Several types of authigenic illite differing in their distribution and morphology are described in Rotliegend (Permian) gas-bearing sandstones of northern Germany. The microtextural environment seems to control the K-Ar age variations of illite-fractions of different sizes. Thin, uncompacted, and radial coatings are 200 Ma old, whereas uncompacted fibrous laths filling enlarged secondary porosity are 180 Ma old, and compacted or recrystallized tangential particles yield intermediate or even younger ages. Such Liassic ages relate to a thermal event, the effects of which on the clay material have not been erased subsequently, despite present-day burial depth of 4500 m and temperatures of 150°C.

Lower Palaeozoic rocks crop out extensively in Wales, the Lake District of northern England and the Southern Uplands of Scotland; they also form the subcrop concealed beneath the English Midlands and East Anglia. These mainly marine sedimentary rocks were deposited in basins created during plate tectonic assembly of the various terranes that amalgamated to form the British Isles, 400-600 Ma ago. Final amalgamation occurred during the late Lower Devonian Acadian Orogeny when the basins were uplifted and deformed, producing belts of cleaved, low-grade metasediments, so-called slate belts, with a predominantly Caledonian (NE-SW) trend. The clay mineralogy of mudrock lithologies - including mudstone, shale and slate - found in these belts is reviewed. Using X-ray diffraction data from the <2 μm fractions of ~4500 mudrocks samples, clay mineral assemblages are summarized and discussed in terms of diagenetic and low-grade metamorphic reactions, and the metapelitic grade indicated by the Kübler index of illite crystallinity.

Two sequences of clay mineral assemblages, or regional assemblages, are recognized. Regional Assemblage A is characterized by a greater diversity of clay minerals in assemblages from all metapelitic grades. It includes K-rich, intermediate Na/K and Na-rich white micas, chlorite and minor amounts of pyrophyllite. Corrensite, rectorite and pyrophyllite are found in the clay assemblages of contact or hydrothermally altered mudstones. K-white micas are aluminous and phengite-poor, with b cell dimensions in the range 8.98-9.02 Å. Regional Assemblage B has fewer clay minerals in assemblages from a range of metapelitic grades. Phengite-rich K-mica is characteristic whereas Na- micas are rare, and absent in most assemblages; chlorite is present and minor corrensite occurs in mudrocks with mafic-rich detritus. Minor amounts of kaolinite are sporadically present, but dickite and nacrite are rare; pyrophyllite and rectorite are generally absent. The b cell dimensions of K-white mica in Regional Assemblage B are in the range 9.02-9.06 Å. The two regional assemblages are found in contrasting geotectonic settings. Regional Assemblage A is characteristic of the extensional basin settings of Wales, the northern Lake District and the Isle of Man. These basins have a history of early burial metamorphism associated with extension, and syn-burial or post-burial intrusive and extrusive volcanic activity. Intermediate Na/K mica probably developed from hydrothermal fluids generated around submarine volcanic centres. Deep diagenetic and low anchizonal clay mineral in these basins may develop a bedding-parallel microfabric. Chlorite-mica stacks also occur in the extensional basins and the stacking planes represent another type of bedding-parallel microfabric. Both types of microfabric are non-tectonic and developed by burial during the extensional phase of basin evolution. Regional Assemblage B is developed in the plate-convergent settings of the Southern Uplands and the southern Lake District. In the accretionary complex of the Southern Uplands the processes of burial diagenesis, metamorphism and tectonism were synchronous events. In both plate- convergent basins, low temperatures and tectonic fabric-formation had an important role in clay mineral reactions, whereas hydrothermal fluids played no part in clay genesis.

Les vermiculites Mg, Ca, Sr et Ba ont été étudiées par diffraction des rayons X et absorption infra-rouge.

Pour les vermiculites monocouches, l'espacement interréticulaire est compris entre 12·2 et 11·45 Å.

On distingue: (i) les vermiculites Ba; le cation, entouré de 5 à 6 molécules d'eau, est à environ 1Å du plan médian et il intéragit avec un hydroxyle structural du feuillet le plus proche, (ii) les vermiculites Mg, Ca et Sr; le cation, entrouré de 3 ou 4 molécules d'eau,est dans le plan médian ou dans son voisinage (0·3 Å), et il n'interagit pas avec les hydroxyles des feuillets.

