Les hydrates à une couche des vermiculites présentent cinq modes d'empilement. Aux cations de petit et de grand rayon ionique (Li et Ba) correspondent des modes d'empilement particuliers. Les cations (Na, Mg, Ca et Sr) conduisent chacun à deux modes d'empilement correspondant à deux stades différents d'hydratation.

Present-day marine sediments around the emerged basaltic areas of Iceland and the Faeroe Islands are characterized by the abundance and predominance of smectites. Smectites increase regularly and systematically near volcanic areas. Their origin from (i) meteoric/deuteric weathering, (ii) hydrothermal products, (iii) neoformation in the marine environment or (iv) transformation during their transport to or stay in oceanic regions is considered in a comparative study of three types of environments on or near the Faeroe Islands. These are (1) a basaltic environment where phyllite minerals of deuteric and hydrothermal origin are abundant; (2) a soil environment formed on basaltic flows; (3) a marine sedimentary environment within adjacent sedimentary basins. This investigation clearly shows the link between hydrothermal and deuteric saponite-celadonite in basalt, Fe,Mg-smectite in Faeroe soils and Fe-smectite in marine sediments.

Diagenetic chlorite is forming as a result of temperature-controlled burial diagenesis in shales from the Sleipner area of the North Sea. Accompanying chlorite diagenesis, kaolinite and illite-smectite decrease in abundance, and illite increases in abundance. These clay mineral transformations occur between 122–126°C at temperatures higher than normally expected for chlorite diagenesis. Kaolinite and ordered illite-smectite are largely unaffected by diagenesis below 100°C. It is proposed that chlorite diagenesis is thus delayed due to the absence of a source of ions resulting from smectite decomposition. Clay mineralogy is of no lithostratigraphic use in the Jurassic sediments of the Sleipner area. However, the zone of chlorite diagenesis is a reliable indicator of maximum burial temperature.

The thickness, length, width, area and perimeter of 575 particles from 16 aqueously dispersed samples of a variety of interstratified clays, smectites and illite have been recorded using TEM techniques. Complete dispersion of the clay material was achieved by saturating the clay with either Na+ or Li+, removal of excess ions by dialysis, and isolation of the <0·1 or <0·2 µm fraction by centrifugation. The samples have mean maximum dimensions of 1900 to 90 nm and the dispersed system can be considered as colloidal in nature. The mean thickness of the clays is about 1 nm for smectites, corresponding to that of elementary 2:1 silicate layers, from 1·9 to 4·9 nm for the interstratified clays, and 9 nm for illite. From these data the volume, total surface area and other parameters have been calculated and compared with independent determinations of surface area and CEC. The total surface area by TEM, assuming a density of 2·6 g/cm3, varies from ∼675 m2/g for smectites to 86 m2/g for illite, and is inversely proportional to the mean particle thickness. The charge density of monovalent cation exchange sites on the surface of the particles as determined for nine of the samples varies from 0·54 to 1·16 nm2/site. The particle-thickness distribution data can be used to calculate interstratified XRD layer-sequence probabilities and composition parameters, and agree with XRD data for interstratified clay with <60% illite layers. The thickness data also provide a rationale for the interpretation of TEM lattice-fringe images. Relationships between the particle area, length, thickness and volume are shown to be potentially useful in assessing the mechanism(s) of crystal growth of these extremely small phyllosilicate particles.

The fluorination of montmorillonite by aqueous ammonium bifluoride solution (NH4HF2) has been investigated by chemical, sorptive, porosimetric and electron microscopic methods. Changes in the chemical composition of the montmorillonite during the fluorination were compared both in the crystal surface and in the bulk sample. The accumulation of fluorine was distinctly zonal, being present mainly in the surface layers. The electron microscope studies showed that the 300–400 nm thick macrodomains in the initial montmorillonite were cracked into small microdomains 20–30 nm in thickness, this resulting from disruption in the continuity of the octahedral sheets. These distinct changes in morphology of the montmorillonite aggregates particularly influenced the porosity and sorptive properties.

Dans le cadre de l'étude de l'interaction eau-kaolinite, des mesures de conductivité (pertes diélectriques) et de permittivités réelles ont été entreprises à fréquence variable (103–107 Hz) et en fonction de la température (27–80°C). La conductivité croît avec la fréquence et la température de mesures. L'utilisation d'un modèle de conduction par déplacement de protons par sauts discrets entre sites localisés est applicable. La loi σ′(ω) ∼ Aωs est vérifiée. L'évolution du paramètre s en fonction de la pression de vapeur d'eau permet de déterminer le mécanisme d'établissement de la monocouche d'eau sur le solide. La permittivité réelle décroît avec la fréquence. C'est une fonction croissante de la pression de vapeur d'eau. Un calcul simple permet de retrouver la loi expérimentale relative à la permittivité ɛ′ ∼ exp(K√P).

