronounced; the slight decrease in the iron diffusivities can be explained by the similarities of Fe2+ and Mg2+ with respect to their metal–oxygen bond lengths. Hence, while increasing the MgO-concentration Fe2+ should occupy energetically less advantageous sites which favors the formation of FeO 4 - -tetrahedra.