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.
Infrared analysis has developed into a useful and reliable method for qualitative and quantitative investigations of cements, clinkers and their hydration products. Thus it is now possible to identify the hydrates that contribute to the setting of cements with fair certainty. Difficulties can arise in the preparation of samples for the infrared, and also because cement hydration leads to a complex mixture of different hydrates which change with time: the similarity of their spectra makes it difficult to distinguish the components. On the other hand it is possible to analyse quantitatively a Portland-cement clinker, for both principal phases and gypsum content, by infrared spectroscopy, the calculations being based on a set of linear simultaneous equations. Difficulties do arise from varying contents of impurities in some clinker phases, and these sometimes require a new calibration taking account of variations in the components present in the raw mix.
In this chapter, the possibilities and limitations of infrared spectroscopy as a tool for identification and quantitative analyses of cements and their hardening products is examined. We shall not discuss problems associated with the assignment of absorption bands and their symmetry classification, but we will sketch the contribution made by IR spectroscopy to our understanding of the structure of the hydrated calcium silicates.
As examples of the possibility of identifying technical cements from their infrared spectra, the spectra of some silicate cements are shown in Fig. 19. 1, and various aluminous cements in Fig. 19.2. It will be seen that the IR-absorption of the