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 spectra of borate minerals show great variety and complexity, arising from the many possible anions which these materials may contain. It is therefore necessary to establish some principles for determining which anions may be present, and then to discuss the spectra characteristic of these ions. The structural principles involved have been discussed by Wells (1960), Christ (1960), Edwards and Ross (1960) and Valyashko and Vlasova (1969), and the following rules have been formulated: 1. The coordination polyhedron around a boron atom will be either a triangle or a tetrahedron. 2. Polynuclear anions can be formed from these coordination polyhedra by sharing corners; such anions may contain only trigonally or tetrahedrally coordinated boron, or both. These anions are described by Christ as “compact insular groups of low to medium negative charge”. The basic unit of these anions is a six-membered ring containing alternate B and O atoms. 3. In the poly-ions of hydrated borates, unshared oxygen atoms attach a proton and exist as hydroxyl groups. 4. The poly-ions may polymerize in various ways with the elimination of water. In practice, they appear to form either infinite chains, with or without crosslinking, or dimers, but other oligomers do not appear to exist.
Study of these materials by infrared spectroscopy reveals two distinctions which can be fairly confidently made. Firstly, the positions of the absorption bands, particularly in the stretching region, are diagnostic of 3- or 4-coordinated boron. Secondly, it is possible to establish