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 use of Raman spectroscopy for characterization and for structural studies of minerals is at present in its infancy. It is hoped that this chapter will show that the technique is a valuable one for such purposes and may, under certain conditions, provide data which can be as immediately useful as those provided by infrared methods, and that indeed Raman spectroscopy has certain real advantages over infrared for mineral studies. Nevertheless Raman and infrared are complementary techniques and, wherever possible, both should be used in mineral studies. The main reason for the slow development of Raman spectroscopy has been the difficulty of obtaining suitable intense monochromatic sources, but the situation has now been completely transformed by the introduction of lasers. The development of these concentrated, high-intensity monochromatic sources, particularly those of the gas or mixed gas laser type, has made possible the non-destructive study of powdered minerals in very small quantities (c. 2 mg or less) or of single crystals of sizes of the order of a few millimetres. The mineralogical literature is almost devoid of any reference to the technique; most of the reports on Raman spectra of minerals (or of Raman spectra in general) are to be found in the journals concerned with chemistry, physics or molecular spectroscopy. The publication Chemical Abstracts covers all of these and reference should be made under the headings, in the abstract indices, to “Spectra, Raman” and of course to the name of the specific mineral itself. Literature searching through back issues is