The Electron-Optical Investigation of Clays
Clay minerals occur most frequently in a state too finely divided for satisfactory observation with the best optical microscopes, or for study with single-crystal X-ray techniques. The higher resolution made possible by electron-optical instruments can therefore be put to good use in the investigation of the morphologies and crystal structures of clays. It is the intention of this monograph to summarize achievements to date, to indicate problems that have perhaps not received the attention they deserve, and, as a result, to suggest lines of investigation that might prove fruitful. The first two chapters explain in some detail the various types of electron-optical equipment that are currently available, the methods of operating them to the best advantage, and interpretation of the results. The techniques for preparation of specimens are reviewed in the third chapter, with emphasis on those most suitable for clay minerals. With the exception of the last chapter, on practical applications of electron-optical methods, each subsequent chapter deals with studies on a particular class of clay minerals. Some chapters include detailed descriptions of specimen preparation or other techniques that have been developed by the authors to resolve specific problems peculiar to the minerals dealt with in those chapters. Electron microscopy and other electron-optical techniques have been used, alone or in conjunction with other methods, to investigate problems that have proved otherwise insoluble. Nevertheless, these techniques have their limitations, which must always be borne in mind, as results can occasionally be misleading. It therefore seems appropriate, at this stage, to review the methods of specimen preparation and examination, and to attempt to assess their value for investigation of clays.
Minerals of the kaolin group were among the first materials to be subjected to study in the early days of electron microscopy when easily obtained, previously studied, morphologically appropriate specimens were sought as much to test the capabilities of the new instruments as to reveal previously unresolved features of the minerals themselves. Light microscope work such as that included in the studies of Ross and Kerr (1930, 1935) had successfully established the characteristic shapes and habits of resolvable single crystals and crystal aggregates of nacrite, dickite, and kaolinite; and had indicated that whereas halloysite often possesses birefringence probably “due to a partly oriented aggregate of birefracting mineral grains of submicroscopic size” (1935, p. 129), allophane is light microscope and X-ray amorphous. Unanswered were questions as to the morphology of halloysite and allophane, and the size range exhibited by the various minerals of the group; and it was for the answers to these and similar questions that clay and soil mineralogists first turned to the electron microscope.
The period 1938 to 1942 produced a significant number of papers describing the results of several groups of workers in Germany and one group in the United States. In the kaolin group of minerals the pseudo-hexagonal morphology of various kaolinite samples and of dickite was confirmed, but it was also revealed (see, for example, Humbert, 1942)