Application of a kinetic model to tracer diffusion of silicon and oxygen in silicate melts

Abstract

In this work we have improved the kinetic model of Kieffer and Borchardt for diffusion in silicate melts, which takes into account both the diffusion of individual polyanions and the reaction of different polyanions through condensation and splitting events. Making assumptions for the individual condensation reactions, similar to those by Masson, the equilibrium polyanion distributions in silicate melts were calculated by solving a system of coupled first-order differential equations, describing the reactions between polyanions of various sizes. The concentration dependence of the SiO44- monomers, determined by our approach, is identical to that resulting from the thermodynamic treatment by Masson. Furthermore, by allowing for local changes in the isotope distributions of the elements, and by assuming that migration of all species present in the system occurs due to random motion, the self-diffusion of silicon and oxygen in CoO-SiO2 melts has been simulated. The parameters for the model have been estimated by fitting me simulation results to our experimental data of Si and O tracer diffusion. While the simulated concentration profiles were in good agreement with our experimental tracer diffusion measurements in the CoO-SiO2 system, their shape could not be described by the standard solution of Fick's law. Conversely, for the CaO-SiO2 and PbO-SiO2 systems, the simulated profiles were in much better agreement with the standard solution. This difference in diffusional transport properties can be qualitatively interpreted as due to the structural differences of CoO-SiO2 as compared to the two other systems.