The finest fractions of Upper Jurassic shales from the North Sea and onshore Denmark contain 80–90% of an illite-smectite-vermiculite (I-S-V) mixed-layer mineral and, in addition a phase which has X-ray diffraction (XRD) peaks at 7.20–7.26 Å and 3.56–3.58 Å in air- dried and glycolated specimens. This phase may be a fine kaolinite with a small thickness of coherent scattering domains (CSDs) or alternatively a mixed-layer mineral which has kaolinite as the dominant component. For one sample from the Norwegian well 9/4-3, these alternatives are investigated using the multi-specimen method by which agreement between the experimental pattern and the pattern calculated for one and the same structure is obtained for each of several specimens saturated with different cations and with/without glycolation. It is demonstrated that the modelled XRD patterns for a kaolinite-illite-vermiculite (K-I-V) structure having 0.94 kaolinite, 0.03 illite and 0.03 vermiculite layers and random alternation fit the experimental patterns.

Van Houten (1973), Walker (1976) and Turner (1980) have documented the authigenic origin of red coatings in continental red beds. Pigmentary hematite, ferrihydrite, goethite and clay-associated Fe-oxide crystallites have been suggested as the principal colouring agents of continental red beds (Walker, 1968, 1976; Walker & Honea, 1969; Larson & Walker, 1975; Turner, 1980; Fine, 1986, Besly et al., 1993). The red pigment in most Mesozoic red beds consists of hematite, whereas ferrihydrite and goethite are more common in younger sediments (van Houten, 1973). This has led several authors to the assumption that poorly crystalline Fe-oxides/hydroxides age into hematite (Walker, 1968, 1976; Walker & Honea, 1969; Larson & Walker, 1975; Turner, 1980).

The redox potential and pH of aerated suspensions of iron(II) hydroxide in sulphate–containing aqueous solutions are measured during the oxidation process. Plateaux corresponding to the equilibrium conditions between Fe(OH)2(s) and Fe(II)-Fe(III) hydroxysulphate GR2(SO2-4)(s) on the one hand, and between GR2(SO2-4)(s) and FeOOH(s) on the other hand, are displayed. Potentiometry, voltammetry, pH-metry and Mössbauer spectroscopy are applied to follow all reactions. The thermodynamic meaning of the measured potential of the first plateau which corresponds to the GR2(SO2-4)(s)/Fe(OH)2(s) equilibrium is demonstrated. The chemical composition of GR2(SO2-4)(s) is found to be FeII4 FeIII2(OH)12SO4·nH2O all along the oxidation process, implying that this compound must be considered as a pure phase with a well-defined composition. The Gibbs standard free energy of formation or chemical potential μ°[GR2(SO2-4)(s)] in ‘anhydrous form’ (n = 0) is determined at −3790±10 kJ mol-1. A consistent value of μ°[Fe(OH)2(s)] at −490±1 kJ mol-1 is obtained.

The Tinto river is one of the most polluted stream environments in the world, as a result of both acid mine drainage and natural acid rock drainage. Two representative samples from the phyllosilicate-rich rocks exposed in the drainage basin (Palaeozoic chlorite-bearing slates and Miocene smectite-rich marls) were treated with acid river water (pH = 2.2) for different times to constrain the effects of extreme hydrogeochemical conditions on clay mineral stability. Illite and kaolinite did not show appreciable variations in their crystal chemistry parameters upon treatment. Chlorite underwent an incipient chemical degradation evidenced by the progressive loss of Fe in octahedral positions coupled with a shortening of the b unit-cell parameter, although no weathering products of chlorite were observed. Smectite and calcite were rapidly and fully dissolved thus neutralizing the water acidity, and subsequently Fe and Al oxy-hydroxides and opaline silica precipitated from the aqueous solution, together with a neoformed amorphous silicate phase largely enriched in Al and Mg.

In order to determine the relative influence of palaeoenvironmental and diagenetic processes in clay assemblages, as well as their significance, both fine- and coarse-grained sediments from the Campo de Gibraltar flysch have been studied by means of X-ray diffraction, optical and electron microscopy, and chemical analysis. Diagenetic modifications appear to be lithologically controlled and mainly affect coarse-grained sediments, where Fe-chlorites, illite and kaolinite are the more characteristic authigenic clay minerals. The evolution of detrital assemblages, determined in fine-grained beds, indicates that, from Cretaceous to Eocene times, clay mineralogy, characterized by the opposite kaolinite+smectite and illite + I-S mixed-layer assemblages, was mainly controlled by sources, climate and transport processes. On the other hand, from the Oligocene, clay mineral assemblages, characterized either by the abundance of kaolinite, or by the illite+chlorite association, mainly reflect the petrology of source rocks, as a consequence of climatic cooling and the increasing tectonic activity, which impede the development of soils.

Both magnetic and electric fields have been used to induce optical birefringence in colloidal suspen- sions. These have formed the bases of fast methods for measuring particle sizes and their distributions (Mekshenkov, 1965; Tsvetkov et al., 1975; O'Konski, 1976; Maret & Dransfeld, 1977; Stoylov, 1991; Jennings & Stoylov, 1992; Trusov & Vojtylov, 1993). Whereas the majority of studies have related to the electric-field procedure, we recently reported a magneto-optic procedure suitable for sizing magnetic particles (Wilson et al., 1996). By increasing the viscosity of the suspensions, we are now able to extend the size range down into the sub-micron region with relative ease. This note reports magnetic birefringence data for a montmorillonite sample and justifies the technique via supplementary data on vermiculite of greater size range.

