The Carbonate Minerals

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Abstract
The carbonates are a complex group of minerals built around the ion. The carbonates and the carbonate double salts are all highly anisodesmic compounds. The carbonate ion and other anions such as sulphate, phosphate, or hydroxyl have strong covalent bonds with high force constants. The separation of the vibrational spectra into internal and lattice modes is therefore a useful approximation. Each vibrational spectrum will consist of a set of internal modes for each crystallographically distinct anion, with frequencies perturbed only slightly from the free ion values, and a set of low frequency lattice modes characteristic of the particular crystal structure. In general, the internal modes will be at higher frequencies and appear in the mid-infrared region while the lattice modes appear at lower frequencies. It is for this reason that the mid-infrared spectra of all carbonate minerals tend to look alike and that the IR spectrum is a useful identification tool for this anion. Spectra of the carbonates have been included in most of the compilations of infrared spectra of inorganic compounds and minerals. These include Hunt et al. (1950), Keller et al. (1952), Miller and Wilkins (1952), Miller et al. (1960), Moenke (1962a, 1966), Alexanian et al. (1966) and Nyquist and Kagel (1971). Infrared spectra have proved useful for the qualitative identification and quantitative analysis of carbonates in rocks and other minerals. Descriptions of these analyses are given by Louisfert and Pobeguin (1952), Pobeguin (1954), Baron et al. (1957), Pobeguin (1959), Chester and Elderfield (1967), 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.