A study of clay coatings in selected soils
Clay coatings from a variety of soils were studied using the petrological microscope, the scanning electron microscope, the transmission electron microscope and various X-ray techniques in order to ascertain their morphology and composition. It was hoped that their mode of formation could also be determined.Thin section results showed that the morphology of the clay can be divided into seven types, according to the extinction patterns seen in crossed polarized light. These patterns indicate that the clay does not usually lie in flat parallel sheets, but that the sheets have a wavy form, with variations in wavelength and amplitude. Within the coatings there are frequently areas of clay which have no obvious orientation relationship to the main clay particle orientation.The scanning electron microscope work confirmed the optical results and also showed that in several cases, at low magnification, the surface of coatings have pseudohexagonal ridges and high magnification sharp ridges and overfolds are present i.e., the surfaces are not smooth. The most significant result from this part of the study was that clay coatings are not composed of tightly packed sheets of clay, but that these features have a very open packing. arrangement, with eliptical pores seen in cross-section.Elemental and mineralogical analysis indicated that the type of clay in the coatings is very dependant on the intensity of the weathering environment and that it is similar to the clay in the matrix.The results imply that clay illuviation is the dominant process responsible for clay coating formation.