A green smectitic mineral from Jordan with 16% Cr2O3 (almost 22% on an ignited basis) has been shown by chemical, X-ray, electronoptical, electron microprobe, infrared and thermoanalytical evidence to be an iron-free volkonskoite of composition:

The octahedral occupancy of 4·7, with only Mg2+ and Cr3+ in octahedral positions (in a ratio of 1·14), suggests that the mineral is intermediate between di- and trioctahedral—an inference supported by the uniformity of composition of individual particles (as revealed by the microprobe) and IR evidence, which indicates that it is predominantly dioctahedral but with certain trioctahedral characteristics. A volkonskoite sample from the type area in the USSR, examined for comparison, contained 23·5% Cr2O3 (about 29% on an ignited basis), but proved on electron microprobe examination to be a mixture of at least three species, one of which was aluminium-rich. IR and thermoanalytical characteristics are discussed in relation to the existence of tri-dioctahedral species.

Clay minerals of several hydrothermally altered zones in a 1200-m biotite-granite core from a drillhole in northern Switzerland were studied microscopically, by XRD and by electron microprobe. The minerals principally affected by the hydrothermal alteration were plagioclase (An5-An20) and, to a lesser extent, biotite. Illite, regularly interstratified illite-smectite and dioctahedral chlorite-smectite, dioctahedral chlorite, trioctahedral chlorite and kaolinite were detected in the alteration products. Commonly, two or more clay minerals occurred together in pseudomorphs after plagioclase. The mineral chemistry of the clay minerals showed a predominance of the substitution KAl for Si and, to a lesser extent, MgSi for Al. Fluid-inclusion data and the absence of pure smectite and epidote indicated temperatures of ∼200°C for the fluid that caused this alteration.

Olique-texture electron diffraction examination of K-saturated dioctahedral smectites after 70–100 wetting-drying cycles allows the determination of cation distribution between trans and cis octahedra of the 2:1 layers. Studies of smectites with different compositions has revealed a wide variety of occupancies of the available octahedral sites.

Continuous nodular limestone beds in north-west European shale sequences developed during early diagenesis via anaerobic, bacterial catabolysis of organic matter which yielded both carbonate and sulphide. The high carbonate content of the nodular limestones (50–90%) demonstrates that they developed in the upper tens of metres of the sediment column. In addition, each of the nodular limestones displays a centrifugal decrease in carbonate, indicating that growth was synchronous with compaction of the host ooze. The chemistry of the carbonate changed with time, becoming more rich in magnesium and iron and reflecting a progressive change in the porewater chemistry from its initially marine composition. The earliest precipitate was calcite containing < 1 mole% MgO; later precipitates contained several percent MgO and ultimately dolomite was precipitated. Similarly, iron content of the carbonate increased as precipitation continued and sulphate reduction, and thus pyrite production, declined. The early diagenetic cementation of many marine sandstones followed a pattern similar to that of nodular limestones and much of the carbonate was also derived from organic matter via bacterial oxidation. Thus, early diagenetic bacterial oxidation of organic matter may not only reduce the source potential of shales but also reduce the reservoir potential of associated sandstones, or those more distant which lie along compactional-fluid pathways.

Synthetic ferrihydrites with Si added at mass ratios of Si/Fe = 0, 10, 20, 40, 80, and 160 × 10−3 have been studied to examine the effect of Si on the properties of ferrihydrite. The Si proportionally reduces the point of zero charge but influences surface area, loss of weight to 800°C and solubility in acid oxalate solution to varying degrees. At a critical concentration of Si (when Si/Fe = 20 × 10−3) these measured properties indicate a greater particle size and a more ordered structure. The relationship between synthetic ferrihydrite and the surface of kaolinite has been studied by a combination of surface area and charge determinations and TEM. An attraction predicted at pH 3 between ferrihydrite particles and the basal surface of kaolinite has been confirmed, and resulted in a stable coating. However, at pH 6 any attraction between the minerals appears to be too weak to produce a coating of the kaolinite surfaces.

