A series of Mn-goethites was synthesized at highly alkaline conditions. The samples were aged for 15 days at a final [KOH] of 0.3 M. Products were washed free from non-goethite phases using 3 M H2SO4. The bulk mineralogy of the samples was determined by X-ray diffraction and verified on selected individual crystals by electron diffraction. The samples had a relatively low magnetic susceptibility (300≤MS≤400×10-9 m3/kg). As revealed by total acid dissolution, the Mn mole fraction X Mn ranged from zero to 0.125. Five samples (X Mn: 0.025, 0.050, 0.077, 0.099, 0.125) were selected to investigate the variability of the X Mn value in single goethite needles (crystals) by analytical electron microscopy (AEM) using rastered and spot analyses. Linear regressions of both as a function of total Mn yielded unit slopes and zero intercepts, indicating that acid dissolution gave the same results as AEM. Spot AEM, however, revealed significant variation of Mn distribution within individual crystals which argues in favour of Mn zoning in goethite. Inhomogeneous transformation of ferrihydrite to goethite may partly explain the Mn zoning.

The petrography, mineralogy and diagenesis of an overpressured Tertiary and Late Cretaceous mudrock sequence from a single well in the East Shetland Basin (North Sea) are described. The clay fraction is believed to be dominated by volcanic glass above 1400 m depth and by smectite below 1400 m. The smectite content increases with depth due to an increase in detrital smectite and to in situ alteration of volcanic glass and grains; this commenced almost immediately after deposition and is still not complete at burial depths >2300 m. There is little evidence of alteration of the smectite to illite despite a local available supply of K+ in the form of K-feldspar and the presence of K in the more deeply buried smectite. Very little leaching or precipitation of quartz has occurred. Microfossil tests filled with authigenic clays such as have been reported in other Tertiary mudrocks in the North Sea were not observed, though a few fossil cavities are filled with carbonate cements, and very rarely with microquartz. The suprisingly limited reactivity of this mudrock sequence may be due to the early development of overpressure, resulting in a “closed” system, and to the absence of interbedded higher permeability layers or open faults through which fluids could be exchanged.

Structural differences which occur on heating La-montmorillonite have been studied and compared with those for montmorillonite saturated with Na(I) and Li(I). Rehydration experiments show that La-montmorillonite swells after preheating up to 500°C. However, FT-IR spectra suggest partial migration of the La(III) cations from the interlamellar region to the structural sheets and(or) deprotonation of hydrated La(III) cations, followed by migration of the resulting protons to vacant octahedral sites. Additional measurements of X-ray absorption spectra (EXAFS, XANES) and heating to 700°C suggest that interlamellar hydrated La(III) partially deprotonates producing polyoxocations.

A chlorite-smectite interstratified mineral in a pottery stone from Niwatorizawa mine, Izushi-Cho, Hyogo Prefecture (Japan), has been studied by means of Fourier transform methods, together with interlamellar sorption of ethylene glycol and glycerol. The calculations were carried out with the INTER program (using an Olivetti M-24 personal computer with MS-DOS operating system) which allows the analysis of two-component interstratified structures by two Fourier transform based methods: the X-ray diffraction intensity function method, and the interlayer distances distribution function method. The material studied contains a 1:1 chlorite-smectite interstratification and shows a marked tendency to alternation, i.e., the interstratification is almost regular.

A number of bentonitic clay deposits from the Ordu area (NE Turkey) have been investigated by optical and electron microscopy, X-ray diffraction and chemical analysis. The clays are the alteration products of Late Cretaceous trachytic and trachyandesitic pyroclastic rocks. They contain volcanogenic sanidine, plagioclase, biotite and pumice, along with the diagenetic minerals opal-CT, cristobalite and rare calcite and zeolite (clinoptilolite). Ca-smectite (montmorillonite) is the dominant clay mineral. The rare earth element patterns of all the bentonitic clays are generally similar, although there are variations in the Eu anomaly indicating formation from different types of pyroclastic rocks. Oxygen and hydrogen stable isotope values suggest that the smectite was formed at 20–40°C in pore water of mixed meteoric and marine origin during argillization. These features, together with mass-balance calculations, indicate the enhanced MgO and Sr content of the smectite resulted from formation at shallow burial depths within a dominantly marine environment.

