Influence of non-tetrahedral cations on Si-O vibrations in complex silicates

The effect of non-tetrahedral cations on the silicon-oxygen vibrational modes in complex silicates was modelled in a small-cluster approximation by introducing a shell containing first- and second-order neighbours of the peripheral oxygen atoms of the Si-O cluster. As an example, the proposed method was applied to study the Raman spectra of different types of tourmaline, which contain in their structure single six-membered 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 cations in octahedral Y-positions and as a result the peaks shift and their shape modifies. Using such changes, one can estimate the occupation of Y positions in tourmalines by various cations.