The Corallian Beds beneath the Harwell Research Site, south Oxfordshire, are a highly variable sequence of sandstones, mudstones and limestones. The more permeable lithologies of the upper part of the sequence constitute an important local aquifer. Diagenetic and post-lithification processes have strongly influenced the porosity, permeability and mineralogy of the aquifer rocks. Calcite cementation reduced the porosity and permeability of the sandstones and limestones in the upper part of the aquifer. Cementation occurred in at least three stages ranging from early diagenesis to post-compactional diagenesis. Late-stage dissolution of calcite took place along fractures and bedding discontinuities, restoring the porosity and permeability of these sediments and developing secondary porosity and permeability where original clastic carbonate was removed. Invasion of the Corallian porewaters by waters charged with CO2 is postulated as a mechanism by which calcite was removed. At the base of the aquifer, early diagenetic dissolution of biogenic silica created a high secondary porosity. Silica was reprecipitated in the dissolution voids and matrix as opal-CT. Authigenic smectite and zeolites are also associated with opal-CT. These phases are believed to have precipitated from pore-waters rich in ions derived from the alteration of volcanogenic detritus associated with the Corallian sediments.

A new conceptual model for interstratified clays is proposed, based on complementary use of the transmission electron microscope (TEM) and X-ray diffraction (XRD). Clays yielding interstratified XRD patterns are shown to be composed of aggregates of fundamental particles (here defined as an individual or free particle which yields a single crystal pattern by electron diffraction) whose interfaces are capable of adsorbing water and organic molecules. In these circumstances, the interstratified XRD character results from an interparticle diffraction phenomenon. Smectites are shown to be primarily composed of particles 10 Å thick corresponding to elementary 2:1 silicate layers. Nearest-neighbour regularly interstratified illitesmectite (K-rectorite) is composed of particles of elementary ‘illite’ 20 Å. thick, corresponding to two silicate layers coordinated by a single plane of potassium ions. Regularly interstratified chlorite-smectite (corrensite) consists of particles 24 Å thick corresponding to two silicate layers coordinated by a single brucitic sheet. The layer sequence examined by XRD is determined by the types and proportions of particles present within the clay material. Thus a sedimented aggregate of 20 Å elementary ‘illite’ particles appears to be a regularly alternating sequence of illite and smectite layers by XRD. Randomly interstratified itlite-smectite and chlorite-smectite can be synthesised by making mixed suspensions of the <0·1 µm fractions of smectite with those of K-rectorite and corrensite respectively, providing experimental confirmation of the proposed model. Unusual associations, such as three-component illite-chlorite-smectite interstratified systems can be synthesised by making mixed suspensions of K-rectorite and corrensite. Long range regularly interstratified illite-smectite of the IIS and IIIS types are composed primarily of fundamental ‘illite’ particles 20–50 Å thick. Conventional illite is composed primarily of particles > 50 Å thick. This conceptual model has implications for the chemistry, behaviour and genesis of interstratified clays which are common constituents of soils and sediments.

Mineralogical properties (XRD, DTG) and chemical compositions of some chlorite-smectite interlayer minerals in alteration products of ophiolitic rocks from the Northern Apennines are presented and discussed. The presence of iron hydroxides and the continuous variation in Al2O3 content suggest that the corrensites are intermediate stages in the process of alteration of chlorite to smectite in an environment characterized by high element mobility. A thermal test to characterize the swelling component of the interlayer species is proposed.

La relation entre la surface développée par les particules d'une poudre et leur paramètre de taille et de morphologie est abordée de façon théorique, rigoureuse et générale. A l'aide d'un nouveau choix des grandeurs mesurées, il est possible de ne prendre en compte qu'une seule distribution statistique, de l'un des paramètres, les autres paramètres n'apparaissant dans la formulation qu'à travers leurs moyennes arithmétiques. Dans le cas de plaquettes, le problème est analysé de façon théorique et expérimentale. La formulation est encore simplifiée et on voit que la formule la plus simple consistant à négliger surfaces latérales et distribution de taille est finalement parmi les meilleures, les deux approximations se compensant.

Double-cation hydroxy compounds, generally with the pyroaurite-type structure, were synthesized by a reaction involving the hydrolysis of a cation in solution by a fully hydrolysed and precipitated hydroxide of a second metallic cation. Synthesis of this type of compound is not limited to reactions between di- and trivalent cations as previously thought but can, in this process, involve divalent-divalent, divalent-tetravalent and trivalent-trivalent cations. Crystalline compounds with reasonably sharp XRD patterns formed generally within a few hours at 25°C at pH 4–8·5 depending on the cations involved. The technique presents a more rapid method of formation of these compounds than previously described syntheses, and has been called induced hydrolysis because the fully hydrolysed cation causes complete hydrolysis of the second cation at a pH below that at which this would normally occur. The minerals takovite, Ni6Al2(OH)16CO3·H2O, and hydrotalcite, Mg6Al2(OH)16CO3·H2O, were amongst some of the compounds formed and the reaction is considered to be more realistic in terms of conditions encountered in natural environments than previous syntheses. The reactions described could also be expected to affect the mobility of heavy elements in the environment. These double hydroxy compounds convert on heating between 300–500°C to double-metal oxides with the spinel structure.