The use of transmission electron microscopy in association with other methods is described for the characterization of Brazilian industrial clays, especially kaolinitic-halloysitic clays. Examples are presented from: (a) tubular 7 Å-halloysites and the characterization of mixtures with ordered and disordered kaolinites in residual china clays; (b) tubular kaolinitic clay from Piedade, São Paulo; (c) platey 10 Å-halloysite from Poģos de Caldas, Minas Gerais; (d) rolled forms similar to 10 Å-tubular halloysite formed by repeated K Ac intercalation in well ordered kaolinite; (e) use of particle shape and size of kaolinite crystals in the São Simão, São Paulo ball clays as orientation for good sanitaryware ball clays; (f) characterization of gibbsite crystals in high alumina gibbsite/kaolinite clays; (g) antigorite as a clay mineral; (h) electron optical studies of thermal phase transformations involving tubular kaolinite, halloysites, antigorite, chrysotile, talc and pyrophyllite mono crystals.

Theories concerning the optical properties of cationic dyes adsorbed on clay surfaces are analysed in detail. An investigation of the aggregation of methylene blue (MB) in montmorillonite dispersions is conducted using visible (VIS) spectroscopy. The effects of the dye/ clay ratio and of the swelling properties of the montmorillonite substrate on dye aggregation are compared in terms of the effect of clay layer charge. The observed influence on dye aggregation was almost negligible for both swelling and dye loading. The layer charge of the silicate determines the extent and the type of dye aggregation in freshly prepared MB/montmorillonite dispersions. Observed spectral changes with time indicate a rearrangement and redistribution of dye H-aggregates (band close to 570 nm) to monomers (660 nm), dimers (605 nm) and J-aggregates (760 nm). Dye aggregates are probably already formed during dye cation migration in the vicinity of clay colloid particles. The extent and the type of initially formed species are probably affected by the electric double layer of clay layers. After reaching the clay surface, dye cation assemblies are rearranged and decompose as described above. Reaching chemical equilibrium, dye cations adjust the distribution of the layer charge, in order that each cation could balance the charge due to one unequivalent substitution.

The main reason for the initial determinations of illite crystallinity (IC) was to support the exploration for liquid and gaseous hydrocarbons. The application in 1960 of the Weaver Sharpness Ratio to core materials of a borehole from eastern France indicated that it was not a reliable tool for identifying well-crystallized illite. This ratio was later replaced by the Full Width at Half-Maximum (FWHM), the value of which decreases regularly and consistently towards greenschist facies. The use of FWHM allowed a precise definition of the anchimetamorphic zone between the upper diagenesis and the epimetamorphism. Afterwards, analysis of weak-tointermediate diagenetic sequences showed that illite crystallinity decreases together with the amount of swelling interlayers in mixed-layer clay minerals. Technological improvements, such as computing and modelling of X-ray diffraction patterns, increased the analytical precision relative to measurements of the plain FWHM. Consequently, illite crystallinity went back to its initial use, namely detection of the transitions between diagenesis, anchi- and epi-metamorphism in smectitefree lithologies, where it can be used as a stratigraphic and mineralogic marker of alteration stages.

Hydrotalcite-like compounds (HTlc) were found as secondary products in two corrosion experiments performed with Al plates in highly concentrated solutions (MgCl2-saturated brine) in the presence of metallic and soluble iron at 90ºC under an Ar atmosphere. The durations of the experiments were ∼10 and 13 months, respectively. To investigate the corrosion experiments, solution and gas samples were collected and the main parameters measured. The solid and liquid samples obtained were analysed by X-ray diffraction (XRD), scanning electron microscopy (SEM) and ICP-OES. The presence of the hydrotalcite-like phase was confirmed by XRD and SEM. In one experiment HTlc was found only after dialysis. Formation of the hydrotalcite-like phase was observed at lower pH than mentioned in the literature. These results are useful in understanding the long-term behaviour of corrosion products of reactor fuel elements in relation to their possible barrier function and to comprehend the long-term behaviour of this type of waste during its final disposal.

Isocitric acid oxidative decarboxylation was realized in the absence and in the presence of homoionic Na+-, Mn2+-, and Cu2+-montmorillonite. The catalytic activity of the clay depends upon the nature of the interlayer exchangeable cation. Isocitric acid is transformed into α-ketoglutaric acid under the action of the clay mineral saturated with Na+ cations which do not form a complex with the isocitrate anion. Nevertheless, the reaction rate is very much lower than in the presence of the enzymatic system (isocitrate dehydrogenase enzyme and nicotinamide adenine dinucleotide phosphate coenzyme). The reaction mechanism in the presence of clay is given showing the different steps of the transformation.

