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.
The smectites (or montmorillonite–saponite group) are defined as layer-lattice minerals with a 2:1 (triphormic) structure that undergo interlayer swelling when exposed to water or water-vapour, or any one of several organic liquids; this swelling can be detected by alteration of the 001 spacing measured by X-ray diffraction.
The structural models for the elementary layers are derived from that of pyrophyllite for the dioctahedral family, or that of talc for the trioctahedral family, depending on the characteristic isomorphous substitution (MacEwan, 1961; Caillère and Hénin, 1963). The four principal minerals of the group are indicated in Table 6.1. To these must be added nontronite, a dioctahedral mineral in which most of the aluminium is replaced by Fe3+. The charge on the nontronite layers could be due to either tetrahedral or octahedral substitution, to give a beidellite or a montmorillonite type, respectively, but current opinion favours classification as the beidellite type, with tetrahedral substitution corresponding to the formula (Si8 -x Alx)iv(Fe4)viO20(OH)4.
The formulae given in Table 6.1 must be considered idealized because there may be other minor substitutions that do not alter the distribution of charge (e.g. Fe3+ for AI3+, or Fe2+ for Mg2+), and because tetrahedral and octahedral substitution can coexist in a mineral. The species is therefore generally classified according to the major substitutions; for instance, a montmorillonite in which more than half the exchange capacity is due to tetrahedral substitution is preferably classed as beidellite (Hofmann et al., 1955).