The classification and nomenclature of chlorites have been critically reviewed. A new classification based on the unified projection system of chemical composition has been proposed in which for the first time all trioctahedral, di-trioctahedral and dioctahedral species are included. The chemical composition of chlorites is controlled by the general crystallo-chemical formula: This is superior to the known formulae for the trioctahedral and dioctahedral chlorites. It has been shown that the chlorite end-member compositions of Bayliss (1975) project in one point of the projection field leaving many end-member compositions unnamed. As they correspond to the names used previously, a proposal is made to restore these names.

Mixed-layer illite-smectites (I-S) have a stable charge on the illite interlayer equal to 0·89/O10(OH)2, as shown by electron microscope measurements of mean fundamental particle thickness, using both Pt-shadowing and high-resolution techniques. This has been verified by independent measurements of total surface area and CEC. Typically, the smectite interlayer charge is close to 0·4/O10(OH)2 but clays evolving in K-deficient environments may exhibit higher values. In the course of illitization, the silicate layer composition becomes more restricted than the smectite composition, AlIV and AlVI increasing while Fe and Mg decrease. These substitutions lead to the composition of non-expandable illite differing distinctly from that of muscovite and phengite, close to FIX0·89Al1·85Fe0·05Mg0·10Si3·20Al0·80. HRTEM data show that XRD systematically underestimates %S layers in I-S due to the small size of coherent scattering domains. An experimental curve is proposed for correcting XRD data. The above results were obtained for I-S formed from volcanic materials in different diagenetic and hydrothermal environments. The illite interlayer charge value close to 0·9 reconciles well the available data on I-S produced by wetting and drying of K-smectites.

The point of zero charge (pzc) of synthetic Fe-oxides is well documented and usually ranges between pH 7 and 9 (Parks, 1965; Schwertmann & Taylor, 1977). In contrast, the pzc of natural Fe-oxides has only rarely been determined. Using electrophoretic mobility, Van Schuylenborgh & Arens (1950) found that a natural goethite had a much lower pzc (∼3) than synthetic goethites. They attributed this to better crystallinity of the natural goethite caused by slower crystallization. Soils dominated by Fe- (or Al-) oxides rarely have pzc values as high as those of pure oxides. This is usually attributed to the presence of negatively charged impurities such as clay silicates and organic matter (Parfitt, 1981).

Ferrihydrite, a natural, poorly-crystalline Fe-oxide mineral of bulk composition 5Fe2O3.9H2O, occurs in hydromorphic soils (Schwertmann et al., 1982) and is the main component in ochrous precipitates formed when Fe-bearing fresh waters come in contact with air (Schwertmann & Fischer, 1973; Carlson & Schwertmann, 1981). Under these conditions the ferrihydrite is reasonably free of other charge-active minerals. The aim of this study was to find out if the pzc of these natural ferrihydrites differed from those of synthetic samples.

Interest in mineral–microbe interaction has grown enormously over recent decades, providing information in a puzzle-like manner which points towards an ever increasingly intimate relationship between the two; a relationship that can be truly termed co-evolution. Clay minerals play a very central role in this co-evolving system. Some 20 years ago, clay scientists looked at clay mineral–microbe studies as a peripheral interest only. Now, can clay scientists think that they understand the formation of clay minerals throughout geological history if they do not include life in their models? The answer is probably no, but we do not yet know the relative weight of biological and inorganic factors involved in driving clay-mineral formation and transformation. Similarly, microbiologists are missing out important information if they do not investigate the influence and modifications that minerals, particularly clay minerals, have on microbial activity and evolution. This review attempts to describe the several points relating clay minerals and microorganisms that have been discovered so far. The information obtained is still very incomplete and many opportunities exist for clay scientists to help to write the real history of the biosphere.

A study of the ion-exchange properties of montmorillonite has been performed in order to facilitate computer predictions of the chemical properties of natural fluids and mineral assemblies. Clay/electrolyte interactions can be described using a technique based on the concept of hypothetical surface complex formation. This technique, which is compatible with ion-association models such as GEOCHEM, can be used to simulate simultaneous ion-exchange, hydrolysis of clay edges and anion adsorption on clay surfaces. Effects such as variable cation-exchange capacity and compositionally dependent exchange constants, normally indicating non-ideal behaviour, can be simulated using different combinations of ideal reactions involving charged surfaces and complexing groups representing clay edges. The modelling procedures are flexible and thermodynamically self-consistent. The techniques were applied to data on the ion-exchange characteristics of Wyoming bentonite to yield thermodynamic data for the reactivity of this clay with alkali metals, alkaline earths and a range of first-row transition metals at 25°C.

