The oxides of iron, aluminium, and manganese
THE oxides (including the hydroxides and hydrous oxides) of iron and aluminium are, along with silica, the most common accessory minerals in clays; manganese oxides, on the other hand, occur more sporadically. Nevertheless, it is appropriate here to consider these oxides together, since there are many fundamental resemblances not only in composition and structure, but also in occurrence and origin. The crystalline oxides and hydroxides of iron and aluminium are well-defined and distinctive in character but those of manganese are more poorly-defined and the validity of certain reputed species is as yet by no means certain: in consequence, only well-defined species can be considered. All three metals also apparently form oxides with too low a degree of order to diffract X-rays or even electrons: these will be termed amorphous. Structural relationships between many of these oxides have been discussed by Rooksby (1961) and thermal characteristics by Mackenzie (1957).
Figures & Tables
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.