The distribution of Fe3+-kaolinite, Al-goethite and Al-hematite and their contents of Fe and Al in bauxites and ferricretes are controlled by water activity, dissolved silica activity, temperature and particle size. The proposed model, based on ideal solid-solution equilibria in the Fe2O3-Al2O3-SiO2-H2O system, takes into account water and silica activities. By using the same considerations as those previously developed for the Fe2O3-Al2O3-H2O system, the model calculates the amounts of coexisting phases, Al or Fe substitution ratios in goethite, hematite or kaolinite, and the stability field distributions of the minerals under various conditions. Thermodynamic equilibrium conditions and element distributions within the mineral constituents are shown to be dependent on the parameters cited above. The model yields results compatible with natural observations on lateritic profiles.

The value of peak width at half-height for the illite 001 XRD reflection is known as the Kubler index or the illite ‘crystallinity’ index. This measurement, which has been related to the degree of metamorphism of very low-grade, pelitic rocks, is a function of at least two crystal-chemical factors: (1) illite X-ray scattering domain size; and (2) illite structural distortions (especially swelling). Reynolds' NEWMOD computer program is used to construct a grid with which these two contributions to illite peak width can be determined independently from measurements of the 001 peak width at half-height and the Środoń intensity ratio. This method yields more information about changes undergone by illite during metamorphism than application of the Kubler index method alone.

Millimetre-sized flakes of a homogeneous, high layer charge vermiculite from S. Olalla (Spain) saturated with l'ornithine cations were made to swell in l'ornithine hydrochloride solutions of various concentrations, and the gel-like structures formed were subjected to uniaxial loads up to 150 g/cm2. Small-angle X-ray diffraction measurements from the swollen flakes show well-modulated intensity patterns from which the equilibrium interlayer distances were obtained for the various swelling conditions. The interlayer spacings can be adequately expressed as normal-logarithmic distribution functions of probability density. The gel texture is described in terms of the average number of elementary layers, all in parallel orientation, constituting independent coherent domains within the swollen flake. Changes in structure and in texture taking place as the gel volume increases, or as water is expelled from the interlayer volume under mechanical compression, are followed by the variation of the corresponding parameters of order.

The kaolin minerals in lateritic weathering profiles on dolerite and granite in Western Australia have been characterized by intercalation tests for halloysites, and also by their particle morphologies. Towards the surface of the profiles there is a decrease in both total kaolin mineral content, and in the expandibility of those minerals to intercalation. Tubular kaolinite that is non-expandible by any of the intercalation tests for halloysite was identified in the upper few metres of both profiles. Hydrated halloysite is most common at the base of both profiles. Formamide expands some anhydrous kaolin minerals while the sequence of hydrazine/water/glycerol (HWG) expands more of these minerals. Non-tubular (subhedral to sub-hexagonal platy) kaolinite is also present. Hydrated tubular halloysite apparently alters to tubular kaolinite via dehydrated halloysite that is expandible by formamide and a tubular ‘kaolin’ that is expandible by HWG but not by formamide.

A technique for preparing vermiculites for examination by high-resolution transmission electron microscopy (HRTEM) has been developed. A TEM-stable expanded phase can be obtained by intercalating n-alkylammonium ions between the silicate layers of a parent biotite. The vermiculite particles were embedded in Spurr resin and centrifuged to improve orientation. Ultra-thin specimens were prepared using an ultramicrotome, the quality and thickness of the sections being monitored by TEM. Lattice images of biotite, Ba-vermiculite and octylammonium-vermiculite, the latter showing a perpendicular arrangement of the alkyl chains relative to the silicate layers, were obtained with a resolution ∼2 Å. The reliability of these images was confirmed by computer simulation.

