Improvements were made, to the Kieffer-Borchardt kinetic model for diffusion in silicate melts, which took account of the diffusion of individual polyanions and of the reaction of various polyanions via condensation and splitting. By making assumptions concerning the individual condensation reactions, the equilibrium polyanion distributions in silicate melts were calculated by solving a system of coupled first-order differential equations which described the reactions between polyanions of various sizes. It was found that the concentration dependence of SiO44- monomers, as determined by using the present approach, was identical to that which resulted from a thermodynamic treatment. By allowing for local changes in the isotopic distributions of the elements, and by assuming that migration of all of the species which were present in the system occurred due to random motion, the self-diffusion of Si and O in CoO-SiO2 melts was simulated. The parameters of the model were estimated by fitting the simulation results to experimental data on Si and O tracer diffusion. Whereas the simulated concentration profiles were in good agreement with experimental tracer diffusion measurements of the CoO-SiO2 system, their shape could not be described by the standard solution of Fick’s law. On the other hand, in the case of 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 could be interpreted qualitatively as being due to the structural differences of CoO-SiO2, as compared with the other 2 systems.

T.F.Young, J.Kieffer, G.Borchardt: Journal of Physics - Condensed Matter, 1994, 6[46], 9835-52