INFLUENCE OF NON-TETRAHEDRAL CATIONS ON SI-O VIBRATIONS IN COMPLEX SILICATES

The effect of non-tetrahedral cations on the silicon-oxygen vibrationa l modes in complex silicates was modelled in a small-cluster approxima tion by introducing a shell containing first- and second-order neighbo urs of the peripheral oxygen atoms of the Si-O cluster. As an example, the proposed method was applied to study the Raman spectra of differe nt types of tourmaline, which contain in their structure single six-me mbered rings of SiO4 tetrahedra. It is shown that in the range 450-750 cm(-1) the Raman spectrum of the ring is sensitive to the type of cat ions in octahedral Y-positions and as a result the peaks shift and the ir shape modifies. Using such changes, one can estimate the occupation of Y positions in tourmalines by various cations.