X-ray Diffraction Procedures for Clay Mineral Identification
PREVIOUS chapters have surveyed the structures of ordered and disordered clay minerals and related layer silicates, their swelling in water and organic liquids and interstratified layer structures. We come now to consider how this detailed information can be used to identify clay minerals. This means that we must consider how to prepare clay materials for X-ray examination. how to utilize X-ray diffraction equipment to obtain the necessary data, and finally how to compare these data with the accumulated information so as to arrive at an identification suitable for the purpose involved. Many aspects of sample preparation, of diffraction analysis, and of material identification apply to the study of crystalline materials generally. Here we shall emphasize those aspects which relate to the particular class of materials under consideration. It will be assumed that readers have access to books dealing generally with X-ray diffraction procedures, particularly powder methods of analysis; the following may be mentioned specifically: Elentents of X-Ray Diffraction, Cullity (1956, 1978); X-Ray Diffraction Procedures for Polycrystalline and Amorphous Materials, Klug and Alexander (1954, 1974).
No single identification procedure, not even X-ray diffraction, gives all the answers on all occasions. Consequently diffraction analysis is combined almost always with other methods, partly chemical and partly physical, Much depends on whether very detailed information is required on a few samples, or less detailed information on a very large number of samples. At one extreme, identification is more or less synonymous with mineralogical and crystallographic study of minerals; at the other extreme, it becomes one aspect of a broad geological or soil survey. In this chapter we enkavour to keep in mind this range of interests.
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Crystal Structures of Clay Minerals and their X-Ray Identification
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.