Mineralogical and water adsorption properties of water dispersible clays extracted from five soils of different typology after treatment with acid to different pH levels, were studied. The mineral composition, total and divalent iron content, surface areas and average adsorption energies of water dispersible clays were complicated functions of pH, reflecting the simultaneous effects of mineral destruction and aggregate disruption processes under influence of protons on soils. The magnetic susceptibilities of water dispersible clays were the lowest at pH 2 and roughly constant at higher pH values indicating the dissolution of crystalline forms of Fe below pH 3. The broadening of the peaks of adsorption energy distribution functions with decrease in pH is related to increasing formation of amorphous material from mineral phases.

Proteins (catalase, albumin, pepsin and lysozyme with different molecular weights and isoelectric points) were differently adsorbed at pH 7.0 on the clay fraction of three raw Na-saturated smectites (Crook and Uri montmorillonites and one hectorite). The adsorption isotherms of proteins on clay minerals showed typical Langmuir characteristics. Lysozyme was adsorbed under the effect of electrostatic interactions between the opposite charges of clay surfaces and protein molecules, whereas catalase and albumin were adsorbed under the effect of non-electrostatic forces. Pepsin was held in relatively high amounts only on the surfaces of hectorite. Proteins were intercalated in the interlayers spaces of smectites, usually undergoing extensive unfolding. Protein-smectite complexes showed different behaviour to heating treatment. Some complexes remained practically unchanged after heating at 200°C. Presence of ‘wrecks’ of interlayered materials was found after heating at 500°C for two hours. The amounts of proteins adsorbed on the external and interlamellar surfaces of clay minerals, partially coated with OH-Al species, were much lower than those fixed on the clean clays. Only lysozyme was intercalated in chlorite-like complexes.

The Carrión lacustrine basin (≈10 km2), central Spain, is filled with Neogene-Quaternary sediments (20–25 m thick) rich in palygorskite. Two clay units of ∼15 m thick are of particular interest in the basin. The lower one contains over 75% phyllosilicates (palygorskite up to 90%, smectites and illite) in marginal areas (facies A), and it is laterally bound by a distal facies (B) also rich in clay minerals (smectites and illite, and some palygorskite) and gypsum, and includes dolomitic marl intercalations. The composition at the top is primarily marly, with some gypsum (C). The upper clay unit, mainly clay-size material, (D) is rich in smectites, palygorskite and sepiolite, and contains carbonate intercalations near the top. A statistical treatment of the mineralogical and chemical composition data from 120 samples, using principal factor analysis, suggests that: (a) illite and smectite are detrital in origin; (b) palygorskite was formed by transformation of illite and smectite involving reaction with Mg; and (c) sepiolite was a neoformation product favoured by evaporation of silica- and Mg-bearing waters.

Measurements of the kinetics of acid dissolution of synthetic aluminous goethites and corresponding hematites produced by heating of parent Al-goethites at various temperatures were carried out in 1 M HC1 at 30, 40 and 50°C. Dissolution-time curves show sigmoidal shapes for the goethites (110°C), whereas deceleratory shapes were obtained for most of the partly and fully dehydroxylated samples. The dissolution rate for all materials decreased with increasing Al substitution and increased with increasing dissolution temperature, specific surface area and heating temperature. On a unit surface area basis, hematite dissolved ∼ 2–8 times faster than goethite. Dissolution kinetics of most heated goethite samples (200–260°C) were quite well described (R2 > 0.96) by the modified first-order Kabai equation. The activation energy and frequency factor for dissolution increased with increasing Al substitution.

Distribution of Fe in the fine fractions of four Czech bentonites was investigated by M6ssbauer spectroscopy. The spectra at room and liquid nitrogen temperatures revealed that most Fe present in the samples is structurally bound in smectite. Microcrystalline and/or Al-substituted goethite was identified in all fine fractions, comprising 8 to 50% of total Fe. This mineral was partly magnetically ordered in the Rokle sample even at room temperature. Two per cent of total Fe was bound in hematite in the Krásný Dvoreček sample. Only octahedral Fe(III) was found in the Rokle and Stebno samples, while Krásný Dvoreček and Hroznětín contained 9 and 38%, respectively, of the total Fe as octahedral Fe(II) and 14% of total Fe was found as tetrahedral Fe(III) in Krásný Dvoreček.