Authigenic clay mineral formation in fluvial sandstones and one brackish sandstone from the lower and middle Coal Measures of the East Midlands has been studied using XRD, optical microscopy and a variety of electron optical techniques. The sandstones are quartz-rich with minor feldspar and mica. The clay minerals present are kaolinite, chlorite and illite; they form 10–15% by volume of most of the sandstones. Both detrital and authigenic illite and chlorite are present but the kaolinite is entirely authigenic. Kaolinite and illite authi-genesis has been important in all the sandstones but authigenic chlorite is only abundant in the phyllosilicate-rich Silkstone sandstone. The controlling factors in the diagenesis of the sandstones were primary composition, environment of deposition, bed thickness, pore-fluid migration and composition, pressure and temperature. These interacting factors have been of varying importance in time and space in the diagenesis of the sandstones. Authigenic clay morphology appears to have been controlled by time and environment of formation, pore-fluid chemistry and, possibly, parent minerals.

A modified HGMS system is described in which the inclusion of a peristaltic pump to continuously circulate a suspension of a soil clay through the magnetic trap greatly facilitates the operation of the technique. Application of this purely physical method of mineral separation to a variety of soil clays—using IR spectroscopy to monitor the progress of the separation—illustrates the ease with which magnetic fractions comprising different iron-containing clay minerals may be separated from a single soil clay, thereby making these minerals more accessible for further study by other techniques.

Bulk mineralogy, clay fraction mineralogy and layer ratio in interstratified illite-smectite were studied in three Upper Miocene profiles, up to 200 m deep and representing very low sedimentation rates (∼80 mm/103 yr) with environments of sedimentation ranging from hypersaline (gypsum deposits) to brackish. No correlation was found between mineralogy and depth, or between mineralogy and chemical environment of deposition, despite the use of precise identification techniques. It is concluded that smectites and interstratified illite-smectites are stable in marine sediments until the elevated temperatures of deep diagenesis are reached.

Dans les encroûtements calcaires sous galet de la Plaine Poitevine, nous avons mis en évidence, par des méthodes microscopiques, une séquence minéralogique évolutive de la calcite. Par spectrométrie d'absorption infrarouge des éléments de la séquence prélevés en hiver, nous caractérisons, à côté de la calcite (bande à 1420 cm−1), du carbonate amorphe hydraté (bandes à 1405 et 1485 cm−1 et un autre type de calcite (bande à 1445 cm−1). Les résultats d'essais de laboratoire nous permettent d'envisager une évolution du carbonate amorphe vers la calcite par l'intermédiaire d'une phase calcitique transitoire, et d'aragonite toujours en faible proportion. La formation de ces carbonates est liée aux conditions bio-physico-chimiques du sol.

Transmission electron microscopic (TEM) examination of the <0·1-µm fraction of montmorillonite and regularly interstratified illite-smectite (I-S) shows that these clays, when dried from suspension, consist primarily of particles 10 Å and 20 Å thick respectively. However, X-ray diffraction (XRD) examination of sedimented aggregates of montmorillonite indicate that the effective number of unit cells that are diffracting coherently is ∼9. This discrepancy can be reconciled by postulating an interparticle diffraction effect from the sedimented aggregates of oriented particles. The interfaces of these particles are capable of adsorbing water, ethylene glycol etc. so that on this basis smectite is composed of elementary silicate particles 10 Å thick, and regularly interstratified I-S is primarily composed of elementary ‘illite’ particles 20 Å thick, values which are in agreement with the TEM observations. This concept is confirmed experimentally by XRD examination of sedimented aggregates from mixed suspensions of both materials; the resulting patterns are identical to those of randomly interstratified illite and smectite layers, which indicates that the layer sequence examined by XRD has been entirely rearranged. It is concluded that the use of XRD peak breadth to determine mean crystal thickness cannot be reliably applied to these systems. Standard XRD data from sedimented aggregates may not be able to distinguish between true interstratification and interparticle diffraction effects of intimate physical mixtures.

