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During the past several years some very important advances in electron microscopy have been made in instrument design and performance as well as in specimen preparation techniques. The resulting improvement in resolution has opened up a new pattern of study, with greater emphasis now being placed on fine structural details which were not able to be resolved in earlier studies. With some of the new microscopes a resolving power as low as 8 Å can be obtained in direct observations of the specimen, while the lowest possible resolution with a replica technique is about 20 to 30 Å. The latter appears to be limited chiefly by the structure of the shadowing metals. With the development of a new replica technique made possible with Bradley's method of forming evaporated carbon films (1), particular attention is being paid to fine structure in clay minerals, glasses, and other ceramic materials. The examination of bulk clays has revealed details that are important in attempting to define the natural state of the mineral and has helped to provide logical explanations for some of the physical and chemical properties of importance when they are used as catalysts and in the manufacture of certain products in which clays play an important part. Surface details on glasses show fracture paths, liquid immiscibility, location and size of ucleating agents, weathering effects, and the effects of chemical surface treatments. In the study of ceramic bodies, electron micrographs of fractured specimens show how surface topography is related to particle size of the raw clay, the presence of impurities, and the firing temperature. This information has been helpful in basic studies of the formation of high-temperature phases in fired clays and has provided information on structural development which may be correlated with properties of the test specimens. Replica studies of other minerals have also been carried out, including studies of structure in crystals grown from solutions containing added impurities. Direct examinations of clays and other minerals are made for the determination of particle size distributions, the determination of the morphology of individual particles, or the determination of the state of aggregation of a specimen. The preparation of specimens for direct examination usually takes less time than that required for replica studies and under normal conditions permits the use of the ultimate resolving power of the instrument. By a simple selection of proper lenses, the electron microscope can now be converted readily for electron diffraction studies using an aperture system which permits one to “select” diffraction patterns from areas of over 30μ down to less than 2μ in diameter. One of its important uses is in the identification of small quantities of impurities in samples which by X-ray diffraction may appear to be “pure.” It is used also in the study of single crystals to obtain information on unit-cell parameters of compounds whose crystal size is so small as to preclude single crystal X-ray analysis.
Comer, Joseph J.
Associate Professor of Mineral Sciences, Pennsylvania State University, University Park, Pa.