GARNET: INTERGROWTHS, MULTI-PHASES, AND OPTICAL ANISOTROPY

Silicate garnets are one of the major constituents in the Earth’s crust, upper mantle, and transition zone. They are commonly found in many igneous and metamorphic rocks, as well as in beach sands and in some sedimentary rocks. Common silicate garnets have high (cubic) symmetry. Optical anisotropy in cubic garnet was reported over a century ago, but the origin remains questionable. Some grossular, hydrogrossular, spessartine, andradite, and uvarovite samples may show appreciable anisotropy under cross-polarized light, which may indicate that they are not optically cubic. Several reasons were given as the cause of the anomalous anisotropy, but the main one appears to be cation order that may cause reduction of symmetry from cubic. The crystal structure of several anisotropic garnet samples were refined using the Rietveld method, space group Ia-3d, and monochromatic synchrotron high-resolution powder X-ray diffraction (HRPXRD) data. Each anisotropic sample contains an assemblage of two or three cubic phases that have slightly different structural parameters. The intergrowth of these cubic phases causes strain-induced optical anisotropy. The intergrowths occur as fine-scale nano-domains and it is not always possible to obtain distinct compositions for the individual phases using the electron microprobe.