The study of structural features of mixed-layer minerals by X-ray powder diffraction (XRD) is based on the analysis of the intensity distribution of basal reflections using direct and indirect Fourier-transform methods. The direct Diakonov method, in contrast to that of MacEwan, provides information on a mixed-layer mineral structure based only on d-values and intensities of basal reflections. The indirect methods are difficult because a great number of independent parameters are needed to model XRD patterns. An a priori precise determination of these parameters is not always possible. A combination of direct and indirect methods may prove fruitful for studying “monomineralic” interstratified clays.
The degree of homogeneity is an important characteristic of mixed-layer samples consisting of thin particles (e.g., containing 5–15 layers). Finely dispersed mixed-layer samples can be classified as homogeneous, quasihomogeneous, and heterogeneous. All particles in homogeneous mixed-layer samples have the same composition and differ from one another only in the pattern of alternation of layer types. Mixed-layer samples whose thin individual particles differ both in composition and in the pattern of alternation of layer types are called quasihomogeneous if the structural features of the samples can be described by a small number of independent probability parameters for each given short-range order factor, R. These parameters are the total number of layers, N, the contents of different layer types, and the pattern of their alternation. If these factors cannot be described by a small number of independent probability parameters, the mixed-layer samples are termed heterogeneous. XRD effects calculated for quasihomogeneous and homogeneous samples and for mixtures of quasihomogeneous samples are generally not sensitive to wide variations in the degrees of homogeneity.
Selected-area electron diffraction is an independent tool for structural studies of poorly crystalline minerals, including mixed-layer phases, and has been used to reveal ordered and random mixed-layer phases among manganese minerals in Fe-Mn nodules.
Analyses of the structural features of interstratified illite/smectite (I/S) from diagenetic and hydrothermal environments suggest a two-stage mechanism for the transformation of smectite into illite. In the first stage, smectite is believed to be transformed into I/S having predominantly smectite layers by a solid-phase mechanism. At higher temperatures, I/S phases having predominantly illite layers and R > 1 may be formed by dissolution-precipitation. Identical azimuthal orientations of 2:1 layers in the crystal favor the irreversible fixation of K during the smectite-into-illite transformation. Illitization of smectite is accompanied not only by the increase in the layer charge brought about by the Al-for-Si substitution in tetrahedra, but also by the redistribution of cations over trans- and cis-octahedra in the 2:1 layers.
Figures & Tables
Proceedings of the International Clay Conference Denver, 1985
The papers included in this proceedings volume are representative of the research on clays being conducted in all parts of the world at the time of publication. Many of the subjects treated are controversial, and although some ideas expressed may not necessarily represent the views of the editors, the referees, or the publisher, they deserve to be brought to the attention of the international clay community.