Par diffraction des Rayons X sur la vermiculite-Li de Santa-Olalla, il a été montré en fonction de la température qu'il existait différentes phases homogènes (d001 = 12·17 Å, T = 25°C; d001 = 12·02 Å, T = 60°C; d001 = 11·89 Å, T = 70°C; d001 = 10·04 Å, T = 100°C). Par ailleurs, pendant le processus de déshydratation entre les phases homogènes à 11·89 Å et 10·04 Å, il a été mis en évidence l'apparition d'états inhomogènes (interstratifiés). L'utilisation parallèle des méthodes directes d'étude des diagrammes de rayons X par transformée de Fourier monodimentionnelle pour les phases homogènes et d'une méthode indirecte de modélisation pour les états interstratifiés a permis de montrer que le passage de la phase 11·89 Å à la phase 10·04 Å se fait conjointement par une ségrégation de ces phases et un interstratifié aléatoire (11·89–10·04). Ce travail a en outre montré que la caractérisation des états intermédiaires entre phases homogènes ne peut être faite sans une étude structurale préalable des phases homogènes.

Sequences of alternating arenaceous and argillaceous lithologies are commonplace in the siliciclastic hydrocarbon reservoirs of the North Sea and elsewhere. Individual lithological units may vary in thickness from millimetres to tens or hundreds of metres. The first part of this introduction will discuss general aspects important for the understanding of the patterns of clay diagenesis associated with these sequences, whereas the second part will deal with the papers presented in this issue of Clay Minerals and how they fit into this general area of interest.

Analytical transmission electron microscopy (ATEM) was used to obtain chemical analyses of single illite crystals from Upper Carboniferous reservoir sandstones of the Bothamsall Oilfield, East Midlands, UK, and from Rotliegendes (Permian) sandstones of the North Sea. All samples were found to be highly aluminous, with only minor Mg and Fe, and to have near-ideal dioctahedral sheet totals. No significant variation in chemical composition of illite was found within the Bothamsall reservoir rocks, irrespective of paragenesis or stratigraphic horizon. Similar results were obtained from the Rotliegendes samples. Variation was found, however, between Bothamsall and Rotliegendes analyses populations. The Rotliegendes illites were distinctly more K-rich, which is the result of greater charge deficiency in the octahedral sheet. These results indicate that all the illite in both reservoirs precipitated in equilibrium with the reservoir pore-fluid. Furthermore, they imply that the physico-chemical composition of the pore-fluid did not evolve significantly between the different illite generations in the Bothamsall samples. These data, when compared with published analyses of authigenic illite, indicate that the compositional field for illite in sandstones is restricted in comparison with that of illite in mudrocks.

Talc is very often an important constituent of suspended matter in marine and lacustrine environments but is rarely detected in sediments. The presence of talc is usually attributed to its industrial and agricultural application. Its recent anthropogenic use, long residence time in the water column due to its small particle size, transport by surface currents and winds, and its tendency to resuspension from the sediment are factors which contribute to the preferential detection of talc in suspended matter.

Porous, high surface area solids have been prepared by intercalation of a trinuclear acetato hydroxy-iron(III) complex cation between the layers of montmorillonite and the resulting materials have been characterized in order to identify the nature of the iron intercalates formed during and after the cation exchange step. Spectroscopic studies demonstrated that at 623 K, the iron complex was partially decomposed and α-Fe2O3 was only detected in samples heated at high temperatures, > 923 K. Under these conditions, the dehydroxylation of the montmorillonite layers led to the collapse of the structure.

Analysis of the smectite content of mixed-layer illite/smectite minerals in 4 wells in the Paris Basin indicates that the sediments have not experienced the same thermal history throughout their burial history. Reconstruction of the total burial depths and thermal maxima for the sediments in the different levels of the wells indicates that the present-day depth-temperature relations reflect the existence of the same continuous sequence of diagenetic change in 2 of the wells. An empirical relation giving smectite content as a function of maximum paleotemperature was derived. A strong break in the clay mineral compositions in the upper levels of 2 wells at a third site indicated an anomaly in the diagenesis process. This break in the sequence of clay diagenetic transformation can be traced to major subvertical faults which cross-cut the wells. These anomalies are attributed to the faults which are assumed to have acted as thermal drains towards the surface, locally increasing the temperature of the sediments for a major portion of the burial history when the faults were active. Closure of the fault system coincides with the return to a normal pattern in clay diagenesis for sediments in the upper portions of the wells. It is concluded that clays may be used to trace thermal histories of sediments in a sedimentary basin.