Transmission powder patterns of random and of highly oriented samples of natural micas have been recorded and compared with those calculated from structural data with the aid of the DIFK computer program modified to correct for preferred orientation. Intensities calculated from structural data for random and for highly oriented samples (symmetrical and oblique settings) are presented for 1M, 2M1, 2M2, 3T polytypes of muscovite, 1M, 2M1, 3T polytypes of paragonite and 1M (C2/m and C2 space groups), 2M1, 2M2, 3T polytypes of lepidolite.

Selective dissolution analysis is widely used to separate various soil minerals (e.g. oxides and oxyhydroxides of Al and Fe, allophanes, phyllosilicates) from each other. Although a wide variety of reagents has been used for some of these determinations, few rigorous comparative studies have been attempted. Too often, reagents used to extract particular soil components are evaluated using geological or other specimens that may bear little resemblance to soil minerals formed by pedogenic processes; the investigations of Borggaard (1982) and Chao & Zhou (1983) are two recent examples of such an approach. Insufficient use has been made of difference infrared spectra (Wada & Greenland, 1970) obtained from soil samples.

Nickel crystal chemistry was systematically studied in various phyllosilicates, mainly the natural phases selected from the ‘garnierites’ of the supergene ore deposits of New Caledonia. Minerals which do not usually occur in New Caledonian parageneses were synthesised, as they could represent intermediate phases of genetic importance. In the kerolite-pimelite series, a linear relationship occurred between the ratio I(13,20)/I(02,11) of the hk bands and Ni-content. Diffuse reflectance spectra were used to derive the crystal chemical parameters of Ni. These confirmed its divalent character and its occupation of octahedral sites; the resulting structural distortion was slight and could not be detected in some minerals. There was no optical evidence for Ni atoms in 4-fold coordination. The two main parameters which showed significant variations among the studied phases were site distortion and crystal field stabilization energy (CFSE). Site distortion was at a maximum in trioctahedral smectites and sepiolite. CFSE depended on the mineralogy, crystallinity and chemical composition (Al-content) of the phase. Finally, clay minerals are classified according to the increasing stability of Ni in the octahedral sheet, which has been tentatively related to the geochemical distribution of this element. Secondary minerals are usually enriched vs. primary ones and among them are nepouite and kerolite which exhibit a high CFSE in contrast to sepiolite.

L'altération latéritique débutante des orthopyroxènes et clinopyroxènes de Niquelandia, Brésil, montre plusieurs stades. Le premier stade facultatif est la formation de produits ‘amorphes’ dont la composition chimique proche de celle des minéraux parentaux, montre déjà un enrichissement en nickel. Le deuxième stade est marqué par l'apparition de phyllosilicates dont la nature et la composition chimique varient selon le degré de porosité de la roche parentale. Cette porosité contrôle également la distribution du nickel dans les différents phyllosilicates, (saponite, talc, pimélite). Les pyroxènes ne contenant pas ou très peu de nickel, il est clair que le nickel qui s'accumule dans les produits de l'altération est totalement importé avec les eaux de percolation. L'origine de ce nickel est à rechercher plus haut dans les profils ou plus en amont dans la séquence, là où les dunites sont présentes.

Mathematically, it is well established that besides the Fourier coefficients, the natural characteristics of any distribution are its moments. The use of the second moment, called variance, as a measure of line broadening has already gained wide popularity because of its several merits relative to other empirically devised parameters. In recent decades, the fourth moment has been recognized as an important parameter; its theoretical significance has been discussed by Mitra (1964), who developed the moment range function for the estimation of crystallite size and distortion in cold-worked metal. However, owing to the requirements of high degree of accuracy of intensity measurement coupled with cumbersome calculations, this parameter did not find much general application. Modern diffractometers and sophisticated computers have helped ease the problem and in recent years several workers (Kagan & Snovidov, 1965; Kulshreshtha et al., 1971; Mukherjee, 1981) have successfully used this parameter for defect analysis in various non-ideal forms of materials.

The occurrence of sepiolite and palygorskite in soils, Eocene limestones and the Asmari dolomitic limestone of Iran has been described by many workers (Bentor et al., 1963; Martin Vivaldi & Robertson, 1971; Abtahi, 1977, 1980). No studies have been carried out on the genesis of these minerals in Iranian soils, however, and the purpose of this note is to describe some experimental conditions under Which sepiolite can form, and which also could apply to the genesis of the mineral in Iranian soils.

The mineralogy of the fine clay fraction (<0·2 μm) of two soil pedons (Aquic and Udic Haplustalfs) representing the wheat-sugarcane belt of the Indo-Gangetic alluvial plain was studied by X-ray diffraction, differential thermal analysis, transmission electron microscopy, and by chemical analysis. The XRD patterns are characterized by unusually high spacings which are interpreted in terms of an interstratification comprising smectite, illite and ‘chlorite’ components. The CEC data (Ca/Mg and K/NH4) also indicate the possible presence of vermiculite. A plateau bridging the 7–10 Å maxima in K-saturated specimens heated at 300°C suggests interstratification of kaolinite and smectite, although this may not be part of the above interstratification. The diffuse bands shown by Mg-glycerol-solvated Ap-horizon clays at ∼21 Å, and the increasing elimination of XRD peaks in the > 10 Å region with distance from the surface, suggests that the expanding layers have a slight tendency to segregate in Ap horizon samples and that randomization tends to increase with depth. Discrete illite and small amounts of kaolinite were also detected.

