The crystal structure of a garnet solid solution (Ca0.9Mg0.1)3Al2[SiO4]3 at 295 K and 100 K: Local site behavior – dynamic and static disorder

Abstract

Abstract The crystal structure of a synthetic solid-solution garnet, Gr90Py10, of composition (Ca0.9Mg0.1)3 · Al2[SiO4]3 was refined at 100 K and 295 K from single-crystal X-ray data. The difference displacement parameters were examined in order to investigate static and dynamic disorder of Ca and Mg in the dodecahedral E-site of 222 point symmetry. The difference displacement parameters for the E-site, derived from the diffraction experiment, are compared to those calculated from two juxtaposed Gaussian distribution functions of different E–O distances. This analysis indicates that Mg in Gr90Pyl0 adopts a quasi four-fold coordination with four short E–O(2) distances of ca. 2.1 Å and four longer E–O(4) distances of ca. 2.5 Å. In contrast to pyrope, Mg3Al2Si3O12, this different bonding of the Mg cation in Gr90Py10 leads to reduced vibrational disorder of Mg. Single-crystal X-ray diffraction refinements and Raman spectroscopic measurements are consistent with the proposal that for grossular-rich solid solutions the grossular substructure remains largely unchanged, but that minor oxygen displacements produce a quasi four-fold coordination around Mg.