Micropore distribution and effective size of the channels of natural sepiolite from Turkey were measured by the BET method. Before the BET measurement, the samples were treated under a water vapour atmosphere at various pressures to fill progressively the sepiolite micropores with water. The surface areas measured by means of N2 adsorption decreased with increased vapour pressures of water. The outer surface area was estimated by comparison of the surface area of the vacuum-dried sepiolite with that filled with adsorbed water. The total surface area was ∼290 m2/g, and the outer surface area was 170 m2/g, the difference being attributed to the structural micropores of the sepiolite. The ratio of the surface areas possessed by the channels and that of the outer surface suggest that the mean thickness of the sepiolite fibre was ∼12 nm. The effective size of the channels was estimated from the number of various-sized molecules sorbed by the sepiolite, the results showing that molecules larger than benzene could not migrate into the channels.

Le dosage de l'opale biogène dans les sédiments marins peut être effectué par diffractométrie X par la mesure des surfaces des bandes de diffusion et par la mesure des surfaces des pics de cristobalite. La surface de la bande de diffusion, observée sur les diagrammes X de sédiments non chauffés, caractérise la concentration en composés amorphes (organiques et inorganiques). Elle permet de quantifier l'opale biogène de sédiments ne contenant que peu ou pas d'amorphes non biogènes. La surface du pic de cristobalite, obtenu par transformation de l'opale biogène par chauffage à 1050–1100°C pendant 12 h, permet de doser la silice biogène de divers sédiments. Ces deux approches méthodologiques ont été appliquées aux sédiments d'une carotte (MD 84–527) prélevée dans le secteur ouest-Indien de l'Océan Austral. Ce matériel, qui a enregistré la sédimentation des 40,000 dernières années, ne renferme qu'accessoirement des constituants amorphes volcaniques. Les concentrations en silice biogène sont obtenues avec une précision de ±10%. Le comptage des diatomées par observation au microscope a été également réalisé sur les mêmes sédiments. Les courbes des teneurs en opale biogène obtenues par les deux méthodes sont comparables et se corrèlent bien à l'abondance totale des diatomées.

In solutions of three different, stable, Al-Fe(III) hydroxy species, reaction with silica particles for one month decreased the solution pH and gave deposits containing Fe, Al and Si over the surfaces of the particles, joining them together. Infrared difference spectroscopy showed the deposits to be similar to imogolite or feldspathoid phases, depending on the composition of the initial solution. Comparative SEM and EDXA elemental analysis profiles supported the conclusion that these laboratory products were similar to naturally occurring soil cementing agents. An explanation is given for the observed reactions and the formation of materials likely to act as cementing agents in soil.

The thermal behaviour of sepiolite and palygorskite treated with butylamine was investigated by DTA, TG and EGA-MS under a flow of air and N2. It was shown that the amine was adsorbed by the clays, penetrating into the pores and replacing zeolitic and bound water. The presence of amine in the pores shifted the dehydroxylation peak to temperatures higher than for the untreated clays. Under N2 the thermal desorption of the amine from sepiolite was observed at 175, 275 and 525°C, and from palygorskite at 170 and 270°C. In addition to desorption, reactions of pyrolysis and condensation to charcoal were detected by the evolution of NH3, CH4 or H2, respectively. In air, oxidation of the organic matter led to the appearance of exothermic peaks, the temperatures of the most intense peaks being determined by the rate of C oxidation, but the shape of the DTA curves and the temperatures of exothermic shoulders were determined by the rate at which H from the organic molecules combined with O from the air.

A thin (15–20 cm) Fe-rich intraformational conglomerate in the fresh- to brackish-water Early Cretaceous succession of south-east England contains pisoids and pseudo-ooids of a green clay mineral which has been identified as berthierine. The pisoids (up to 0·5 cm in size) are concentrically laminated, have complex structure, and are frequently nucleated around a sandstone fragment. They show evidence of weathering due to a change in their environment after formation. The ooids have well to moderately well developed concentric lamination, are very often multi-nucleated and do not always completely cover the nucleus. They suggest formation as a result of mineral precipitation as opposed to agitation and accretion. The climate and environment during the Wealden were suited to the formation of Fe-rich soils and it is likely that the pisoids and ooids were originally composed of iron oxyhydroxide and kaolinite, having formed in soil in the Weald Basin. They were then eroded and deposited in the base of a large scour structure as part of the conglomerate. On burial, and the attainment of low Eh conditions, berthierine formed by diagenetic transformation of the pre-existing minerals.

It has been shown that clay minerals transform into (Mg, Al) or (Ni,Al) serpentine-like phases if treated hydrothermally in the presence of MgCO3 or NiCO3 (Shitov et al., 1974, Frank-Kamenetskii et al., 1978, 1983a,b; Kotel'nikova et al., 1976; Varela et al., 1983; Kotov et al., 1985; Ryumin et al., 1978) and that the polytypism of the initial kaolins is inherited by the products. It has also been shown that (Mg,Al) serpentine-like phases react with KC1 to form dioctahedral micas and Mg-rich serpentine phases, but the latter do not react to trioctahedral micas (Kotel'nikova et al., 1976). Consequently, it was desirable to look for mechanisms for obtaining trioctahedral micas from serpentine-like phases.