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.
It is proposed in this chapter not to list all the available instruments, techniques and procedures used in infrared spectrometry, but to outline, in the light of experience at the Macaulay Institute for Soil Research, what are considered to be the basic instrumental and sample requirements for obtaining meaningful infrared absorption spectra of minerals, and to indicate the type of ancillary equipment which has been found to be most useful in achieving this end. A good extended treatment of the general field of infrared instruments and methods is given in the book edited by Miller and Stace (1972).
Commercial infrared spectrometers first became readily available in the mid 1940s and have proliferated greatly since then. The handbook by White (1964), although out of date, contains a good introduction to commercial spectrometers and their capabilities. A more recent listing appears in the book by Stewart (I 970). Choice of instrument will always be governed by funds available and specific needs, but in order to obtain good spectra with adequate resolution over a wavelength range capable of giving the best opportunity of characterizing the mineral, the spectrometer should meet certain minimum requirements. These are, that resolution, particularly in the 4000-2000 cm-1 region, should be better than 2 cm-1 and requirements employ diffraction gratings as the dispersing elements, and if the coverage is to 200 cm-1", will have facilities for purging the radiation path of water vapour which absorbs strongly below 300 cm", For critical work in the range 650-680 cm-1