OPTICAL ANISOTROPY, MULTIPHASE INTERGROWTHS, AND ZONING IN GARNETS

Garnets are useful and well-known minerals that are found in various environments. Common silicate garnets have cubic symmetry, so their anomalous optical anisotropy has puzzled researchers for over a century. Many garnets are anisotropic under cross-polarized light, which may indicate that they are not optically cubic. Several reasons were given as the cause of this anomalous anisotropy, but the main reason appears to be cation ordering that causes a reduction in cubic symmetry. We have refined the crystal structure of several anisotropic garnet samples using the Rietveld method, space group Ia-3d, and monochromatic short-wavelength synchrotron high-resolution powder X-ray diffraction (HRPXRD) data. Each anisotropic sample contains an assemblage of two or three cubic phases with slightly different structural parameters. The fine-scale intergrowths of these cubic phases occur as oscillatory compositional zones and cause strain-induced optical anisotropy. The compositions of the individual phases can be resolved with the electron probe microanalyzer if the crystal is viewed in a suitable orientation. Examples from various garnet members will be given.