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.

Oedometer swelling and compaction tests up to a maximum pressure of 32 bars were made on the Wyoming montmorillonite SWy-1 with a range of mixed (Na,K) exchange ion compositions. The permeabilities and water diffusivities of the compacted clays were calculated using soil physics theory. Significant inhibition of swelling from that of the Na-montmorillonite is developed for K fractions in the range 0·30 < K+ < 0·56. However, the water diffusivity increases by a factor of approximately six as the K exchange fraction increases from 0 to 0·7. Permeabilities to water flow show a power-law dependence on the void ratio e.

XRD measurements at controlled humidity yield equilibrium interlayer spacings which in general increase with both increasing relative humidity and increasing Na exchange fraction. The extent of swelling is strongly inhibited in clays having Na exchange fractions of 0·44 or less, in good agreement with the oedometer studies.

Aluminium substituted goethites (Fe, Al)OOH were synthesized from Fe(III), Al systems (ferrihydrites) in m KOH at room temperature. A linear correlation existed between degree of Al substitution and log[Al] in solution. Incorporation of up to 12 mole % Al was obtained. The rate of conversion of ferrihydrite to goethite decreased as [Al] increased. The goethite consisted of thin elongated plates approximately 0·1 µm long with serrated edges and reduced growth in the crystallographic X-direction. The presence of salts and the nature of the starting material either an Al-ferrihydrite or a ferrihydrite in aluminate solution did not influence the degree of substitution, but only the rate of formation of the goethite.

The procedure used for fractionation of soils, rocks, and sedimentary materials involves separation of the clay (less than 2 microns) and silt (2 to 62 microns) by centrifugation. Randomly oriented powders and oriented aggregates of these materials are examined using X-ray diffraction as the principal method of analysis.

The nature, distribution and origin of clay minerals in the hydrocarbon-bearing Permian Rotliegend sandstones of the North Sea and the adjacent areas of the Netherlands and Germany are reviewed. The clay minerals occur as detrital coatings of smectite and smectite-illite on the surfaces of sandgrains, and as later diagenetic cements of kaolinite, chlorite (two varieties), and illite in the pore spaces of those sandstones. Two diagenetic clay mineral assemblages are predominant in the Rotliegend of the North Sea. The kaolinite-illite assemblage is restricted to the Rotliegend of shelf areas which underwent shallow burial followed by strong Jurassic/Cretaceous (Late Cimmerian) structural inversions, whereas the illite-chlorite assemblage is associated with basinal areas that underwent deep and rapid burial throughout the Mesozoic.

The factors controlling mineralogy, crystal chemistry and morphology of those diagenetic clay minerals, as well as their regional distribution and origin, are numerous, complicated, and inter- related. Evidence suggests that the following aspects were important parameters: (1) variations in the original depositional arid desert environment; (2) the chemistry and flow patterns of the porewaters; (3) temperature and timing of clay mineral formation; (4) local burial history; (5) the presence or absence of meteoric water; and (6) the structural setting of the Rotliegend sandstones.

Oxygen isotope data indicate that the illite cements formed over a wide range of temperatures (24–140°C) that is consistent with the deep burial conditions prevailing in the palaeo-basins. In contrast, oxygen isotopes indicate that kaolinite cements formed over a more restricted temperature range (40–80°C) and under the influence of meteoric water penetrating the sandstones of the shelf areas as a result of their Late Cimmerian uplift and associated erosion. Hypotheses suggesting that the absence of kaolinite cement from the deeply buried Rotliegend sandstones is caused by its illitization during burial, and that the chlorite cements have formed by the alteration of earlier smectite, smectite-chlorite and corrensite cements, are not supported by evidence.

Authigenic kaolin is a major occluder of porosity at all depths in the Hutton-NW Hutton Brent Group reservoirs. Two polytypes, kaolinite and dickite, are present. Kaolinite occurs to a depth of ∼10,600 ft and is interpreted to be eogenetic. Dickite increases in abundance with burial depth, concomitant with increasing K-feldspar dissolution. Kaolin petrography indicates that kaolinite transforms to dickite via dissolution-reprecipitation at depths of ∼11,000 ft. Dickite has δ18O values characteristic of modified meteoric water for growth at temperatures of contemporaneous quartz authigenesis.