The formation of FeII,III hydoxy salts (green rusts) from FeII sulphate and chloride solutions and their transformation to lepidocrocite, γ-FeOOH, was studied at ambient temperature, constant pH of ∼7.0 and under controlled air flow. The formation of green rust and its subsequent transformation to lepidocrocite could be distinguished by the reaction rate and solution chemistry. The SO42− and Cl− concentrations in solution attain a minimum at the break between the two reaction steps and are completely restored to their initial values after complete transformation to lepidocrocite. While the green rust is being formed, its FeII/FeIII ratio shows a minor decrease and its FeIII/anion ratio a minor increase. These ratios, however, deviate from the ideal pyroaurite-type structure (M2+/M3+ = 3; M3+/A− = 1; M: metal; A−: anion) and possibilities to accommodate this deviation are discussed. A period during which no protons were produced (i.e. no base consumed) was found to separate the two reactions in the chloride, but not in the sulphate system. During this period a partial solid state oxidation of the green rust took place, leading to its destabilization and eventual decomposition.

The rate of dissolution of synthetic Al-goethites with 0–10 mole% Al in 6 m HCl at 24°C decreases markedly with increasing extent of Al substitution. Most of the dissolution-time curves are S-shaped, suggesting some increase in surface area during the initial stages of dissolution. Based on electron microscopic examination, the increase in surface area is explained by preferential dissolution of less ordered ‘interdomainic’ zones and the consequent production of isolated ‘domains’. Thin sections of crystals provided some support for defect zones oriented parallel to the crystallographic c-direction. Stepwise multiple correlation analysis employing various properties of the goethites was used to investigate the large variation in half-dissolution time (1–96 h). 94% of the variation could be explained by the variation of the mean crystallite dimension perpendicular to the planes (110) and (111) (MCD110, MCD111). Inclusion of Al-substitution increased R2 to only 97%. As shown in a separate experiment, increasing Al concentration in the system retarded crystal growth. It is therefore believed that Al affects the dissolution rate of goethites not directly, but indirectly, by influencing crystal growth rate which, in turn, affects crystal size and order as measured by MCD110 and MCD111.

2-line ferrihydrite stored in water at ambient temperatures from 4 to 25°C and at ten different pH values between 2.5 and 12 for up to 10–12 y transformed to both goethite and hematite at all temperatures and pH values except at pH 12 where only goethite was formed. The rate and degree of transformation (20–100%) increased with increasing pH and temperature. The hematite/ (hematite+goethite) ratio varied between 0 and ~0.8, increased with increasing temperature and showed a strong maximum at pH 7–8 which increased from 0.1–0.2 at 4°C to 0.7–0.8 at 25°C. The maximum coincides with the zero point of charge of ferrihydrite where its solubility and, thus, its via-solution transformation rate to goethite are minimal. We assume, therefore, that in this pH-range the (slower) via-solution transformation to hematite can more efficiently compete with that to goethite.

The densities of two allophane samples measured by displacement with water were 2·72–2·78 g/cm3, while those measured by a float-sink test in CH3OH-, CH3 COCH3-, C6H6- and CCl4-C2H2Br4 mixtures were 1·84, 1·94–1·98, 2·35 and 2·35–2·39 g/cm3, respectively. The results of the density determinations for montmorillonite, kaolinite, halloysite, imogolite and volcanic glass are also reported. The data were interpreted in terms of the accessibility of the displacement liquids to void spaces within and between the layers or ‘structure units’ of the minerals and a model of the ‘structure units’ of allophane is proposed.

Sodium smectite clay, commercially available in the form of bentonite powder, serves as a very effective water flow barrier and cation exchanger. At bulk densities between 1·9 and 2·2 g/cm3 after water uptake to reach complete saturation, the hydraulic conductivity is of the order of 10−14 to 10−13 m/s. The swelling potential produces a tight contact with confining structures, which makes bentonite ideal as overpack of rock-deposited radioactive canisters and for borehole and shaft sealing. At bulk densities between 1·1 and 1·3 g/cm3, the hydraulic conductivity of smectite gels is still sufficiently low to make them useful as effective grouts for sealing rock fractures. The longevity in a granite environment is very significant and the ultimate transformation residue—hydrous mica (“illite”)—preserves some of the excellent sealing properties of smectites.

