The Infrared Spectra of Minerals
The principal concern of this book is the use of vibrational spectroscopy as a tool in identifying mineral species and in deriving information concerning the structure, composition and reactions of minerals and mineral products. This does not mean that the approach is purely empirical; some theoretical understanding of the vibrational spectra of solids is essential to an assessment of the significance of the variations in the spectra that can be found within what is nominally a single mineral species, but which usually includes a range of compositions and defect structures. Theory alone, however, can give only limited support to the mineral spectroscopist, and careful studies of well-characterized families of natural and synthetic minerals have played an essential role in giving concrete structural significance to spectral features. The publication of this book represents a belief that theory and practice have now reached a state of maturitity and of mutual support which justifies a more widespread application of vibrational spectroscopy to the study of minerals and inorganic materials. The wide area of theory and practice that deserves to be covered has required a careful selection of the subject matter to be incorporated in this book. Since elementary vibrational spectroscopy is now regularly included in basic chemistry courses, and since so many books cover the theory and practice of molecular spectroscopy, it has been decided to assume the very basic level of knowledge which will be found, for example, in the elementary introduction of Cross and Jones (1969). With this assumption, it has been possible to concentrate on those aspects that are peculiar to or of particular significance for mineral spectroscopy.
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