Single-crystal X-ray structure study of synthetic pyrope almandine garnets at 100 and 293 K

The crystal structures of synthetic pyrope (Mg3AI2Si3012),almandine (FeAI2Si3012), and the solid-solution garnet compositions PY8o-Alm2o, PY6o-Alm4o, and PY2o-Alm8o have been refined in space group Ia1d from high-precision X-ray diffraction data with sin 0/>..> 0.4 A-I measured at 100 and 293 K. There is no indication of lower symmetry for pyrope, almandine, or solid-solution members. Experimentally determined atomic coordinates and displacement parameters for the solid-solution compositions are in good agreement with those linearly interpolated from the end-members. Thus there are no apparent structural features that could account for substantial nonideal enthalpies of mixing in the system pyrope-almandine. The tetrahedral rotation angle is inversely correlated with the X-O distance. Fe2t substitution on the eight-coordinated X site of pyrope, or increasing temperature, decreases the rigid tetrahedral rotation in garnet. The large and anisotropic displacement parameters for the X-site cations in garnet are mainly a result of anisotropic thermal vibrations along the longer X-O bonds, which produce nonrigid polyhedral behavior for the dodecahedral site. The tetrahedra and octahedra behave as rigid bodies. These strong vibrations of the former give rise to the relatively large heat capacities and third-law entropies in garnet. Previous proposals concerning subsite dodecahedral ordering in pyrope must be revised.