Chemical and mineralogical data on Holocene coastal plain soils from Thailand suggest that during formation of pyrite in tidal swamps Fe(III) is extracted from smectite and replaced by Mg. During drainage of these sediments pyrite oxidizes and the reaction is reversed. Replacing Fe(III) by Mg in clay minerals stores alkalinity that can neutralize three-quarters of the acidity released during oxidation of pyrite, and thus lowers the potential acidity of pyritic sediments. Possible causes for variation in the amounts of Mg taken up during pyrite formation are discussed.

La mesure de l'intensité à l'échelle absolue consiste à déterminer le pouvoir diffusant de l'objet rapporté au motif élémentaire de la structure, c'est-à-dire exprimé en nombre d'électrons libres équivalents diffusant indépendamment et qui donnerait, en chaque point de l'espace réciproque, le même effet que l'objet. Les avantages de cette méthode sont les suivants: (i) elle permet de corriger la courbe expérimentale de la diffusion incohérente et ainsi d'augmenter le nombre de réflexions quantitativement exploitables; (ii) elle permet une meilleure caractérisation de la nature des défauts structuraux et, pour des systèmes hétérogènes tels que des particules en suspensions, elle rend possible la séparation des effets de diffraction spécifiques de chaque phase; (iii) elle rend possible l'interprétation des fonds de diffusion. Ces propriétés sont illustrées sur quelques exemples d'application (kaolinite, sépiolite mal cristallisée et suspension de montmorillonite-Na dans l'eau).

Measurements have been made of the transient changes in the polarized components of fluorescence when aqueous suspensions of dye-tagged sepiolite particles were subjected to pulses of high frequency alternating electric field. The dye was acridine orange, a planar, cationic molecule. From these measurements, the size and electrical prolarizability of the sepiolite particles was evaluated and the directions of the absorption and emission transition moments responsible for the fluorescence of the dye located. The dye molecules were shown to adsorb on the clay surface with their long axes parallel to the sepiolite rods. Adsorption of the dye molecules in the surface channels of the clay was indicated. The electrical polarizability was considered and compared with the cation exchange capacity of the mineral. A novel, rapid means of studying clay minerals is indicated hereby.

The < 2 µm fractions of two fluvio-glacial sediments from East Greenland have been analysed chemically and by XRD and Mössbauer spectroscopy. The bulk of each < 2 µm fraction consists of feldspars and micas (biotite) which are important constituents of the Precambrian igneous and metamorphic rocks of the area. Significant amounts of vermiculitic and smectitic minerals are also present. Compared with similar minerals present in Danish soil clays, the Greenland vermiculite and smectite are characterized by a high degree of crystallinity and it is concluded that these minerals have been formed from biotite through K release and lattice expansion. These processes appear to have proceeded at a sufficiently high rate to yield significant amounts of vermiculite and smectite despite the cold climate prevailing in the area.

The adsorption of the Asulam herbicide (4-aminobenzensulphonyl-methyl-carbamate) by Al- and H-montmorillonite has been studied by IR spectroscopy, thin layer chromatography and chemical analysis. This compound is adsorbed from CCl4 solution at room temperature through protonation of the NH2 functional group by the ‘acidic’ water coordinated to the interlayer cation of the clay. After heating the complex at 75°C for 20 h, a hydrogen-bond between the C=O group of the Asulam and the water is established and the organic molecule decomposes to sulphanilic and carbamic acids. If the complex is equilibrated at 100% relative humidity, the carbamic acid becomes unstable, and the IR spectrum shows only the bands typical of the acidic montmorillonite-sulphanilic acid complex. The catalytic properties of the clay surface are demonstrated by comparison with the transformations occuring in the organic molecule on heating in concentrated mineral acids.

The clay mineralogy of a sequence of 36 andesitic tephra and associated paleosols dating from 320 to > 100,000 years BP has been studied using XRD, IR, DTA and TEM techniques. The mineralogy and weathering sequence is compared with that of rhyolitic tephra of central North Island, New Zealand.

The morphology of volcanic glass particles in rhyolitic and andesitic tephra of central North Island and Taranaki areas of New Zealand has been studied by scanning electron microscopy. Electron probe analyses of the glasses are compared with those of the clays to which they weather. Loss of silica characterizes the weathering of both glasses. The rapid rate of weathering of andesitic glass is attributed to its occurrence as fine, soft microlites and extensive substitution of Al for Si in the structure. Rhyolitic glass weathers more slowly because it occurs as hard and brittle particles containing relatively little alumina. It is suggested that the structure, chemical composition and chemical activity of allophane is governed largely by the chemical composition and bonding characteristics of the parent glass.

Two well-characterized soils containing substantial amounts of interstratified mica-smectite have been examined by a combination of techniques in an attempt to correlate the characteristics of the interlamellar surfaces with other properties. The techniques used included XRD and chemical analyses, determination of surface area by nitrogen sorption and by desorption of water and ethylene glycol monoethyl ether vapour, and determination of cation exchange capacity. Reasonable agreement was found between the results obtained for the proportion of expanding or smectite-like surfaces, although estimates by XRD, especially when coarser material (> 2/µm) was included, were generally lower than those obtained by the other methods. The mean density of charge on the surfaces of the soil clays appeared to be similar to that of smectite. The decrease in porosity observed for both soils dried by the critical point method was largely accounted for by the decrease in volume associated with the loss of interlamellar water from the smectite-like interlamellar surfaces included within the clay fraction.

L'interprétation des diffusions en dehors des taches de Bragg dans une vermiculite saturée par du baryum (d001 = 12·20 Å) permet de préciser l'organisation de l'espace interlamellaire. Le motif élémentaire est constitué par un groupement Ba-6 H2O où le baryum est engagé dans une cavité ditrigonale et les 6 molécules d'eau sont dans le plan médian, l'ensemble forme une pyramide très aplatie de base hexagonale. La répartition de ces motifs est bipériodique avec une maille 2 a,b centrée en projection. Cette organisation se développe dans des domaines identiques, orientés à 120° les uns des autres.