Les vermiculites magnésiennes chauffées durant plus de 24 h à une température de 700°C dans l'air, donnent une phase non gonflante qui est un biopyribole. Les diagrammes de diffraction X obtenus avec un monocristal montrent que ce matériau est constitué de deux phases: une enstatite en épitaxie sur le plan (010) d'un phyllosilicate. Le phyllosilicate a un empilement des feuillets semi-ordonné. Les rangées du plan (h0l) éant entièrement discrètes, la projection x0z a été étudiée par une analyse de Fourier classique à deux dimensions. Les autres rangéés du réseau réciproque sont plus ou moins diffuses dans la direction c*. Le désordre est creéé par une distribution aléatoire de trois translations entre les feuillets adjacents. Les relations structurales existant entre le phyllosilicate et l'enstatite expliquent l'absence de propriétés de gonflement de la phase phyllosilicate du biopyribole.

Mössbauer spectra of Fe2+Fe3+ and Fe2+Al hydroxycarbonates taken in the paramagnetic state were fitted using two Fe2+ and Fe3+ doublets each. These spectra can be interpreted in terms of an ordered cation arrangement in the pyroaurite-type structure. The inner Fe2+ doublet seems to be the outcome of excess Fe3+ in the samples studied. The outer Fe2+ doublet (quadrupole splitting ∼2·8 mm s−1, FWHM = 0·26 mm s−1) is relatively well-defined and can, together with the fact that this doublet disappears on oxidation, possibly serve to identify these compounds in natural systems.

The thermal decomposition of stearic acid in the presence of various clay minerals was studied in an open, inert system under isothermal (250°C) conditions. The distribution of hydrocarbons and the amount of CO2 evolved were determined. The degree of decarboxylation depended on the composition of the octahedral sheets of the clays. It was greater with trioctahedral than with dioctahedral minerals. The presence of dioctahedral minerals tended to reduce the amount of CO2 produced below that obtained with the neat acid. The nature of the hydrocarbons formed depended on the structure of the minerals. Different products were obtained with minerals that have accessible interlayers and channels (smectites, palygorskitesepiolite) than with the 1:1 clays and pyrophyllite or talc. Some minerals caused cracking to light-weight hydrocarbons. Higher surface acidity of the clay interlayers enhanced this process but was not the only decisive factor. In the systems studied, evolution of CO2 did not necessarily precede the formation of light- or medium-weight hydrocarbons from the parent acid.

L'intérêt porté à la synthèse de la kaolinite a été suscité par des préoccupations d'ordre pédologique, qui nous ont amenés à comparer les expériences réalisées en laboratoire aux situations naturelles considérées comme propices à la formation de la phyllite 1/1. Il apparaît essentiel, pour le succès des synthèses, de privilégier la formation d'ions hydroxylés monomères de Al, pouvant réagir avec l'acide orthosilicique. La neutralisation lente et partielle de solutions très diluées semble garantir ces conditions. Deux séries d'essais ont été entrepris. Les résultats expérimentaux tendent à démontrer la possibilité d'obtenir une précipitation de kaolinite à partir des ions en solution et par simple neutralisation. Le mécanisme proposé par Siffert (1962), mettant en jeu la formation d'éléments T-O, semble le plus probable. Ces éléments prendraient naissance par réaction de l'acide orthosilicique et de l'ion Al(OH)2+. Le rapprochement entre l'importance du rôle joué ici par ce monomère hydroxylé et la place qu'il occupe dans le schéma de dissolution établi par Sarazin (1979) peut permettre de comprendre que les néoformations de kaolinite correspondent à des sols relativement bien drainés.

Middle Jurassic sandstones contain a variety of authigenic clay materials. The origin and distribution of these are related to the influence of several factors, principally depositional pore-fluid chemistry, sandbody geometry and the migration of aggressive fluids. Pore-lining illite, pore-lining chlorite and pore-filling vermiform kaolinite in the Ravenscar Group mutually exclude each other because of depositional pore-water chemistry; seawater in the case of the illite, anoxic freshwater in the case of chlorite and oxygenated freshwater in the case of the kaolinite. Blocky pore-filling dickite occurs ubiquitously within the large connected sandbodies. Its origin may be related to the migration of aggressive fluids and its distribution to depositional sandbody geometry. Mixed-layer chlorite-vermiculite also occurs, and is believed to have formed from chlorite during Recent weathering. Similar patterns occur in Ninian Field Brent Group sandstones, although the situation is more complicated (in these sandstones the kandite subgroup minerals are undifferentiated in this paper). Here too, pore-lining illite occurs in marine sandstones and pore-filling vermiform kandites in non-marine sandstones. However, vermiform kandites also occur in the marine sandstones, perhaps due to freshwater-table development following progradation. The more blocky kandites occur in large connected sandbodies. Finally, a second phase of illitization occurs, postdating blocky kandites, perhaps caused by alkaline formation waters. The occurrence of chlorite in the Broom Formation is anomalous and its possible origin is discussed.