Influence of Non-Tetrahedral Cations on Si-O Vibrations in Complex Silicates

Abstract

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.