The aim of this study was to determine the influence of the various source areas on the Neogene sediments of the Alpujarran Corridor, to ascertain the type and intensity of the sedimentary processes involved, and to establish some generally applicable criteria. At the bottom of the sections the detrital pebbles are derived from the highest nappes of the Alpujárride Complex, whereas those at the top are from the deepest unit of the Nevado-Filábride Complex (the Veleta nappe); between these the entire range of materials from the lower-Alpujárride Complex and the upper-Navado-Filábride Unit (the Mulhacén nappe) is represented. Comparison of the micas and chlorites in the pebbles with those in the conglomerate matrix reveals the effect of the sedimentary processes on every aspect of the crystallochemistry. The d001 of the chlorites and the b0 of the micas are scarcely affected and can be used to determine the source area. The crystallinity, the Fe content of the chlorites and the basal spacing and angular difference (Δ2θ M-P) of the white micas, on the other hand, are all modified, especially in the clay fraction, according to the degree and nature of the alteration of the sediments, and consequently can be used as a guide to the extent of this process.

Authigenic illites and chlorites from clastic reservoirs, offshore Norway, have been studied by analytical transmission electron microscopy (ATEM). Present-day reservoir conditions range from 1600 m burial depth and 70°C to 4400 m and 160°C. The reservoir units have experienced continuous subsidence since early Cretaceous, and are presently at maximum burial. Textural and morphological evidence indicates that the authigenic illites and chlorites investigated are late diagenetic products, except for an early-formed berthierine in the shallowest reservoir. While the illites show very limited chemical variability at temperatures between 140° and 160°C the chlorites exhibit definite compositional trends with increasing temperatures. Tetrahedral Al increases substantially from 100°–160°C, and the octahedral vacancy decreases correspondingly. These observations indicate that continuous recrystallization of authigenic clays has taken place. This is also supported by the chemical composition of present-day porewaters, which are close to equilibrium with the clay mineral assemblage.

Differential heat of water vapour adsorption for Cs+-saturated montmorillonite, kaolinite and allophane was measured over a low relative humidity range from 10−4 to 10−1. Two interactions (clay surface atom-water and counterion-water) are mainly involved in the water vapour adsorption process. To assess the affinity of water to the clay surface, the amounts of adsorbed water and the differential heat due to Cs+ were estimated from equilibrium constants in the gas phase reactions. It is tentatively concluded that (i) a dominant role of water adsorption due to the Cs+-water interaction in the adsorption process is evident only for montmorillonite; (ii) the affinity of water to the clay surface is strongest for allophane and weakest for montmorillonite.

Des phases transitoires ont été préparées par activation thermique de deux montmorillonites, l'une calcique et l'autre sodique, dans le domaine de température 650–950°C, puis étudiées par spectroscopie infrarouge. Le coefficient de désordre à courte distance, défini à partir de l'élargissement et l'intensité de certaines bandes caracteristiques, présente ses valeurs maximales avec les phases transitoires obtenues à 800°C et 820–850°C respectivement pour la couche de silice et la couche d'alumine. Ce résultat a pu être corrélé à plusieurs données expérimentales concernant la cinétique de dissolution de ces phases transitoires dans l'acide fluorhydrique dilué ou dans des solutions saturées d'hydroxyde de calcium. En outre, l'accroissement particulièrement marqué du désordre à courte distance mis en évidence au niveau de la transition V→IV de la coordinance de l'aluminium, confirme l'hypothèses émise antérieurement par d'autres auteurs pour expliquer le troisième effet endothermique de faible intensité observé sur les courbes ATD des montmorillonites entre 800° et 900°C.

