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.
SILICA in fine particulate and mainly crystalline form has been studied by many different methods including both optical and electron microscopy and electron diffraction. Probably the most important reason for investigating silica in the micron and sub-micron range has been its importance as the causative factor in the lung disease known as silicosis. Optical methods have been used for identification, size measurement, and the study of surfaces. Some forms of amorphous silica have a recognizable shape and this may have some diagnostic value in identification, but crystalline silica appears to be either devoid of cleavage planes or to have insufficiently distinct weaknesses to break into anything other than randomly shaped particles. These cannot be identified by shape, and electron microscopy is of limited value in identification. This is partly due to the tendency for an amorphous or very disordered layer to form on particle surfaces, and to affect the structure to a depth of a similar order to that of the penetrating power of the electron beam. X-ray diffraction techniques for the quantitative determination of quartz must also take account of the effect of the amorphous layer on the quality and intensity of the diffraction pattern.
Since crystalline silica particles are of random shape, reasonably reliable statistical correlation can be established between particle sizes as measured by electron microscopy and other parameters such as the surface and mass as measured by other techniques. This can be of importance in respirable particles of less than 2 μm diameter, whose size can be reliably measured only by electron microscopy.