Smectites are one of the most important groups of phyllositicates found in soils and sediments, and certainly one of the most difficult to study. New information about the formation mechanisms, impact of structural features on surface properties, and long-term stability of smectites can best be gained from the systematic study of single-phase specimens. In most instances, these specimens can only be obtained through synthesis under controlled conditions. Syntheses of smectites have been attempted (1) at ambient pressure and low-temperature ( 1000~ or pressures >10 kbars), and (4) in the presence of fluoride. Of these approaches, syntheses performed under moderate hydrother- mal conditions are the most numerous and the most successful in terms of smectite yield and phase- purity. Using hydrothermal techniques, high phase-purity can be obtained for beidellites and several transition-metal smectites. However, synthesis of montmorillonite in high purity remains difficult. Starting materials for hydrothermal syntheses include gels, glasses, and other aluminosilicate minerals. The pres- ence of Mg 2+ seems to be essential for the formation of smectites, even for phases such as montmorillonite which contain low amounts of Mg. Highly crystalline smectites can be obtained when extreme temper- atures or pressures are used, but other crystalline impurities are always present. Although the correlation between synthesis stability fields and thermodynamic stability fields is good in many instances, metastable phases are often formed. Few studies, however, include the additional experiments (approach from under- and over-saturation, reversal experiments) needed to ascertain the conditions for formation of thermody- namically stable phases. Thorough characterization of synthetic products by modern instrumental and molecular-scale techniques is also needed to better understand the processes leading to smectite formation.

Synthesis of smectite clay minerals; a critical review

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https://pubs.usgs.gov/wsp/1616/report.pdf

Ion-exchange minerals and disposal of radioactive wastes-- A survey of literature

A mineral belonging to the kaolinite group from Pogos de Caldas, M.R., Brazil, is described rvhich, in its naturally humid state, has a basal spacing o-f 10.05 A. Exposed to open air conditions, or placed in boiling water, it collapses to 7 3-7 .2 A, and in ethylene giycol it expands to 10.90 A. Electron micrographs of freshly collected material show very thin, platy forms ranging in size from 0 5 micron downwards. The platy morphology deteriorates when the material is aged; the plates appear to aggregate and to curl, producing cabbageiike forms. The classification of minerals resembling kaolinite and halloysite, in terms of their r-ray diffraction and electron-optical characteristics, is discussed. It is shown that a divariant rather than univariant classification scheme is recuired.

Mineralogical studies of kaolinite-halloysite clays: Part IV. A platy mineral with structural swelling and shrinking characteristics

Study of chemical and thermal treatment of kaolinite and its influence on the removal of contaminants from mining effluents.

Improvements in replica techniques have made possible the high magnification study of textural characteristics and surface features of clay aggregrates found either in nature or in the laboratory. The most successful method of sample preparation involves pre-shadowing the specimen with platinum and backing this with a layer of carbon prior to removal of the clay with a suitable solvent.

Electron Microscopy of Clay Surfaces

A stabilized halloysite-glycerol complex was studied by electron microscope and electron diffraction techniques. The basal reflection was found to be 10.7 A by both electron diffraction and X-ray diffraction analyses. Electron micrographs of the expanded lattice (halloysite-glycerol complex) were not different from those of the collapsed lattice (halloysite). On the basis of the tubes evident in the micrographs and the fact that the basal reflection was present in the transmission electron diffraction pattern, it was established that the halloysite-glycerol complex is tubular. Since this complex has an expanded lattice 0.6 A greater than hydrated halloysite, it is believed that hydrated halloysite (endellite) is tubular as postulated by Bates, Hildebrand and Swineford.

A Study in Morphology by Electron Diffraction

A number of methods were tested for their applicability to the problem of determining the content of halloysite in kaolin clays. These analytical methods included the thin-section point-count technique, base-exchange determinations, electron microscope techniques and infra-red absorption spectra differences.

Evaluation of Methods for Quantitative Analysis of Halloysite-Kaolinite Clays

Comparison was made between a sample of Wyoming bentonite and its synthetic analogue. Equivalent decomposition temperatures and mineral phases were obtained in hydrothermal environments and by calcinations.

L. B. Sand

Papers

A Study in Morphology by Electron Diffraction

Evaluation of Methods for Quantitative Analysis of Halloysite-Kaolinite Clays

Comparison of a Natural Bentonite with its Synthetic Analogue