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.
IN the course of their studies on soil allophane Yoshinaga and Aomine (1962a, b) have noted that certain Ando soils contain, in addition to crystalline clay minerals and free sesquioxides, two different mineral colloids. One of these, which was completely amorphous to X-rays, was called allophane; the other showed some degree of order and was termed imogolite—after Imogo, a brownish yellow volcanic ash soil in the Kuma basin in the Kumamoto Prefecture of Japan. They observed that after deferration of the soil, allophane dispersed in both acidic and alkaline solutions, whereas imogolite dispersed in acid and flocculated in alkali. On electron micrographs imogolite appeared as thread-like particles several μm long and 100 to 200 Å wide.
A mineral component showing similar features was later found in macroscopic gel films filling the interstices of weathered pumice grains (Miyauchi and Aomine, 1966) and in soil clays derived from volcanic ash (Aomine and Miyauchi, 1965; Kawasaki and Aomine, 1966). Early observations were confined to Japanese volcanic ash soils, but imogolite has since been identified in pumice tuff soils of Western Germany (Jaritz, 1967), volcanic ash soils of Papua (Greenland et al., 1969), and Ando soils of Chile (Besoain, 1969).