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.
The apparatus and techniques of transmission electron microscopy and diffraction have much in common. Both require a high vacuum, a source of electrons with high velocity, and electromagnetic lenses for focusing the electron beam. In both cases, specimens are usually thin films or dispersions of thin particles supported on a thin substrate. For many years, however, the two disciplines progressed independently. and attracted entirely different groups of scientists. Although it is often stated that the possibility of an electron microscope with high resolution was suggested by de Broglie's (1924) hypothesis of the wave nature of electrons, reviews of the early history of the electron microscope by Mulvey (1962) and Freundlich (1963) clearly indicate that the wave properties of electrons were not seriously considered during development, from 1928, of the first instrument of this type built by Knoll and Ruska (1932a). The concept of an electron microscope arose from attempts to construct an improved cathode-ray oscilloscope, and at the time it was believed that there were no theoretical restrictions to the ultimate resolution. Later in 1932, however, Knoll and Ruska (1932b) made the remarkably accurate prediction of 2·2 Å as the resolution limit for 75 keV electrons. As the resolving power of production microscopes has improved, consideration of the wave nature of electrons has become increasingly important for interpretation of micrographs, especially from crystalline specimens such as minerals.