Palaeohydrodynamic models involving meteoric water penetration down to 2 km through faults or Palaeozoic rocks of the East Shetland Platform are unlikely to provide the vigorous fluid throughput required for kaolin precipitation. Reverse post-rift modelling indicates that the crest of the Hutton-NW Hutton fault block was not sub-aerially exposed, precluding meteoric water ingress via this route. Eogenetic kaolinite formed in a meteoric water flush driven by topographic head on the Brent Delta. Dickite is, in part, derived from kaolinite dissolution, with additional sourcing from K-feldspar dissolution and Si-Al in pore-waters expelled from mudstones during burial. This model precludes the need for extensive fluid-flushing at intermediate burial depths.

Heating experiments were carried out with ferrihydrite, in the presence of organics, to gain more insight into the intermediate formation of a ferrimagnetic cubic phase often observed in soils as the result of fire. Without an organic reductant, no intermediate cubic phase was formed when ferrihydrite was transformed into hematite, regardless of whether the ferrihydrite DTA curve showed one or two exothermic peaks. Addition of glucose or charcoal as reductant caused the initial formation of a cubic magnetite and/or magnetite/maghemite phase in samples heated above 300°C in both air and N2 atmospheres. Ferrihydrite reduction, as determined by increasing cubic unit-cell edge lengths between 0.832 and 0.840 nm, increased with reductant concentration and heating time, providing excess reductant remained. Following consumption of the reductant, FeII may be partially or completely reoxidized, forming maghemite and then hematite. The occurrence of maghemite, but not of magnetite, in soils where forest fires have occurred suggests that sufficient oxygen was available, while temperatures remained elevated, to oxidize magnetite to maghemite.

The clay mineralogy and chemistry of a green lacustrine marl that has been pedogenically modified in the upper part was investigated in order to better understand the formation of low-temperature Fe-rich 10 Å clay. Twelve samples in a vertical sequence have been investigated using X-ray diffraction (XRD), chemical analysis, scanning electron microscopy (SEM) and laser particle size analysis. The clay assemblage has a range of overall illite-smectite (I-S) compositions (64–100%) resulting from several I-S phases that, for the sake of modelling, have been simplified to one to three I-S phases of increasing illitic content. Where the lacustrine marl has been pedogenically modified, the smectite-rich I-S is much reduced in abundance or absent and the 10 Å -rich component is both more abundant and more illitic. These assemblages are a consequence of illitization of detrital I-S in the lake and soil, and dissolution of other clays (kaolinite and chlorite) in the hypersaline lake. Interlayer K, octahedral Fe and octahedral + interlayer Mg increase with intensity of illitization (increase range 0.32–0.63, 0.68–1.67, 0.18–0.24 per O10(OH)2, respectively), first in the increasingly saline lake, and latterly as a result of wetting and drying in a gley soil. In the soil environment, reduction of Fe(III) to Fe(II) resulted in increased layer charge but, as by this stage very few smectite interlayers remained, this did not result in an equivalent increase in illite. Laser particle-size analysis, supported by SEM observation, shows the existence of a bimodal distribution of clay particle size (maxima at 0.2 and 1.5–1.8 μm) in which the finer fraction increases largely in the pedogenically affected samples, probably due to particle break-up caused by seasonal wetting and drying. This ‘dual action’ illitization, first in a hypersaline lake and latterly through wetting and drying, may be responsible for both the intensity of illitization and exceptionally high (for the Solent Group) Fe content of the authigenic illite. The chemical characteristics of the illitic I-S and the illite end-member correspond to glauconite. Hence, this is an example of onshore, non-pelletal glauconite formation.