Pour les vermiculites anhydres, l'espacement interréticulaire est inférieur à 10 Å, le cation est dans le plan médian et il intéragit avec les hydroxyles de 2 feuillets opposés.

A partir de ces résultats, complétés par les données de la bibliographie, des modéles sont proposés pour représenter l'organisation de l'espace interlamellaire.

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.

Mixed-layer illite-smectite from Lower Palaeozoic sedimentary rocks, on both sides of the Teisseyre–Tornquist tectonic zone (TTZ) in northern Poland, was studied by X-ray diffraction and dated by K-Ar means.

The percentage of smectite layers in illite-smectite (%S) indicates maximum palaeotemperatures of 125–135°C at the surface of Lower Palaeozoic rocks on the craton (NE of TTZ), and 110 to ≤180°C in different tectonic blocks to the SW of TTZ (area of Caledonian consolidation). The vertical changes in the %S indicate that the maximum palaeotemperatures were reached before Permian time on the craton, and before Jurassic, Triassic, Permian or Carboniferous periods but after the beginning of Devonian time in the Caledonian zone. The K—Ar ages of bentonites indicate that the maximum palaeotemperatures were reached by 370—390 Ma or even slightly earlier (Middle—late Devonian). A maximum of 3—6 km of Silurian–Devonian cover on the craton and in the TTZ is suggested by the data.

Le mécanisme de dispersion dans l'eau d'une smectite a été étudié en fonction de 2 paramètres: la nature du cation interlamellaire, la nature du feuillet de smectite. Pour cela, on a observé l'évolution du diagramme de diffusion aux petits angles en fonction de la température (entre −15°C et la température ambiante) d'une série de dispersions contenant 20 g d'argile dans 100 g d'eau. La méhode d'interprétation des diagrammes est basée sur la comparaison entre courbes expérimentales et calculées. Les résultats obtenus montrent le rôle, dans l'aptitude au gonflement de la smectite, du cation interlamellaire, de la charge cu feuillet et de sa localisation. Ils montrent également quelles caractéristiques de la structure sont conservées entre la dispersion finale et le solide initial, ce qui permet d'en déduire le mécanisme de dispersion pour chaque smectite.

Fluid inclusion studies from two contrasting fields in the Northern North Sea reveal hydrostatic pressure conditions during the formation of secondary quartz overgrowths which trapped both brine and petroleum inclusions. In these fields, dating of authigenic illite, part of the diagenetic sequence and broadly coeval with quartz cementation, permits comparison with modelled pressure conditions. Pressure modelling indicates the likelihood of several periods of overpressure in the past, interspersed with normal pressure. The timing of diagenesis coincides with one of these periods of hydrostatic pressure. Furthermore, as both fields are now highly overpressured, the cause and timing of overpressure generation post-date the diagenesis. A model is proposed to link the pressure history and diagenesis.

In the Maláiguide Complex (Betic Cordilleras, Spain), dickite is widely developed in clastic Permo-Triassic sequences. The lateral extent stretches at least 300 km along the Betic range, whilst vertical extent is variable and appears limited to the lowest 20–150 m of these sequences. Textural, chemical and crystallochemical characteristics of the dickite, illite/mica and chlorite in dickite-bearing rocks and in the overlying (Permo-Triassic) and underlying (Carboniferous) rocks have been investigated to determine the approximate conditions in which dickite has developed. Using the chlorite geothermometer (based on A1IV contents), temperatures of 146±28°C and 169±12°C have been deduced for two Permo-Triassic members, and 305±12°C for Carboniferous. The Si content in illites has been used as a geobarometer, and pressures of 4.8±2 kbar have been estimated in Carboniferous rocks and tentative pressures of 2.7±3 and 2.1±2 kbar in Permo-Triassic members. Chemical evolution of phyllosilicates is accompanied by increasing illite crystallinity and % 2M1 polytype.

