Order–Disorder in Clay Mineral Structures

Abstract

CHAPTER 1 has reviewed the regular or ordered structures of layer silicates determined mainly by single crystal, X-ray diffraction methods. The present chapter and the two following chapters are concerned with irregular and disordered structures mainly in the 0.1–10 μm particle size range. X-ray powder diffraction methods are commonly used in studying these clay-grade materials, but single crystal, electron diffraction analysis furnishes additional important information (see Gard, 1971; Zvyagin, 1967). Structural disorder is so prevalent in clay minerals that its recognition and evaluation are important aspects of the identification process. Some acquaintance with the fundamental concepts of diffraction by disordered systems is very helpful in understanding the phenomena involved. A full treatment would go far beyond what can be given in the present monograph, but from the simple treatment presented it is hoped that the main features of powder patterns from disordered layer structures will be more clearly recognized and interpreted than would be possible from a purely descriptive treatment. The theoretical discussion is presented in Sections 3, 4, 5 and 6 and its application is given in Sections 7 and 8. Inevitably there will be some overlap between the theoretical sections and their application to different mineral groups. It may be remarked that macro-crystalline minerals are not devoid of structural disorders, but in single crystal methods of structure analysis the investigator can usually select “good” crystals with a minimum of disorder. In studying clay minerals it is usually necessary to study them as formed in Nature and without the option of selecting “good” materials. 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.