A trioctahedral, ferrous smectite has been located in the uppermost deposits of Atlantis II Deep, SW basin, Red Sea. This clay is very unstable when removed from its environment of formation and, for example, oxidizes during desiccation under laboratory conditions. Only X-ray transmission diffraction study of the mud itself demonstrates the trioctahedral character of the clay. Electron-optical investigation shows that oxidation within the octahedral sheet creates a juxtaposition of dioctahedral and trioctahedral sub-lattices in the same particle. Ferrous smectite authigenesis is located near the hydrothermal spring outlets of the Atlantis II Deep.

Aluminium-substituted goethites are found in many soils and can also be synthesised readily in the laboratory. In recent years, synthetic substituted goethites have been examined by various techniques including XRD, IR, TEM and dissolution kinetics (Thiel, 1963; Jonas & Solymar, 1970; Fey & Dixon, 1981; Fysh & Fredericks, 1983; Schulze & Schwertmann, 1984; Schwertmann, 1984). Transmission electron microscopy (TEM) studies have shown that as Al substitution rises above 10%, the goethite needles become shorter and also thicker in the a direction. Furthermore, crystals which at zero substitution consist of domains parallel to the c axis become less domainic with increasing Al substitution (Schulze & Schwertmann, 1984).

The ability of clay minerals to adsorb heavy metal cations is an important property in the context of the increasing contamination of the aquatic environment and soils by toxic waste. Laboratory studies of the sorption of heavy metals by clays have mostly been carried out using montmorillonite and illite (Chester, 1965; Egozy, 1970; Bourg & Filby, 1974; Helios-Rybicka & Schoer, 1982), less frequently for kaolinite, and only rarely for mixed-layer illite-montmorillonite (Helios-Rybicka, 1983) and the fibrous clay minerals sepiolite and palygorskite. This note describes the sorptive behaviour of sepiolite towards Ni, Zn and Cd.

In 1947, based on field observations and some chemical data, the Leping white earth was first recognized as a sepiolite clay of marine origin and of early Permian age. Later investigations in the 1960s using DTA, XRD and EM confirmed this identification (Zhang Renjun, 1984). Recently, in the ‘Science Spring’, new sepiolite clay deposits have been found in south China and it has become apparent that sedimentary sepiolite in this region is closely connected with the marls of the Lower Permian system.

The object of this study was to find a structural explanation for the corrosion figures produced in biotites having different chemical compositions and to apply this information to other di- or trioctahedral micas. The biotites investigated were used in earlier studies of epitaxial growth, so that previous knowledge of their surface behaviour was available. The biotites studied were from Evje, Norway (number 1), Guomum, Czechoslovakia (number 2), and Tanokami Yama, Japan (number 3). All were very dark in colour due to their high iron content.

The presence of palygorskite in soils and sediments of the Middle East countries has been widely reported: in Syria (Muir, 1951), Israel (Yaalon, 1955; Barshad et al., 1956; Ravikovitch et al., 1960; Singer, 1971; Singer & Amiel, 1974; Yaalon & Wieder, 1976), Iraq (Al-Rawi et al., 1967; Eswaran & Barzanji, 1974), Lebanon (Lamouroux et al., 1973), Jordan (Wiersma, 1970; Shadfan, 1983; Shadfan & Dixon, 1984; Shadfan et al., 1985) and Saudi Arabia (Aba Husayn & Sayegh, 1977; Elprince et al., 1979; Mashhady et al., 1980; Viani et al., 1983; Lee et al., 1983; Mackenzie et al., 1984; Shadfan & Mashhady, 1985). In Egypt, Elgabaly (1962) indicated the predominance of palygorskite in some of the soils in the western desert and proposed that the mineral had formed from calcareous argillaceous material in a saline lagoon. Hassouba & Shaw (1980) found palygorskite to be a major component of the clay fraction in caliche crusts in the coastal plain of north-western Egypt and considered that the palygorskite had formed authigenically in that environment.

L'analyse semi-quantitative, avec une microsonde électronique CAMEBAX, met en évidence des variations chimiques significatives dans des assemblages simples de minéraux argileux. La méthode est décrite et deux applications sont présentées et interprétées en termes de genèse et de paléoenvironnement. La microgéochimie d'illites et de chlorites du Crétacé inférieur (Marge du Maroc) évolue vers le haut de la série (diminution du magnésium) et semble influencée par une diagenèse salifère transformant des argiles détritiques. Des chlorites hypermagnésiennes apparaissent dans les sédiments les plus profonds, et autorisent l'hypothèse d'une authigenèse en milieu confiné. Les différentes smectites étudiées a la limite basalte-sédiment des sites 105 et 384 du DSDP (NW Atlantique) peuvent aisément être distinguées par leur microgéochimie avec Mg, Na, K, Al, Fe, Ti. Elles reflètent diverses sources volcaniques et leurs caractères sont très différents de ceux des smectites terrigènes communes.