MULTICOMPONENT DIFFUSION IN MGO-AL2O3-SIO2 AND CAO-MGO-AL2O3-SIO2 MELTS

The process of multicomponent diffusion in silicate melts has been explored through a series of diffusion experiments in MgO-Al2O3-SiO2 and CaO-MgO-Al2O3-SiO2 melts. Experiments were performed in a variety of compositional directions in both systems at temperatures close to 1773 K. These experiments demonstrate a variety of uphill diffusion effects. The first-order features of our composition profiles can be represented using a constant n-1 dimensional diffusion matrix, however, many second-order features cannot. A more complex model, in which the melt is postulated to be made up of a number of diffusing melt species, is more successful at reproducing flux reversal features. Unfortunately, this scheme suffers from a surplus of unconstrainable parameters. Detailed compositional maps and computer simulations suggest that convective fingering processes are not responsible for the unusual. features in OUT profiles. Best fit values for diffusion matrices are estimated for each of the two melt systems. All eigenvalues of diffusion matrices in both systems are on the order of 10(-1.1) M2 sec-1. The uniformity of diffusion coefficients suggests that the rate of diffusion of all melt species in the melts considered may be governed by the same rate-limiting step.