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 tables list the site symmetries which can be present in crystals belonging to each of the 32 crystallographic point groups except the trivial Cl. Above the site group symbol appears the number of equivalent sites in a primitive unit cell. The crystal species are the same as those of the site of highest symmetry, and the site-group species which contribute to each of the crystal species appear in the body of the table, together with the number and direction of the permissible displacements for a set of atoms lying on these sites. With minor easily recognized modifications, the tables also serve to correlate the symmetry species of an isolated molecule with those appropriate to the site symmetry. These applications are illustrated in Chapter 4.