The clay fractions from a 1191-cm long sediment core in the SW Basin of the Atlantis II Deep, Red Sea, were investigated by XRD, electron microscopy and chemical analysis. Talc dominates in the botton portion of the core, near the brine discharge vent. At 1183 cm depth, the clay consists of vermiculite/chlorite and chrysotile. These minerals are of hydrothermal origin and two possible formation pathways are proposed: (i) vermiculite/chlorite and chrysotile formed by the submarine alteration of previously deposited talc; (ii) vermiculite/chlorite and chrysotile authigenically precipitated as a result of changes in the chemical composition of the brine. At 1170 cm depth, a new depositional sequence results from the progressive alteration of swelling 2:1 minerals into vermiculite. At 1025 cm, Mg-rich clay minerals such as chlorite, chrysotile and talc again become prominent. The upper part of the core is characterized by a transition from non-expanding Mg-rich clay minerals to Fe-rich expanding clays, principally nontronite. Periodically, the content of well-crystallized oxides such as hematite in the layers increases. At 1025 cm, some of the Fe-oxides have a morphology similar to that of akaganéite. In the uppermost part of the core, iron oxides appear to consist of a poorly crystalline hydrothermal hematite. An attempt has been made to correlate the various mineralogical assemblages geochemically.

A new approach to understanding illite-smectite interstratification is formulated which incorporates the concepts of interparticle diffraction and elementary illite particles. The analysis shows that nearest-neighbour ordering, non-nearest-neighbour ordering, ordering at the illite end of the composition scale, and the lack of ordering at the smectite end can be explained by expandable interfaces between elementary, i.e. two silicate layers, and larger illite particles. Ordering in I/S clays is significant only when the probability of occurrence of illite is ∼80%. New concepts proposed are an order parameter (ω) and an expansion parameter (β). Interparticle diffraction implies that I/S clays are unusual among all crystalline materials since their XRD size is equal to or larger than the size of their constituent physically separable particles.

Absorption lines in the g ∼ 4 region of ESR spectra of three New Zealand halloysites were found to originate from similar paramagnetic centres as in kaolinites. Resonances in the g ∼ 4 region were not common to all the halloysites, although the signal at g = 2·0 for the Matauri Bay halloysite is probably due to a trapped positive hole or trapped ion. A 6-line resonance produced by Te Puke halloysite and removable by Na-saturation is attributed to exchangeable Mn on the clay surface. Low-temperature lines exhibited by all samples are due to V4+ or VO2+ replacing Al3+ in the octahedral sheet.

Sorption isotherms of ethylene glycol monoethyl ether (EGME) on homoionic Na−, K−, Mg− and Ca-montmorillonites at 25°C were determined. The extent of sorption was found to depend on the exchangeable cations, being larger for the bivalent ionic forms. Uptake of EGME was compared with the retention value obtained by evacuation of the sample wetted with EGME and also interpreted from XRD data. The results indicate that the montmorillonite-EGME complex formed by sorption at the saturation vapour pressure is stable and less ambiguous than after wetting by EGME. Montmorillonites are not suitable reference materials for surface area measurements.

Hofmann & Klemen ont observé qu'en chauffant modérément (200-220°C) une smectite dioctaèdrique à substitutions octaàdriques saturée par du lithium, celle-ci perdait sa capacité d'échange cationique et son pouvoir de solvatation. Les auteurs ont attribué cet effet à la migration des cations Li vers la couche octaèdrique. Nous avons, en utilisant essentiellement la diffraction X aux grands angles et la microscopie électronique, caractérisé de manière quantitative la structure d'une smectite à déficit de charge mixte (c'est-à.-dire ayant 30% de substitutions dans les couches téraèdriques et 70% dans les couches octaèdriques) saturée par du nickel, avant et après chauffage. Nous avons ainsi pu montrer en éudiant les réflexions 00l qu'après un chauffage à 400°C, les cations interfoliaires migrent vers la cavité octaèdrique vacante pour compenser la totalité des charges négatives dues aux substitutions Al → Mg et qu'il existe une distribution inhomogène de la charge entre les couches octaèdriques et tétraèdriques au sein d'un même cristal. Certains feuillets ont des substitutions uniquement dans la couche octaèdrique, d'autres uniquement dans la couche tétraèdrique, et toutes les possibilités peuvent exister entre ces deux pôles extrèmes.

A lamellar macroscopic Mg-vermiculite from Santa Olalla, Spain, which is accompanied by some talc and mica, shows a reversible expansion on glycolation due to the presence of layers with two different charges. The most probable origin of the vermiculite is by low-temperature hydrothermal transformation of phlogopite.

The reductive dissolution by Na-dithionite of 28 synthetic goethites and 26 hematites having widely different crystal morphologies, specific surfaces and aluminium substitution levels has been investigated. For both minerals the initial dissolution rate per unit of surface area decreased with aluminium substitution. At similar aluminium substitution and specific surface, goethites and hematites showed similar dissolution rates. These results suggest that preferential, reductive dissolution of hematite in some natural environments, such as soils or sediments, might be due to the generally lower aluminium substitution of this mineral compared to goethite.

The volatiles produced when various clay minerals were pyrolysed under a stream of He or in vacuo were condensed and analysed. They contain a wide range of cations and anions in concentrations ranging from ppm to hundreds of ppm. The reactivity of these volatiles was demonstrated by their reaction with calcite. It is suggested that the water liberated on heating acts as a scavenger of various impurities present in the clay.