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 idealized chemical formula for the serpentine minerals is Mg3Si2O5(OH)4, and the idealized structure consists of superimposed sheets, which are trioctahedral analogues of those that form the kaolinite minerals (fig. 5.1). Each of these sheets has two components; one is a network of linked SiO4 tetrahedra, (Si2O5)n, and the other a brucite type octahedral layer. Two-thirds of the hydroxyl ions at the base of the brucite layer are substituted by oxygens at the apices of Si-O tetrahedra. The repeat distances of an Si2O5 network are considerably smaller than those of a brucite layer, and it is this that is in great part responsible for the fact that serpentine minerals often show departures from the simple form of the structure. The two components may accommodate one another by distortion of their simple networks, or by changes in the network parameters through chemical substitution of larger or smaller ions, or by curvature of the composite sheet with tetrahedra on the inside and octahedra on the outside of the curve. A combination of all three methods of accommodation may, of course, occur.
The main kinds of serpentine mineral are chrysotile, antigorite, and lizardite. The latter seems to approximate most closely to the simple structure, since it has the simple unit cell a ⋍ 5.3, b ⋍ 9.2, c ⋍ 72 Å. Lizardites generally show some chemical substitution of trivalent ions for magnesium or silicon or both, but although they do have platy morphology their crystallinity is in general poor and grain size small. Optical properties are not usually measurable, but the coarser grained lizardite is nearly uniaxial (negative) with α perpendicular to the crystal plates.