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The spectra of orthosilicates and pyrosilicates are of interest both in their own right and as a key to understanding the spectra of more complex silicates. Orthosilicate spectra should indicate the range of variation of the frequencies of OS–iO stretching and OSiO bending vibrations, and should allow us to judge to what extent these vibrations are modified by cation-oxygen forces, or, indeed, can be considered as distinct from vibrations involving translations of the cation, or translations and rotations of the anions: pyrosilicate spectra should reveal the additional features which must arise from SiOSi linkages.

It is not possible to answer these questions as conclusively as might be hoped, partly due to the conceptual difficulties associated with these ideas, and partly due to the almost total absence of single crystal IR and Raman studies, which are usually essential to unambiguous band assignments. These deficiencies are, however, largely compensated by careful systematic studies of isomorphous series of orthosilicates and pyrosilicates by Tarte and co-workers in Liege, and Lazarev and co-workers in Leningrad.

After examining progress in rationalizing the spectra, we shall briefly review applications to characterizing ortho- and pyrosilicates, and finally list, in an appendix, sources of published spectra for this group of minerals, including there silicates containing Si3O10 anions, zunyite containing the unique [Al(SiO4)4] anion, and thaumasite containing the unique [Si(OH)6] anion.

An isolated tetrahedral Si04 molecule would have nine internal degrees of freedom but only four modes of vibration due to degeneracy. These vibrations and their frequencies are labelled VI

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