Raman spectroscopy of silicate rings: Benitoite and the three-membered ring

Polarized single-crystal Raman spectra of benitoite (${\mathrm{BaTiSi}}_{3}$${\mathrm{O}}_{9}$) were gathered and atomic displacements have been assigned to each line in the spectra from our interpretations of the Raman data, from determinations by Adams and Gardner in an earlier study, and from determinations by lattice-dynamics calculations. The spectra can be naturally divided into three frequency ranges, where lattice-dynamics calculations show that the 50 to 300 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ range contains Ba, Ti, ring translational, and ring rotational modes, the 300 to 800 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ range contains three-membered silicate ring breathing, stretching, and deformation modes, and the 800 to 1200 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ range contains Si-O bond-stretching modes. From our conclusions for benitoite, Raman features in the 300 to 800 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ range may be sensitive to changes in Si-O-Si bond angles, or to the number of silicate tetrahedra within planar rings.