Quantitative X-ray Mineral Analysis of Clays

Abstract

QUANTITATIVE mineral analysis of clay materials is logically the next step after a qualitative examination has identified the components present. X-ray methods of quantitative phase analysis of mixtures are very versatile and probably have the widest capabilities of the various methods available. X-ray intensities reflected by components in mixtures are obviously related to the proportions of the components and it is a reasonable expectation that quantitative analyses can be made on this basis. The development of X-ray diffractometers greatly simplified the measurement of X-ray intensities as compared with photographic methods and many texts are available describing procedures for accurate measurements among which may be mentioned those of Cullity (1956; revised 1978), Klug and Alexander (1954; revised 1974), and Peiser, Rooksby and Wllson (1955). The present chapter will show that many methods have been developed for quantitative phase analysis some using external standards, others using internal standards, and some which eliminate standards. The application of these methods to clay analysis is rendered difficult because of the variability of clay minerals, chemically and structurally. This variabilrty affects not only the intensities diffracted by the components but also makes the selection of reference minerals very difficult. The suggestion that the reference minerals should be taken from the materials to be analyzed is excellent in principle but often exceedingly difficult in practice. The intensity of X-ray diffraction by a component P in a mixture is given by the equation (see Klug and Alexander, 1974, p. 533): where ρ_p, is the density of component P and ΣW_pµ_p is the average attenuation coeficient; in equation (1), ρ_p, is incorporated in the constant K_p. In the years 1930—1950 clay mineral identification involved mainly a combination of X-ray powder diffraction and chemical analysis with some assistance from other techniques, notably differential thermal analysis. In the period 1950—1970 additional procedures have emerged including infrared analysis, electron optical methods and a variety of thermal methods. These procedures are now treated in other monographs sponsored by the Mineralogical Society and in many other publications. Despite the availability of other techniques, X-ray diffraction remains a basic tool for studying minerals and we hope that this monograph will continue to serve, as did the previous editions, both those concerned with the more academic aspects of clay mineralogy and also those, such as geologists, civil engineers and soil scientists, for whom identification and quantitative estimation of the minerals in natural clayey materials is a practical requirement.