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.
To identify the vibrations that give rise to features in infrared and Raman spectra, we can be guided by a number of qualitative considerations. For example, lighter atoms will vibrate at higher frequencies than heavier atoms, if their bonding is of similar strength; higher bond strengths, usually associated with higher valencies or greater covalency lead to higher bond-stretching frequencies; in covalent structures, bond-stretching vibrations lie at frequencies higher than bond-angle deformations. These considerations, when combined with the results of the symmetry classification described in the last chapter, can permit intelligent guesses at the kinds of vibrations in each symmetry species. If single crystals are available, the direction of the transition moments associated with infrared absorption bands can permit up to three symmetry species to be distinguished, but often only two or one. Raman studies on single crystals can be more productive, allowing, for example, the differentiation of three symmetry species in cubic crystals, where only one species is infrared active.
The recognition of some crystal vibrations is greatly simplified if there exists in the crystal some atomic grouping, a molecule or complex ion, which can be considered as dynamically isolated, i.e. when some or all of its internal vibrations lie at frequencies substantially higher than its external vibrations. Features common to the spectra of a series of compounds containing this atomic grouping can then be correlated with its internal vibrations. Here the effect of the crystal environment can be considered as a perturbation, and we have already seen that two