Powder X-ray diffraction (XRD) curves were calculated for the different structural models so far proposed for feroxyhite (δFeOOH). The influence on XRD features of different structural parameters, including site occupancy of Fe atoms, atomic coordinates, content and distribution of stacking faults, and dimension of coherent scattering domains, were considered. On the basis of agreement between experimental and simulated curves it is shown that δFeOOH is a mixture of feroxyhite proper and ultradispersed hematite in the 9 : 1 volume ratio. Feroxyhite proper consists of hexagonal close packing of anions containing 5% stacking faults. Iron atoms occupy only octahedral sites and are distributed in such a way that face-sharing filled octahedral pairs regularly alternate with vacant octahedral pairs along the c axis. This distribution of Fe atoms is quite similar to that established by Patrat et al. (1983), but in each pair, Fe atoms are displaced by the same value of 0.3 Å in opposite directions away from the centre of their octahedron. Nearest Fe-Fe distances calculated for the model proposed (2.88, 3.01, 3-39 and 3-73 Å) practically coincide with those found by EXAFS spectroscopy for the same sample (2-91, 3.04, 3.41 and 3.7-3.8 Å).

Hematite exists ubiquitously in soils and sediments, and is commonly aluminium (Al)-substituted. This study investigated systematically the effects of Al substitution on the visible diffuse reflectance spectrum (DRS) of hematite by using several sets of synthetic samples. We found that the position and amplitude of the characteristic absorption band of hematite (estimated from the first- and second-order derivative curves of the Kubelka-Munk remission function spectrum derived from the DRS) was significantly affected by the degree of Al substitution as well as by sample grain morphology. Therefore, there are ambiguities in quantifying the degree of Al substitution and the mass concentration of hematite using DRS. Nevertheless, if hematite forms under similar environmental conditions, it is possible to establish a transfer function between the DRS parameters and hematite concentration as discussed here for a Chinese loess-palaeosol sequence.

The adsorption of Rhodamine 6G in aqueous suspension on Laponite B was investigated by electronic absorption and emission spectroscopies. Fluorescence spectra suggest that the monomer is adsorbed at two different surfaces, the external and the internal. A monomer is intercalated in the interlamellar space at low loading of dye (<3% CEC), whereas the monomeric state of the dye seems to be at the solid-aqueous interface in suspensions with high loading (>12% CEC). The metachromatic effect observed in the absorption spectra, for the loading interval between 1% and 15% CEC of Laponite B, is attributed to the dimerization of the dye, which seems, from X-ray diffraction measurements, to be formed at the clay interlayer. The formation constant and the absorption spectrum of the aggregate were obtained and the dimer was structurally characterized by applying the Exciton Theory. The observed fluorescence quenching for loadings lower than 15% CEC is attributed to energy transfer from monomer to the dimer, which obeys the Perrin model.

A reaction path model was constructed in a bid to simulate diagenesis in the Magnus Sandstone, an Upper Jurassic turbidite reservoir in the Northern North Sea, UKCS. The model, involving a flux of source rock-derived CO2 into an arkosic sandstone, successfully reproduced simultaneous dissolution of detrital K-feldspar and growth of authigenic quartz, ankerite and illite. Generation of CO2 occurred before and during the main phase of oil generation linking source rock maturation with patterns of diagenesis in arkosic sandstones and limiting this type of diagenesis to the earlier stages of oil charging. Independent corroborative evidence for the model is provided by formation water geochemical data, carbon isotope data from ankerite and produced gas phase CO2 and the presence of petroleum inclusions within the mineral cements. The model involves a closed system with respect to relatively insoluble species such as SiO2 and Al2O3 but is an open system with respect to CO2. There are up to seven possible rate-controlling steps including: influx of CO2, dissolution of K-feldspar, precipitation of quartz, ankerite and illite, diffusive transport of SiO2 and Al2O3 from the site of dissolution to the site of precipitation and possibly the rate of influx of Mg2+ and Ca2+. Given the large number of possible controls, and contrary to modern popular belief, the rate of quartz precipitation is thus not always rate limiting.