Adsorption isotherms for an homologous series of primary aliphatic alcohols from aqueous solution by montmorillonite and kaolinite saturated with either sodium or calcium ions have been determined. The results show that adsorption is a function of chain length and exchangeable cation in the case of the more insoluble alcohols. Exchangeable cation does not affect the adsorption of the soluble alcohols to any significant extent. Adsorption of the longer chain alcohols occurs in voids in the mineral aggregates.

This paper presents various aspects, revealed by Mössbauer spectroscopy, of structural and magnetic properties of Al-substituted small-particle soil-related oxides. For goethite we focus on the relations between the hyperfine fields on the one hand, and crystallinity and Al content on the other. It is argued that these relations only provide a rough estimate of the Al content in natural samples. The ferrimagnetic-like behaviour reflected in the external-field Mössbauer spectra (4.2 K, 60 kOe) of certain Al goethites is presented. The spectra obtained for lepidocrocites are not spectacular, but confirmed that up to ∼10 at.% Al can be incorporated in the structure. Three differently-made series of hematites are considered. The Morin transition and spin structures in hematite are very sensitive to crystallinity and Al content, and probably to the presence of structural OH–. Integral low-energy electron Mössbauer spectroscopy on non-substituted samples indicates that the Morin-transition temperature in the surface layers (2 to 5 nm) is not significantly shifted from the bulk value. Measurements in extremely high magnetic fields (140 kOe) have shown that a spin-flip transition is induced in highly-substituted samples which exhibit no Morin transition in zero field. The use of external fields is crucial for the characterization and precise determination of the hyperfine parameters and site occupancies for maghemites, and for phase analyses of magnetic soils.

Silicifications and smectite-dominated clay assemblages are associated in the marine Cretaceous sediments of England. Both are neoformed, and have developed at approximately the same time in the host sediment. Two silicification types, opal-CT and α-quartz, have a distribution pattern unrelated to the stratigraphy or lithology of the Lower Greensand, Upper Greensand and Lower Chalk. The petrography of five examples suggest that the two silicification types were both precipitated from the porewaters of their host sediment, and their silica mineralogy and their associated silicate minerals were controlled generally by the silica concentration of the porewaters and more locally by the pH gradients.

K-Ar ages of Eocene glauconite pellets from the Isle of Wight are related to quantified amounts of older glauconite pellets and to occurrences of detrital mica/illite particles that might have been added to synsedimentary pellets during reworking processes. Addition of older glauconite did not significantly bias the K-Ar dates, as the results most often provide the expected stratigraphic reference ages or even significantly lower ages. Alternatively, K-Ar dates significantly greater than those suggested by the stratigraphy appear to result from non-glauconized detrital mica in the pellets which is not always removable, even by high-gradient magnetic separation.

Unexpected ‘old’ K-Ar glauconite ages do not result from misapplication of the method, but from inability to systematically identify and remove the `contaminant' particles from glauconite splits. Analysis of highly evolved glauconite separates is definitely appropriate for reliable isotopic age determinations, but it might not be enough for the final selection, as long-lasting diagenesis might have taken over the synsedimentary process. Much if not all depends on the separation and characterization of the separates, but also on the completion of the glauconitization process which effects cannot always be anticipated.

A sepiolite from Toledo, Spain, was activated with dilute solutions of HNO3 (0·25, 0·50 and 0·75 m) followed by heat treatment at 110, 200, 300, 400 and 500°C for 10 h. The retention of n-butylamine in cyclohexane solution was used to follow the variation of surface acid centres of the sepiolite as a function of the acid and heat treatments, these reaching a maximum for the product treated with 0·75 m HNO3 and heated at 300°C. Acid treatments within the range 0·50–0·75 m and subsequent heat treatment at 200–300°C for 10 h yielded products with the highest surface activity.

