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 minerals which form the subject of this work are the pyroxenes and pyroxenoids, the amphiboles, and beryl, cordierite and tourmaline. The less common chain, ribbon, and ring silicates, including all ring silicates with fewer than six tetrahedra in the ring, are omitted. Considerable emphasis has been placed on the interpretation of the fine structure of hydroxyl stretching bands in amphiboles, for these have yielded much useful information on the composition, structure and reactions of these minerals. By contrast, relatively little attention has been devoted to the vibrations of the silicate anions, or of the cation-oxygen polyhedra which link these anions. This selection of subjects reflects in part the existence of Lazarev's (1972) comprehensive work on silicate spectra and the absence of a comparable review of the hydroxyl spectra, and partly the relative paucity of mineralogical applications of the spectra of the complex anions. Sources of published spectra are listed in a supplementary bibliography to this chapter.
Detailed descriptions of the crystal structures of the minerals considered here will be found in Deer, Howie and Zussman (1962).
Taking bond strengths to be proportional to zlnr, where z is the formal charge on the cation, n its coordination number, and r the cation-oxygen distance, bond strengths in [Si04], [AI04], [AI06], [Mg06], and [NaOs] polyhedra in mineral structures are in the approximate ratios 12:9:5:3: 1. Thus the [Si04] groups which polymerize to form the complex chain, ring and ribbon anions present in minerals are joined to each other by bonds