L'arrangement des cations décylammonium échangés entre les feuillets de la vermiculite et de l'hectorite a été précisé à l'aide des spectrométries infrarouge (IR) et Raman. Les expériences de deutération et de dichroïsme faites par spectrométrie IR montrent que l'axe C3 du groupe NH3+ est confondu avec l'axe de la cavité hexagonale sous-jacente, dans le cas de la vermiculite comme dans celui de l'hectorite, et que dans la vermiculite, les groupes NH sont engagés dans des liaisons hydrogène avec les atomes d'oxygène des tétraèdres substitués. La spectrométrie Raman permet de comparer l'arrangement des chaînes alkyles dans les deux phyllosilicates. En effet, cette technique donne accès à des modes de vibration très sensibles à la conformation des chaînes carbonées, en particulier grâce à la décomposition des bandes de vibration de valence des groupements CH2 et CH3. Ainsi, dans la vermiculite, on montre que les chaînes sont linéaires, et que leur arrangement interdigité est proche de celui du chlorure de décylammonium cristallisé. Les spectres Raman obtenus pour l'hectorite mettent en évidence l'existence de conformations gauches dans les chaînes alkyles, qui caractérisent une rotation de la chaîne autour de la liaison C-C voisine du groupe NH3+ terminal. Les résultats obtenus en Raman montrent par ailleurs que les interactions entre les chaînes alkyles sont beaucoup plus faibles que dans le cas de la vermiculite et confortent le modèle proposé ici pour le complexe hectorite-décylammonium, pour lequel les chaînes sont parallèle aux surfaces des feuillets.

Rare earth elements are mobilized, fractionated and precipitated during weathering processes of basaltic rocks of the Lisbon Volcanic Complex. In general there is an increase in the REE contents in the whole samples with increasing weathering and fractionation between LREE and HREE. Cerium is partially oxidized and Ce4+ is retained resulting in negative Ce anomalies in the whole samples of the base and middle profile zones. In the clay fraction there is an increase in the REE contents from the top to the bottom of the profiles, except for Ce, giving rise to positive Ce anomalies in the top and negative anomalies of this element in the middle and bottom of the profiles. Significant positive Ce anomalies occur in the finer fractions of palaeoweathered (Upper Cretaceous/ Lower Eocene) basalts associated with low REE contents relative to the whole samples indicating more intense leaching conditions in the Upper Cretaceous/Lower Eocene than at the present day. However, in both cases no positive Ce anomalies were found in the whole samples. There appears to be a negative correlation between the (La/Yb)ch ratio variation and the clay mineralogy/drainage conditions.

Any X-ray diffraction (XRD) pattern of mixed-layered maximum R1 ordered chlorite-smectite has an exact equivalent R0 smectite-corrensite or R0 chlorite-corrensite. The identification of a mineral as chlorite (0.8)-smectite R1 is, therefore, not evidence that the fundamental layers in such an interstratified structure are actually chlorite and smectite. Furthermore, there are no obvious differences between XRD patterns of R0 and R1 chlorite-smectite at high or low smectite contents. Therefore the frequently supposed identification of R0 chlorite-smectite with high smectite contents is also questionable. It follows that the identification of R1 chlorite (0.8)-smectite by Robinson & Bevins (1994) and of R0 chlorite (≈0.2)-smectite in an earlier publication (Robinson et al.,1993) cannot be used as evidence regarding the debate over the nature of the component layers in interstratified minerals intermediate between trioctahedral smectite and chlorite.

Diffraction patterns of natural mixed-layer chlorite minerals from metabasites often show a strong reflection at 31 Å yet a lack of resolution of other mixed-layer peaks from those of discrete chlorite. These features cannot be matched by any single mixed-layer mineral consisting of layers of chlorite and smectite and were matched by Robinson & Bevins (1994) using a mixture of different chlorite-smectite minerals. However, they can be matched by a single chlorite-corrensite mineral in which there is an element of segregation of layer types. These different possibilities need to be investigated further.

Samples from deep wells of the Palaeozoic Illinois Basin show the evolution of the illite-smectite (I-S) mineral transformation under low temperatures for very long periods of time. Curve decomposition of the XRD traces was used to identify and follow the evolution of the different clay minerals. All samples showed the presence of a large amount of illite (fine and coarse grained micaceous material) and a much smaller quantity of I-S. In a vertical section the R=1, I-S diffraction peak initially changes with depth indicating change from 80 to 95% illite content but it changes little with further depth. The I-S intensity decreases with depth compared with the illite peaks. These relations indicate that the clays in the Palaeozoic sequence of the Illinois Basin evolve continuously in the deep well, It appears that the final stages of I-S mineral transformation can reach a compositional stagnation (smectite content) as the I-S phase is dissolved to produce illite.

Using a kinetic model to describe the smectite to illite clay transformation, it is deduced that the Illinois basin had a longer period of deposition than indicated by present stratigraphy and that almost 1.5 km of sediments deposited in the Mesozoic were eroded in later Tertiary times. These conclusions are confirmed by published data on the maturation of organic matter.