Well-cuttings and core samples from four Gulf Coast areas have been examined by high-resolution scanning-transmission electron microscopy and other techniques to detail the chemical, mineralogical and textural changes during diagenesis. Wells in each of the four areas exhibit a distinct smectite-to-illite (S-I) transition, but the estimated depths for the transition vary significantly: 5600 to 8800 ft for the onset of the transition, and 7500 to 13 600 ft for the completion (70–80% illite). Calculated temperatures also vary significantly: 58° to 92°C for the onset, and 88° to 142°C for the completion. No pattern is evident for characteristic temperatures of the S-I transition. These data suggest that it is inappropriate to use the S-I transition for determining absolute diagenetic temperatures in geological settings similar to Gulf Coast sequences. TEM data imply the presence of two distinctly different kinds of domains, interpreted as smectite (with some mixed-layer component?) and illite. Initially, smectite domains dominate and contain subparallel layers of variable thickness. As the transition proceeds, abundant dislocations provide avenues for diffusion of both necessary reactants (K and Al) and products (Na, Si, Fe, Mg); illite packets grow within a shrinking matrix of smectite. Variable temperatures for the transition may be due to several factors, including chemical heterogeneity of the original smectite, variation in water/rock ratio, porosity differences, diverse chemical character of available fluids, and the particular physical environment for diagenesis. It is suggested that actual depths and temperatures for the transition are primarily a function of kinetic factors associated with a reaction for which at least one of the principal phases is metastable; the transition therefore is highly dependent on local geological factors.

Dehydroxylation of synthetic and natural goethites with a range of Al-substitution from 0–28 mole% was investigated with a view to predicting the behaviour of soil goethites heated by bush fires. Hematites formed at temperatures ≤ 500°C retain the initial Al-content of the precursor goethite up to a maximum of 28 mole% Al. Loss of Al from the hematite structure occurred at 700°C for synthetic hematites with levels of substitution ≥18 mole% Al, but no crystalline alumina phase was present. Crystallization of corundum occurred for synthetic Al-goethites with levels of substitution ≥18 mole% when heated at 900°C. Formation of corundum reduces the maximum level of Al-substitution in hematites to ∼ 12 mole%. The exsolved corundum occurs as aggregated, platy crystals, 40–45 nm in diameter, containing a maximum of 7 mole% Fe. Al-substituted maghemite (7 mole% Al) formed from high Al-goethites heated at 900°C. Although some corundum in soils may be produced by heating of aluminous goethite by fires, the absence of corundum in natural Al-goethites calcined at 900°C, and the very high temperature (900°C) at which corundum formed from synthetic goethite suggest that other sources of Al may be required as precursors for the corundum formed by heating of soils.

The oxidation state and mineralogical distribution of Fe in two different shale sequences have been studied by Mössbauer spectroscopy and chemical methods. Kimmeridge Clay Formation sediments proved to be richer in total Fe than Gulf Coast shales. In such sediments, Fe may be present in clay minerals (Fe(II) in chlorite, Fe(III) in illite and illite/smectite), pyrite, and ferroan carbonates (siderite, dolomite and ankerite). Pyrite accounts for a much greater proportion of the total Fe in the Kimmeridge Clay samples in which it is difficult to reconcile chemical data with Mössbauer data. There is major doublet overlap of Fe(III) in silicates with Fe(II) in pyrite and spectra cannot be satisfactorily deconvoluted. This would appear to be a fundamental limitation for simple applications. In the pyrite-poor Gulf Coast material, however, chemical and spectroscopic evaluations of overall valence state are much more consistent. Confidence in both determinative techniques is generated, and useful information documenting progressive reduction and redistribution of Fe with burial is obtained.

The formation of kaolinite from Al hydroxy interlayered Garfield nontronite has been carried out at 225°C in hydrothermal conditions. The kaolinitization process, which proceeds through mixed-layer kaolinite/smectite intermediates, was followed by XRD, IR and ESR spectroscopy, chemical analysis and charge properties. The smectite content of the clay products decreases regularly with the duration of the hydrothermal treatment. The CEC and the structural Fe content of the deferrated products show a similar trend. IR features specific to nontronite disappear and are barely detectable as the smectite content of the mixed-layer clay falls below 30%. In contrast, the ESR spectrum of nontronite is characterized by a broad g2 signal that remains even after prolonged hydrothermal treatment. The calibration of the g2 ESR signal, due to Fe-smectite, shows that the synthetic kaolinites have low Fe contents (∼ 1% Fe2O3) indicating that the kaolinitization process involves destruction of the 2:1 layers and the subsequent neoformation of kaolinite and Fe oxides. As illustrated by the study of deferrated soil clay samples, representing a weathering sequence Fe-smectite → kaolinite + Fe oxides, ESR spectroscopy proved to be a powerful Fe probe for detecting Fe-rich smectite in kaolinite/Fe-smectite mixed-layer clays.