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 study of non-clay minerals in clays is very important, both industrially and scientifically. Because of the influence of these minerals on the properties of clays, even when they occur in small amount as fine-grained particles dispersed throughout the clay, it is usually desirable to identify them, to estimate their particle size and distribution, and to observe their morphology. The study of minerals constituting dusts is also very important and here the quantity of the original sample available is often extremely limited.
For such investigations electron-optical methods have some distinct advantages over others. Among recent developments high-resolution, powder, or selected-area electron-diffraction patterns and dark-field images from Bragg reflections (Talbot, 1956) are especially effective for the identification of mineral particles because they can clarify morphological characteristics as well as the details of crystal structure for each small crystal.
Electron-optical data for minerals that are likely to be found in close association with clays are given below. More electron-optical information on other non-clay minerals may be found in the publications of Gorbunov (1957), Bates (1958, 1961), Gritsaenko, Rudnitskaya, and Gorshkov (1961), Rekshinskaya (1966), and Beutelspacher and van der Marel (1968).