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 most natural grouping of minerals for vibrational spectroscopy is based firstly on the mono-atomic anion present, whether oxide, fluoride, or sulphide, etc., and then subsequently subdivided according to such complex anions, e.g. carbonate, sulphate, fluorosilicate, etc., as may be distinguished within the structure. Most minerals can be formulated AaBb---Xx where A, B, --- are cations carrying real or formal positive charges, and X is a mono-atomic anion. The importance of this anion resides in the central role it plays in binding the cations together, so that it is necessarily involved in the strongest bonds within the structure, and participates in the vibrations of highest frequency. Moreover, the X atom is frequently the lightest atom in the crystal, so that these vibrations of highest frequency are largely localized on the X atom. Consequently the masses of the cations play only a minor role, compared to bond strengths, in determining frequencies; for example, if the bond strengths and structures of simple MO oxides were all the same, then variation in the atomic weight of the cation, M, over the range 24–250 would cause a variation of only ± 11% in the M–O stretching frequency, about a mean corresponding to a cation mass of 50. This variation is smaller than that associated with a change in valency or coordination number of the cation (see Tables 1.I, 1.II, 1.III and 7.I).
Returning again to the generalized mineral formula AaBb---Xx, the next most important consideration is whether the B-X bonds are so much