Scaling of thermodynamic mixing properties in garnet solid solutions

The volumes and enthalpies of mixing, DeltaV(Mix) and DeltaH(Mix), of binar y solid-solution aluminosilicate garnets have been studied by computer simu lation. The use of "average atoms" to simulate solid solution was found to give results that are considerably different from those obtained by calcula ting and averaging over many configurations of cations at a given compositi on. Although we expect mineral properties calculated from model calculation s to be correct only on a qualitative rather than a quantitative scale, fai r agreement with experiment was obtained where carefully tested potential p arameters were used. The results show that mixing behaviour in these materi als is controlled by local strain and relaxation effects resulting from the atomic size mismatch of the mixing divalent cations. In particular, DeltaV (Mix) and DeltaH(Mix) shown to scale quadratically with the volume differen ce between the end members, and to vary essentially symmetrically with comp osition, with a moderate dependence on the degree and nature of cation orde r. We conclude that computer modelling should be useful in providing detail ed qualitative information about the mixing properties of solid solutions, which can help to better constrain and interpret experimental results.