Diffusion and activity of NiO in CaO-Mg0-Al203-Si02 melts considering effects of αo2− and γNi2+

Variations in activity coefficients for oxides in silicate melts are complex functions of silicate melt composition. For example, the activity coefficient (y) for NiO shows a minimum when plotted against a parameter reflecting the degree of polymerization (or basicity or fraction of bridging oxygens) (e.g., Pretorius and Muan, 1992). In this paper, we propose that this complexity occurs in part because NiO is not an actual species in silicate melts. Variations in y can be better understood and predicted if the activity of NiO is treated as the product of activities of Ni2+ and 02− ions. Using voltammetric methods, we have measured independent activity coefficients for Ni2+ and O- for compositions between diopsidic and anorthitic melt and have found that variations in these values are more easily understood in terms of the melt composition and structure and permit qualitative variations in γNiOo (activity coefficient of NiO relative to the free energy of formation reported in Robie and Waldbaum, 1968) to be predicted in compositions other than those studied. We suggest that a similar consideration of ionic behavior might improve our understanding of activity coefficients for other oxides in silicate melts as well. In addition, we report diffusion rates for Ni and free energies for the reaction Ni2+ + 02- ⇌ Ni0 + 1202, for melt compositions along the compositional joins CaMgSi206-CaAl2Si208 and CaMgSi206-MgAl2Si 2O8.