The interplay of overpressure and diagenesis is an important factor in developing reservoir quality and controlling the movement of pore-fluids and hydrocarbons within sedimentary basins. The first part of this introduction deals with petrographic features by which certain highly overpressured reservoirs can be recognised. The second part considers the 20 papers, presented at the 1993 Cambridge Conference, published in this special issue of Clay Minerals and how they fit into the broader context of the subject.

The electron microprobe has been used in petrographic research for almost 20 years and more recently as a tool for investigations of clay minerals. However, to the author's knowledge no information has been published concerning the reliability of such analyses―especially those of the alkali elements. Since the alkali contents of clays are often critical, e.g. in determining the smectite content of mixed-layered minerals, it is important to have some idea of the reliability of an analysis made using an electron microprobe.

Three variables can be controlled in electron microprobe analysis—time, beam-current intensity and beam size (the diameter of the beam which excites the sample). Exaggerated conditions of any one of these variables tend to diminish the signal received by the detectors for the alkali elements, especially sodium. As an example, the effect of varying two of these parameters on the intensity of sodium radiation for sodium-aluminumsilicon glass is shown in Fig. 1. The signal decreases (% loss Na) over a 200 s counting time. Beam intensity and beam diameter were varied in this example.

Evidence obtained from ESR and IR studies is presented for the presence of structural Cr in two natural kaolinites (MILO and GEY) formed in an hydrothermal environment in Sonoma County, California. The XRD patterns show a greater structural disorder for GEY than for MILO, but both have the usual hexagonal shapes as shown by TEM observations. On the basis of EDX analysis of different particles, GEY, on average, appears to be richer in Cr (2.1% Cr203) than MILO (0.6% Cr203). The presence of Fe oxide particles containing some Cr, Ni and V is also indicated by EDX analysis. By FT-IR observation, the octahedral Cr 3+ position was easily detected by a well resolved spectral feature at 3586 cm -1. The adsorbed Cr 3+ kaolinite (KMRXCR) presents no band at 3586 cm -1, but two other bands situated at 3527 and 3477 cm -1. The main features of the ESR spectra of these two kaolinites are a set of resonances near g eff4; a broad resonance centred near g eff2 with some modulations; and a set of resonances near gcffl. The broad resonance centred near g eff2 is interpreted as the free iron oxide signal with modulations due to VO 2+. The set of resonances near g eff4 is similar to that observed for octahedral Fe 3+, but the position is shifted compared to that of octahedral Fe 3+. This set of resonances near g eff4 is, therefore, interpreted as belonging to Cr 3+ in octahedral position. The resonances at g eff1 are also interpreted as belonging to Cr 3-. Comparison with the Cr 3+ surface-adsorbed ESR signal of a Cr-saturated kaolinite (which is a broad resonance centred near g eff2) strengthens the interpretation of the g eff4 resonances belonging to Cr 3+ in the octahedral position. The results obtained by the combination of FT-IR and ESR spectroscopics indicate that Cr 3+ is present in the octahedral position of the MILO and GEY kaolinites.

A series of experiments has been carried out to investigate the possible formation of magnetite, Fe3O4, under ambient soil-forming conditions. Rapid and easy synthesis of magnetite was achieved through controlled oxidation of Fe2+ solutions at room temperatures and near neutral pH values. The synthetic products were found to range in size between 0·01–0·07 µm (mean diameter) and hence span the theoretical superparamagnetic-single-domain grain-size boundary.

The choice of kaolinite sample for surface chemical studies is examined. An adsorption apparatus, particularly suited for those studies where the adsorbate acts as a deflocculant, is described. Some preliminary results are given for the adsorption of total phosphate from tetrasodium pyrophosphate solutions and of polyacrylate from sodium polyacrylate solutions. The release of aluminium and silicon from the kaolinite was measured; with tetrasodium pyrophosphate, aluminium-phosphate complexes are formed.

Competitive adsorption measurements suggest that the polyacrylate is adsorbed on the positively charged edges of the kaolinite.