The extent of diagenesis in Lower Jurassic shoreline sandstones of the Beatrice oilfield was controlled primarily by their detrital clay content. Sandstones rich in detrital clay had low depositional permeabilities; these show preservation of detrital feldspars to the present day and have no extensive diagenetic quartz overgrowths. Sandstones poor in detrital clay had high permeabilities and show large quartz overgrowths as part of a normal sub-arkosic diagenetic sequence. Such quartz occurs preferentially below, but not above, impermeable mudstones. These low- and high-permeability features suggest that pore-fluid flow was important during diagenesis. Fluid inclusions trapped in diagenetic quartz overgrowths formed between 68° and 94°C. This silica was probably supplied in solution from the temperature-driven illitization of smectitic clays surrounding these sandstones deeper in the basin. Fluid volumes from clay dewatering, from mechanical compaction, or from influx of overlying seawater were too small to transport diagenetic silica volumes. Silica-rich fluids were probably transported up-dip by convective cells within the sandstones and authigenic quartz precipitated as these fluids cooled. This episode of quartz diagenesis could have been completed within 1·6 × 106 yr if fluid velocities were 3·1 m/yr. Similar convective fluid flows could have transported the fluids responsible for secondary porosity development and the hydrocarbons expelled from mudstones later in the basin burial history.

Various physical and analytical techniques (XRD, XRF, SEM, FTIR and EPR) have been used to investigate the effects of chemical and/or physical modification of Ranong kaolin, which has been proposed as a potential bleaching clay for vegetable oils. Acid treatment after grinding resulted in major changes compared with acid treatment of the original mineral sample or mechanical treatment alone. Previous work has shown that the combined treatments produce increases in surface area and new porous structures, and the present measurements show reductions in Al:Si ratios. These are accompanied by a major reduction in O–H stretching vibrations as a result of grinding, although acid treatment produced little subsequent effect on the O–H bands in the FTIR spectra. However, acid treatment resulted in a reduction in the Al–OH–Al bending vibrations and the appearance of Si–O bands associated with newly synthesized material; these effects were much greater with samples that had been ground prior to the acid treatment. There were appreciable qualitative differences in the way in which the EPR spectra of Fe and Mn were affected; the Fe signal was sensitive to mechanical treatment, but little subsequent change was induced by acid extraction, whereas the Mn peaks were sensitive to the both the pH and the chemical nature of the acid used. These results therefore indicate that the Fe and Mn are in different types of site in the kaolin structure. Little change was observed in the main oxygen-based free radical centre associated with Si atoms, but that associated with Al was lost as a result of the treatments. Such mineral characterization is of fundamental importance to understanding the modification of kaolins and their uses as adsorbents in the food and environmental sciences.

Adsorption isotherms have been determined for mixed Ca-Na montmorillonites, with various values of f, the proportion of (½ Ca) to Na (Fig. 1). These are coincident from f=0 to f=0·3, and then diverge. This is interpreted as meaning that “demixing” occurs at f=0·3, a system being formed in which some interlayer spaces are occupied mainly by Ca, others mainly by Na.

X-ray analysis confirms this view. Plots of the water uptake against f, for various relative humidities (Figs. 2–5) show that, for R. H.'s lower than 75%, the demixing is incomplete.

The observations can be explained by the occurrence of a limited number of doubly-charged sites (by Al-for-Si exchange spots being accidentally contiguous). For the montmorillonite used, the statistical probability of occurrence of such sites is found to be 0·2 if the layers have symmetry C-2/m and 0·33 for C-2, so that the observations are in favour of the latter symmetry.

A quantification technique useful in analysis of clay-rich deposits containing sepiolite and/or palygorskite has been developed by combining diffractometric and chemical data and represents a considerable improvement on classic quantification based on X-ray diffraction. The required data are: intensity ratios between certain diffraction peaks of calcite-dolomite and sepiolite-palygorskite-smectites, and also the CaO, MgO, K2O, Al2O3 and SiO2 contents. The validity of this method was tested by analysis of artificial mixtures of pure phases of the minerals found in such deposits.

Seven samples from a chronosequence of soils developed in historically created polders on the Atlantic coast (Marais Poitevin, Vendée, France) were investigated in order to illustrate the rate of mineralogical change in a clay-dominated system. The oldest polder was constructed in 1665, the last one in 1912; thus the time span of soil evolution is from 80 to 330 years. All the samples had more than 50% clay (<2 μm). The most reactive, fine clay sub-fraction (<0.1 μm) was investigated in detail by X-ray diffraction and chemical analysis. The observed mineralogical changes with increasing age followed the schematic reaction:

smectite + mica = illite + mixed-layer minerals.

The progress of reaction in time appears to be non-linear. This reaction seems to occur in a chemically constant system, and the mineralogical change can be seen as a readjustment of species to a given chemical composition.