RAMAN-SPECTRA OF SILICATE GARNETS

The single-crystal polarized Raman spectra of four natural silicate ga rnets with compositions close to end-members almandine, grossular, and radite, and uvarovite, and two synthetic end-members spessartine and p yrope, were measured, along with the powder spectra of synthetic pyrop e-grossular and almandine-spessartine solid solutions. Mode assignment s were made based on a comparison of the different end-member garnet s pectra and, in the case of pyrope, based on measurements made on addit ional crystals synthesized with Mg-26. A general order of mode frequen cies, i.e. R(SiO4)>T(metal cation)>T(SiO4), is observed, which should also hold for most orthosilicates. The main factors controlling the ch anges in mode frequencies as a function of composition are intracrysta lline pressure (i.e. oxygen-oxygen repulsion) for the internal SiO4-vi brational modes and kinematic coupling of vibrations for the external modes. Low frequency vibrations of the X-site cations reflect their we ak bonding and dynamic disorder in the large dodecahedral site, especi ally in the case of pyrope. Two mode behavior is observed for X-site c ation vibrations along the pyrope-grossular binary, but not along the almandine-spessartine join.