The differences noticed in the porosity-permeability values and the clay mineralogy of the Brent and Statfjord reservoirs of two closely related wells from the Central Viking Graben appear to be due essentially to diagenesis. According to their type, nature and relationships, the diagenetic phases are organized into three distinct sequences, each induced by distinct hydrodynamic systems. These systems are related to three main episodes of the geological history of the reservoirs, which were: the compaction-lithilication phase, the Kimmerian to Lower Cretaceous emersive phase, and the hydrologic loading of the deep parts of the basins and/or fault panels. The late diagenetic sequence is expressed by quartz overgrowths, blocky kaolinite and illite authigenesis. With a probable ‘catalytic’ effect of temperature, this sequence is the most intensely developed and so greatly damages the porosity and permeability. In the oil-bearing reservoirs of Well 25/4-1, the stage of evolution of this third diagenetic sequence is not well advanced: illitization was rapidly stopped by the hydrocarbon charge so that permeabilities remained satisfactory (up to 500 mD). In the water-zone of Well 25/4-5, illitization is extensive and permeabilities considerably reduced (lower than 5 mD). K/Ar dating of authigenic illites provides evidence of a late Eocene phase (40–44 Ma) as the maximum age for the hydrocarbon entrapment in Well 25/4-1, whereas the last illite authigenesis from the water-zone of Well 25/4-5 gives an Oligocene phase (28 Ma in the Statfjord Formation). These data indicate that fluid circulation was effective until this time in the down-dip faulted panel of Well 25/4-5, and has therefore been responsible for the drastic permeability reduction.

The principal clays of the northern and central North Sea are illite (sometimes with interlayered smectite) and kaolin. Chlorite is only locally important. Although it has been proposed that kaolin within North Sea sandstones is detrital in origin, the majority of workers have concluded that it is authigenic, largely the product of feldspar alteration. Kaolin is found within a wide range of sedimentary settings (and within shales) apparently defying the notion that kaolin is an indicator of meteoric water deposition. Within sandstones, the earliest authigenic kaolin has a vermiform morphology, the distribution of which is controlled by the availability of detrital mica to act as a nucleus, and the composition of the post-depositional porewaters. This vermiform kaolin formed in meteoric water, the presence of which is easily accounted for below sub-aerial exposure surfaces in non-marine formations, and below unconformities over marine units. In fully marine sands, and even marine shale units, kaolin still occurs. It has therefore been suggested that even these locations have been flushed with meteoric water.

Early vermiform kaolin recrystallizes to a more blocky morphology as burial proceeds, at least in the Brent Group. Blocky kaolin has been reported as growing before, synchronously with, and after the formation of quartz overgrowths, though oxygen isotope studies support low-temperature growth, pre-quartz. Blocky kaolin may form during meteoric flushing associated with lower Cretaceous uplift and erosion, though it is found in fault blocks that are thought to have remained below sea level. Here, the kaolin may form in stagnant meteoric water, relics of the post-depositional porewater. It has also been proposed that the blocky kaolin grew in ascending basinal waters charged with carboxylic acids and CO2, though this hypothesis is not supported by stable oxygen isotope data. Some of the blocky kaolin is dickite, the stable polymorph above ∼100°C.

Fibrous illite occurs almost ubiquitously within the clastic sediments of the North Sea. An early pore-lining phase has been interpreted as both infiltrated clastic clay, and as an early diagenetic phase. Early clays may have been quite smectite-rich illites, or even discrete smectites. Later, fibrous illite is undoubtedly neoformed, and can degrade reservoir quality significantly. Both within sandstones and shales, there is an apparent increase in the K content deeper than 4 km of burial, which could be due to dilution of the early smectite-rich phase by new growth illite, or to the progressive illitization of existing I-S. Much of the ‘illite’ that has been dated by the K-Ar method may therefore actually be I-S.

The factors that control the formation of fibrous illite are only poorly known, though temperature must play a role. Illite growth has been proposed for almost the entire range of diagenetic temperatures (e.g. 15–20°C, Brent Group; 35–40°C, Oxfordian Sand, Inner Moray Firth; 50–90°C, Brae formation; 100–110°C, Brent Group; 130–140°C, Haltenbanken). It seems unlikely that there is a threshold temperature below which illite growth is impossible (or too slow to be significant), though this is a recurring hypothesis in the literature. Instead, illite growth seems to be an event, commonly triggered by oil emplacement or another change in the physiochemical conditions within the sandstone, such as an episode of overpressure release. Hence fibrous illite can grow at any temperature encountered during diagenesis.