In the region of Illapel, Chile, dioritic and andesitic rocks of Cretaceous to Paleocene age from the Unidad San Lorenzo Formation have suffered strong alteration resulting in the formation of a kaolin-rich zone, which is mined in the La Vanguardia open pit near Combarbalá. Detailed mineralogical and chemical investigations of channel samples from this mine, involving a combination of optical microscopy, XRD, SEM-EDX, XRF and TEM, permitted recognition of a zone of silicification and a zone of advanced argillic alteration, both of which contain alunite s.s. and aluminium-phosphate-sulphate (APS) minerals of the woodhouseite group with subordinate amounts of gorceixite, florencite and goyazite. These minerals are found to have originated from infiltration of meteoric waters. The passage from silicification into advanced argillic alteration was associated with an increase in sulphate activity as manifested by the increase of SO42— at the expense of PO43— in the alunite s.s. and by an increase of the Au content from 0.01 to 0.03 mg/kg Au. This high sulphidation type of wallrock alteration may be roughly compared with the Nansatu type of White (1991).

Authigenic chrysotile, stevensite, calcite, aragonite and pectolite have formed together in a Holocene ophiolitic debris flow in Southland, New Zealand. Mineral growth occurred about 4700-5700 years ago. The temperature of formation of these minerals is estimated from climatological data to be 5–10°C Surface water and groundwater δ180 is currently about –10‰, and was estimated to be about –9.5±1‰ during mineralization. Coexisting calcite (δ180 = +23‰) and aragonite (δl8O = +24‰) were in equilibrium with each other and with the groundwater at 5–10°C Stevensite δ180 is +14 to +16‰, chrysotile has δ180 = +5.5‰, and authigenic pectolite has δ180 near +10‰. Carbon isotope ratios for calcite and aragonite are strongly depleted (δ13C = –13 to –18) which suggests that dissolved CO2 had δ13C below -27. This isotopically light carbon probably resulted from a high organic component of carbon dissolved in the groundwater.

According to XRD, DTA, geochemistry and electron microscopy data of a few selected samples, three categories of smectitic clay minerals occur in the Cenomanian and Turonian chalks sampled in several sections of northern France. The first one corresponds to detrital illitesmectite mixed-layers reworked from pedogenic blankets. The second category, characterized by crystalline overgrowths around detrital particles, is especially well-developed in slowly deposited sediments. The third category, located in thin marly horizons, corresponds to Cheto-montmorillonite derived from the submarine alteration of volcanic glass. Smectitic minerals having distinct origins may therefore occur at different levels of a given formation. From these examples, the smectite-rich clay sedimentation of the Late Cretaceous is tentatively explained by the combination of favourable factors induced mainly by highstand of the sea-level. The climatic conditions, the reduction of continental areas, the development of carbonate platforms and the decrease in sedimentation rates constitute the most relevant of these factors.

The definition of ‘clay’ was first formalized in 1546 by Agricola. It has been revised many times since, although the fundamentals involving plasticity, particle size, and hardening on firing were retained by most. For an exhaustive account of the history of the definition to 1963, the reader is referred to Mackenzie (1963). More recent developments may be found in Weaver (1989).

The definition of clay raises the important issue of clay constituents and, implicitly, the definition of ‘clay mineral'. Mackenzie (1963, p. 15) noted the inappropriateness of defining clay mineral as “any mineral which occurs in clay” since, among several reasons, it would include many accessory minerals that are not characteristic of clay.

Two profiles in Devonian marine deposits have been studied, consisting of pelites, subgreywackes, greywackes and quartzites. Quartz and clay minerals are major components and feldspar and calcite are minor ones. Phyllosilicates in the fine fractions are kaolinite and illite; kaolinite has a high degree of ordering; illite is predominantly of a 1Md polytype, with low Na content and poor crystallinity and has a phengitic composition in greywackes, whereas in pelites it is muscovitic in composition. Both phyllosilicates may be inherited from a source area with intensive weathering processes, although illite may also be a diagenetic phase. These mineral characteristics indicate that the Devonian rocks did not reach the anchizone boundary in their post-depositional evolution.

The chemical composition of pelites and subgreywackes reveals a high degree of chemical maturity. Chondrite-normalized REE patterns indicate a higher degree of weathering of these Devonian sediments than of Post-Archaean Australian Shales (PAAS), possibly as a consequence of sedimentary recycling processes. The REE patterns of the Devonian rocks in addition to the high Th/Sc, La/Sc and Th/Co ratios suggest a felsic composition of the primitive source area, probably a K-rich granite.