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

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