Multicomponent diffusion in aluminosilicate garnet: coupling effects due to charge compensation

Profiles that display a subtle maximum in Ca concentration are explained by the prevailing ideal mean-field theory of multicomponent diffusion, but models implementing that theory cannot replicate inverted profiles for Li and internal maxima for Nd, Sm, and Eu.

The anomalous profiles are quantitatively reproduced, however, by numerical simulations employing a model based on coupled movement of charge-compensating groups during diffusional transport of yttrium and the rare-earth elements (Y+REEs). An alkali-type charge-compensation mechanism for the heterovalent substitution of Y+REEs on dodecahedral sites in garnet produces direct charge coupling between Li + and (Y+REE) 3+ and leads to co-diffusion of Li + -(Y+REE) 3+ pairs, with the result that Li profiles closely mimic those for Y+REEs.