MULTICOMPONENT DIFFUSION AND CONVECTION IN MOLTEN MGO-AL2O3-SIO2

Multicomponent diffusion and convection in molten MgO-Al2O3-SiO2 at 15 50 degrees C and 0.5 GPa were examined experimentally in diffusion cou ples formed around 22.5 (wt%) MgO, 17.5% Al2O3, and 60% SiO2. The diff usion matrix obtained from a simultaneous least squares inversion of c onvectively stable intersecting chemical diffusion profiles has large off-diagonal terms indicating, for example, very strong coupling of th e flux of SiO2 with gradients in MgO. In contrast to an earlier report on multicomponent diffusion involving the same compositions studied h ere, we found no features along the stable diffusion profiles that cou ld not be explained in terms of a 2 x 2 diffusion matrix. X-ray concen tration maps of the sectioned diffusion charges showed that double-dif fusive convection had occurred in a number of experiments along the di rection of constant SiO2, even though the more dense melt had been pla ced below the less dense composition. The occurrence of such double-di ffusive fingering instabilities is shown to be consistent with the pre dictions of linear stability theory. Copyright (C) 1998 Elsevier Scien ce Ltd.