Although there is an extensive dataset of K-Ar ages of authigenic illites from the Jurassic of the North Sea, there is no consensus as to whether the data are meaningful, or whether the purified illite samples prepared for analysis are so contaminated with detrital phases as to render the age data meaningless. At present it is unclear about how to resolve this problem, though there is some indication that chemical micro-analysis could help. It is a common belief that illite ages record the timing of oil charge, and so can be used to calibrate basin models.

Grain-coating Fe-rich chlorite cements can preserve exceptional porosity during burial. They are found in marginal marine sandstones, formed during diagenesis from precursor Fe-rich clays such as berthierine or verdine.

The kinetics of clay formation in buried paleosols developed from late Quaternary rhyolitic tephra layers near Rotorua, New Zealand, can be described in terms of a combination of parabolic and linear kinetics, reflecting the hydration of glass, and the formation of clay minerals, respectively. Such a model is consistent with the formation of clay minerals showing an Arrhenian temperature dependence and suggests, on the basis of calculated activation energies, that the process of formation of Al-rich allophane (imogolite) is diffusion controlled, whereas the rate of formation of Si-rich allophane is controlled by the chemical processes at the site of reaction.

A 29Si MAS-NMR one-pulse and ﹛1H﹜ CP comparative study of sepiolite, folded sepiolite, aluminated sepiolite, and palygorskite was conducted. The three main resonances of sepiolite at -92.0 ± 0.1, —94.3 + 0.1, and — 97.8 ± 0.1 p.p.m. were assigned with respect to the position of the Si atoms within the three pairs of symmetrical sites in the basal plane. The line at —97.8 p.p.m. was attributed to the central sites by comparison with the resonances and chemical environments of Si in talc and palygorskite. The line at —94.3 p.p.m. was attributed to the edge sites, because it is strongly reinforced by cross-polarization. Alumination of sepiolite led to a broadening of the peaks and new downfield resonances in line with what can be observed in other phyllosilicates and zeolites.

Organoclays are sorbent materials prepared from clays by exchanging inorganic with organic cations. Their properties depend on the loading and conformational structure of the organic cations, but little information is available about the surface structures of organoclays. In this work, X-ray photoelectron spectroscopy (XPS) and classical molecular dynamics (MD) simulations are combined to characterize the external interface of an organoclay prepared from hexadecylpyridinium (HDPy+) and bentonite. The XPS survey spectra show well the varying elemental composition of the surface with increasing amount of surfactant, showing a decreasing contribution of clay-derived elements with increasing organic coverage. The high-resolution C 1s XPS spectra depict sensitively the surface arrangement of the surfactant. In combination with MD simulations, the results implied a monolayer coating for low surfactant coverage and a disordered bilayer arrangement at high surfactant uptakes. Molecular dynamics simulations showed that for very high cation uptake a quasi-paraffin-like configuration is also possible. The combination of experimental and modelling methods yielded congruent information on the molecular-scale arrangement of organic cations at the organoclay surfaces and the controlling mechanisms.

In Scandinavian podzols a swelling dioctahedral mineral, probably derived from dioctahedral mica and iron-rich chlorite, has been found in the clay (<2µ) fractions from A2-horizons. This mineral, or group of minerals, is shown by X-ray examination and differential thermal analysis to be of a vermiculite-smectite type, thus making the differentiation between vermiculite and smectite still more problematical. The pretreatment of the samples and choice of interlayer cations is shown to be of great significance for the identification of soil clay minerals.

Ferrihydrite transformed to crystalline Fe(III)-oxides in salt solutions but the degree of transformation generally decreased with increasing concentration (decreasing water activity) and haematite was favoured over goethite. More transformation occurred for chloride than for nitrate or sulphate and more for calcium than for magnesium or sodium. Calcium and magnesium favoured haematite over goethite. The results support the hypothesis that haematite not only can be formed but will be favoured over goethite in natural environments with low water activity, for instance dry or saline soils or sediments. The ionic environment modifies the general water activity effect.