CHLORITES are hydrous silicates of aluminium, magnesium, iron, and rarely chromium, and occur as metamorphic minerals in crystalline schists. They are also found extensively as hydrothermal minerals in igneous rocks, particularly in association with metallic ores in veins or replacement bodies, or in alteration zones around ore bodies. Mother materials are usually mafic minerals or basic glass, but felsic minerals or rhyolite glass have also altered to chlorites. Furthermore, chlorites are found extensively in sedimentary rocks such as shale, mudstone, and Recent marine sediments. Weathering processes and diagenetic changes have played important roles in formation of chlorites in sediments, but it is also considered that in some cases chlorites in sediments are fragments of chlorites transported from mother rocks.
Size of chlorite crystals ranges from macroscopic flakes, as exemplified by chlorites in crystalline schists or in hydrothermal rocks, to extremely fine-grained powders such as chlorites in metamorphic rocks or sediments.
Chlorites normally have a trioctahedral structure, in which talc-like silicate layers alternate with brucite-like layers (see, e.g. Brindley, 1961). Variations are, however, possible, in which the layers are either wholly or partially dioctahedral, or in which the octahedral cations are unequally distributed between the brucite and talc-like layers. Brindley and Gillery
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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.