POLARIZED SINGLE-CRYSTAL RAMAN-SPECTROSCOPY OF SINHALITE, MGALBO(4)

Polarised single-crystal Raman spectra for sinhalite have been collect ed at 80 K over the range 1300-10 cm-1. All 36 of the first-order Rama n bands predicted by the factor group analysis (FGA) of the olivine st ructure type have been observed and assigned. Sinhalite has the smalle st unit cell volume of any crystal with the olivine structure and is s ignificantly more distorted than are the silicate olivines. This leads to distortion-related consequences in the vibrational spectra, such a s greater wavenumber ranges for each type of mode and reduced Raman in tensity for symmetric BO4 modes. Intense Raman bands in the region 796 -468 cm-1 are assigned to BO4 rotations/translations which deform the Al-O bond without displacement of the aluminium atoms which lie on the M1 sites. Raman-active modes involving Ml cation displacements are in compatible with the FGA.