THE CRYSTAL CHEMISTRY OF BIREFRINGENT UVAROVITE GARNET, IDEALLY CA3CR2SI3O12

Extensive research on the anomalous birefringence phenomenon of the garnet-group minerals continues to be the focus and interest of many geoscientists today. Previous studies from over a century of investigations attribute the occurrence of optical anisotropy mostly but not limited to, cation ordering on the dodecahedral {X} and octahedral [Y] sites, lattice mismatch at compositional boundaries caused by strain, and from the presence of hydrous components within the tetrahedral (Z) site. However, the origin remains debatable and is still unclear. Uvarovite, an elusive mantle-derived garnet, ideally, Ca₃Cr₂Si₃O₁₂, is the least studied silicate end-member species. Under crossed polarized light, this beautiful emerald-green mineral displays distinct birefringence of 0.001-0.006 and some crystals exhibit additional anisotropic features such as ‘bowtie’ structure with well defined extinction positions. Birefringent uvarovite from Finland, California, Russia, and Switzerland are investigated using electron-probe micro-analysis (EPMA) and synchrotron high-resolution powder X-ray diffraction (HRPXRD). Our results are consistent with other anisotropic garnets that support the cause of anomalous birefringence in uvarovite garnets, as derived from strain that arise from cubic uvarovite phases consisting of slight variations in chemical compositions.