Textural, mineralogical and chemical parameters of clays belonging to the Upper Cretaceous ‘Argilas de Aveiro’ formation were determined and correlated, using geostatistical analysis, with some relevant geotechnical parameters. The data obtained made it possible to subdivide the region under study into distinctive sectors on the basis of geotechnical behaviour. Clay fraction content and composition control geotechnical parameters such as the expandability and the Atterberg plastic and liquid limits. A positive correlation was found between both expandability and the Atterberg plasticity index values and the total phyllosilicates, smectite, MgO and Al2O3. However, Atterberg plasticity index values were relatively independent of the diverse clay mineral assemblages.

An artificial neural network with back-propagation architecture has been applied to the problem of the quantitative analysis of simulated clay mixtures from synthetic X-ray diffraction (XRD) data. A ‘clay characterization function’ (CCF) has been devised that combines information from two clay XRD peaks into a single peak that simplifies the problem which the neural network must solve. In addition, it eliminates peaks from non-layered minerals. A neural network with 17 neurons in the hidden layer and log-sigmoid transfer functions in each layer is sufficiently successful at predicting the compositions of binary and ternary mixtures of the three model clays to demonstrate the potential of the method. Moreover, training is accomplished in relatively short times using a Levenberg-Marquardt algorithm.

Although the problem ‘solved’ by this neural network is rather simple, the approach has the potential for the much more complex problem of the quantitative phase analysis of mixtures of real clay minerals. In this study, only the CCF peak heights have been used, but the positions and peak widths also contain important information. It may be practical to include as variables not only the concentrations of clay phases, but also compositional information to which XRD is sensitive.

Ferric oxides or hydroxides are common minor components of kaolinites. If free, these minerals can be removed by Fe reduction and leaching (Mehra & Jackson, 1960), but Fe3+ ions can substitute for Al3+ ions in the octahedral sites of kaolinite (Weaver et al., 1967; Malden & Meads, 1967). Moreover, Fe2+ ions can also substitute trioctahedrally in place of dioctahedral Al, leading to a local composition of Fe-antigorite (Cuttler, 1980).

The Font-Bouillant quarry (Charentes, France) is the source of disordered kaolinite known as ‘FU7, U-7-10 or FBT’ (Cases et al., 1982, 1986). The samples used in this study were considered in an earlier genetic analysis of the deposit (Delineau, 1994; Delineau et al., 1994) and were analysed previously by X-ray diffraction (XRD), infrared (IR) spectroscopy, electron paramagnetic resonance (EPR) and UV-visible spectroscopy by Delineau et al. (1994) (Table 1).

The deuteration technique was used to assign unknown infrared (IR) bands of Ge-bearing talcs. Procedures to achieve partial or complete H/D replacement in synthetic Ge-bearing (Mg, Ni and Co)-talcs have been investigated. From the spectral shift of IR absorption bands in the 4000–300 cm-1 region resulting from the H-D substitution, the IR bands of vibrations of Ge-bearing talcs could be identified and synthetic Ge-bearing talcs showed significantly different IR spectra from those for Si-talcs. After deuteration of synthetic Ge-bearing talcs, the unknown bands were attributed either to hydroxyl groups or to Ge–O vibrations of the clay skeleton, and the spectra were interpreted fully.

Diagenetic evolution of illite-smectite in shales corresponds more closely than in other rocks to the thermal history of a sedimentary basin. In bentonites, illitization can be significantly retarded by the diffusion of K+ (Altaner et al., 1984; Clauer et al., 1997), and in sandstones it can be related to a localized hot fluid flow (e.g. Whitney & Northrop, 1987). Thus, extracting the diagenetic ages of illite-smectite from shales remains an important opportunity for the verification of basin history models.

The construction and operation of a spray drier is described where the spray is produced using an air brush, essentially a miniature spray gun. The spray-dried products consist of spheres 50–60 µm in diameter and typical product recoveries are 80%, a marked improvement over simple two-nozzle systems. The spray-dried samples are easy to load into XRD powder holders and present a smooth surface and relatively constant bulk density to the X-ray beam. Problems of preferred orientation are effectively eliminated and the resulting X-ray powder patterns are completely reproducible by different operators.

Back-scattered electron images of clay minerals from sandstones are used, together with complementary micro-analytical methods, to identify and quantify mineral microporosity and geochemistry. Clay minerals typically have a range of microporosity from 10 to >90% dependent on texture and paragenesis. Fibrous clays are highly microporous; detrital clays have low microporosity but specific clay minerals have broad ranges of microporosity. The often quoted mineral-chemical association between thorium (Th) and kaolinite cannot be substantiated by micro-analysis. The Th content of clay minerals is associated with micro-inclusions within the kaolinite which form diagenetically or are derived from precursor minerals.

Early Cambrian sediments on the East European platform in North Estonia, represented mostly by clays and silty clays, were deposited under normal marine conditions. The sediments were never affected by significant tectonic or thermal events after sedimentation 530 Ma ago, and the clays still have high water contents. The clay fraction, divided into four sub-fractions, was studied using X-ray methods and Rb/Sr dating. Decomposition of the XRD curves was used to quantify the amounts of illitic minerals in the sub-fractions. Dating by Rb/Sr showed that the finest fraction (<0.06 µm) was formed 50–150 Ma after sedimentation. The coarser fractions also contain considerable amounts of diagenetically formed minerals. This shows that neoformation of illitic minerals in marine sediments with high water/sediment ratios is a very important process even at temperatures <35°C.