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.
Within this group we shall consider the oxides of single metallic and semi-metallic elements, and complex oxides involving two or more elements of formal ionic charge not exceeding four (silica and the silicates are treated separately). These oxides are therefore distinguished from the majority of sulphates, phosphates, etc., by the fact that no separate oxy-anion can usually be distinguished in their structure. Oxides have received considerable attention from physicists concerned with the solid state: their approach, procedures, and results are described by Hadni (1967) and Mitra (1969). Since the simpler structures involve only one or two different types of bond, it becomes feasible to calculate the complete vibrational spectrum, and to check the results, at the long wavelength limit, against infrared reflection and Raman spectra obtained from single crystals. Oxide spectra are now well understood in principle, as is illustrated by calculations of the vibrations of quartz (Elcombe, 1967), rutile structures, including T1O2 and SnO2 (Katiyar et al., 1971), and perovskites (Cowley, 1964), all of which achieve favourable agreement with experimental data.
In contrast, most of these oxides present special difficulties to the chemical spectroscopist, who aims to obtain information on structure and composition from spectra derived from specimens which can usually only be examined in powdered form. Because of the intense dipole oscillations induced by the vibrations of highly ionic oxides, their powder spectra are profoundly modified by the shape and size of the particles, as discussed in Chapter