Structural properties of garnets under pressure:: An ab initio study

The structural properties of end members of the aluminosilicate garnet family (pyrope, grossular, almandine and spessartine) have been investigated as a function of applied pressure. The study has been performed with the density functional theory code CASTEP which uses pseudopotentials and plane-wave basis sets. The geometrical parameters of unit cells containing 80 atoms have been fully optimized. The calculated static geometries (cell parameters, internal coordinates of atoms, bond lengths), bulk moduli and their pressure derivatives are in good agreement with the available experimental data. The results of a systematic study of the effect of the cation size on compressibility are presented. It is suggested that the bending of the Si-O-Al angle between the octahedra and tetrahedra is the main compression mechanism.