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.
FOR many industrial purposes the mineralogical composition of a product is of great importance, and as a diagnostic tool the electron microscope is often effective; for not only can individual particles be positively identified by electron diffraction but many of them can be recognized with a fair degree of confidence by their morphology.
Kaolinite can usually be recognized, for although it is rare to find all of the crystals with six sharp corners and no re-entrant angles, nearly all kaolins have particles possessing one or more sharp-angled corners in sufficient numbers to ensure identification. Halloysite is also very recognizable since it is in the form of troughs, tubes, or toroids. Nor can one often be in error when sepiolite or palygorskite laths are present; since one cannot by morphology alone distinguish between these minerals one may be forgiven for referring to the presence of a hormite. Muscovite and vermiculite may be indistinguishable, but may be referred to as mica. Illite cannot be recognized with confidence, but where the platelets are very thin and have irregular outlines, illite may be suspected. Some iron oxides are distinguishable, e.g., hematite with curved faces or goethite, which can either have its classical, elongated shape or may be so fine as to be seen as a pepper in the electron micrographs. Some of the hydrated aluminas are recognizable but others could be mistaken for illite. Of the smectites nontronite is the most easily recognized by its ribbons; but even one or two per cent of any of the smectites may be recognized by the way their single layers plate on to the carrier foil.