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 arrangement of this chapter is designed to reflect the structural rather than the chemical similarities between groups of minerals. The most important similarity is that which involves the nature of the anion. We can consider three types of anion, these being:
Isolated tetrahedral ions, as formed by P, As and V, and also pentavalent Cr and Mn.
Isolated trigonal ions, as formed by N.
Anionic groupings of stoichiometry MO3 or MO4 formed by sharing of octahedral coordination polyhedra. These are formed by Sb, Nb and Ta.
The first group is by far the largest, and requires further subdivision in two ways. In the first place, cations should be arranged in groups so that different cations leading to the formation of isostructural compounds are in the same group. The available crystallographic evidence suggests that the following five groups are appropriate:
- Group A.
The cations of the alkali metals, ammonium, silver (I), copper (I) and thallium (I).
- Group B.
The cations of calcium, strontium, barium, lead (II) and cadmium.
- Group C.
The divalent cations of the first row transition elements, and magnesium.
- Group D.
The trivalent cations of boron, aluminium, gallium, indium, thallium, manganese, chromium, iron and bismuth.
Group E. The trivalent cations of the rare earths, yttrium and scandium.
The only elements forming compounds with group V tetrahedral anions and so far excluded from the classification are beryllium