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 disappearingly small proton radius (∼10−5 Å) imparts peculiar properties to the hydrogen atom. Even the most polar A-H bonds (A is an electronegative atom) are mainly covalent. However, in interactions between an acid group A-H and a basic atom B, a "hydrogen bond" A-H…B is formed in which the hydrogen atom plays the role of an electron acceptor.
Current concepts concerning the nature of H-bonds and the influence they exert on the structure and physical properties of substances have been discussed in a number of reviews and monographs (Sokolov, 1964; Pimentel and McClellan, 1960; Hamilton and Ibers, 1968; et al.). For this reason, we shall restrict ourselves to a few remarks on the mechanism of H-bond formation in inorganic compounds.
It is known that for the formation of a strong H-bond A-H…B, the atom B must possess an unshared electron pair on an extended hybrid orbital. It is necessary also that the proton screening by the unshared electron pairs of the A atom be sufficiently weak.1 A schematic representation of the two most frequently observed mechanisms of increasing the proton-donor ability of the A-H group may be given as follows:
Variant I corresponds to the case when the unshared electrons of atom A are accepted by another proton-donor molecule A'-H.
Variant 11, more effective under certain conditions, corresponds to the case when one or several X atoms connected with the A-H group possess vacant orbitals which are well overlapped by the orbitals of