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.
Strunz (1966) arranged 15 groups of minerals in a tectosilicate division of his system, describing about 72 of these three-dimensional structures in detail. The 10 minerals of the quartz group are listed in an oxide and hydroxide class, but in considering their vibrations and spectra these silica minerals are more naturally grouped with the tectosilicates.
In a first comprehensive survey (Milkey, 1960), one can find IR spectra (1400–667 cm−1) of 57 tectosilicates. Of the 473 spectra (1800–400 cm−1) included in Moenke's Mineralspektren (1962, 1966), 40 are of tectosilicates. Lyon (1962a) refers to 93 tectosilicates in his bibliography, which includes 269 silicates in all. Thus the field, which includes the important sub-groups of felspars, zeolites and silica has been well surveyed, although the restricted range of older spectra (down to 670 cm−1) limits their value.
It has also been considered desirable to include the borosilicates and beryllium silicates in this chapter. Although some of these cannot be regarded as tectosilicates, they show some distinctive features which make it convenient to deal with them in one place.
The strongest absorption bands of the tectosilicates lie in the 950-1200 cm region, and these can be characterized as antisymmetric Si-O-Si and Si-O-(AI) stretching vibrations. This range lies significantly higher than that found for nesosilicates (820-1000 crrr") but overlaps the range found for layer and chain structures. The second strongest region of absorption usually lies between 400-550 cm-I, and can be broadly characterized as O-Si-O bending