Double-cation hydroxy compounds, generally with the pyroaurite-type structure, were synthesized by a reaction involving the hydrolysis of a cation in solution by a fully hydrolysed and precipitated hydroxide of a second metallic cation. Synthesis of this type of compound is not limited to reactions between di- and trivalent cations as previously thought but can, in this process, involve divalent-divalent, divalent-tetravalent and trivalent-trivalent cations. Crystalline compounds with reasonably sharp XRD patterns formed generally within a few hours at 25°C at pH 4–8·5 depending on the cations involved. The technique presents a more rapid method of formation of these compounds than previously described syntheses, and has been called induced hydrolysis because the fully hydrolysed cation causes complete hydrolysis of the second cation at a pH below that at which this would normally occur. The minerals takovite, Ni6Al2(OH)16CO3·H2O, and hydrotalcite, Mg6Al2(OH)16CO3·H2O, were amongst some of the compounds formed and the reaction is considered to be more realistic in terms of conditions encountered in natural environments than previous syntheses. The reactions described could also be expected to affect the mobility of heavy elements in the environment. These double hydroxy compounds convert on heating between 300–500°C to double-metal oxides with the spinel structure.

The determination of the cation exchange capacity (CEC) of clays by exchange with the cationic copper complexes [Cu(en)2]2+ and [Cu(trien)]2+ is revisited. The procedures reported by Bergaya & Vayer (1997) and Meier & Kahr (1999) are modified slightly. The concentration of the copper complexes in the equilibrium solutions is measured by spectral photometry. Correct CEC values of the [Cu(en)2]2+ exchange are only obtained when a buffer (‘tris’, tris (hydroxymethyl) aminomethane, pH = 8) is added to the equilibrium solution after separation of the clay, because the molar extinction coefficient of this complex depends on the pH of the solution. A standard procedure is recommended as a reference method. In most cases, tris addition is not needed for the determination with [Cu(trien)]2+ cations. Nevertheless, tris addition is recommended in the standard procedure. Determination of CEC for 40 samples (kaolins, ‘common clays’, bentonites, montmorillonites and beidellite) shows a good agreement between measurements using [Cu(en)2]2+ and [Cu(trien)]2+ cations and with results by the ammonium acetate method.

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.

Boehmite and lepidocrocite, in the form of thin plates with well-developed (010) faces parallel to the constituent layers, show one predominant absorption band due to OH stretching of two-coordinated surface hydroxyl groups, which are shown to be unreactive towards phosphate. In very fine grained preparations of lepidocrocite, two other species of surface hydroxyl are detected, presumably associated with (100) and (001) faces. Marked differences in the frequencies of bulk vibrations between platy and rod-shaped lepidocrocite preparations can be explained partly by their crystal morphology, and partly by degree of crystalline order.

In recent years, experimental and theoretical methods have provided new insights into the size, shape, reactivity, and stability of clay minerals. Although diverse and complex, the surface chemistry of all clay minerals is defined spatially on a common scale of nanometres. This review is organized around the nanoscale architecture of clay minerals examined at several different length scales. The first, and perhaps most important, is the length scale associated with Hbonding in clay minerals. Hbonding interactions define the size and shape of 1:1 phyllosilicates and dominate the surface chemistry of many clay minerals. Structural and surface OHgroups contained within and on the surface of clay minerals provide a type of ‘molecular reporter group’ and are sensitive to subtle changes in their local environment. Examples of OH-reporter group studies in clay minerals, and the spatial scales at which they provide diagnostic information, are examined. The second length scale considered here is that associated with clay–water and clay–organic interactions. Inorganic and organic solutes can be used to explore the surface chemistry of clay minerals. Similar to the use of reporter groups, molecular probes have diagnostic properties that are sensitive to changes in their molecular environment. Clay–water interactions occur at a length scale that extends from the size of the H2O molecule (~0.3 nm) to the larger scales associated with clay-swelling (>10 nm). Similarly, clay–organic interactions are also defined, in part, on the basis of their molecular size, in addition to the type of chemical bonding interactions that take place between the organic solute and the clay surface. Examples illustrating the use of clay–water and clay–organic solute interactions as molecular probes are presented. The largest scale to be considered is that of the particles themselves, with scales that approach micrometres. Recent developments in the synthesis and characterization of ultrathin hybrid films of clay minerals provide complementary information about the nature and distribution of active sites on clay minerals, as well as providing new opportunities to exploit the surface chemistry of clay minerals in the design of functional materials.

The thickness, length, width, area and perimeter of 575 particles from 16 aqueously dispersed samples of a variety of interstratified clays, smectites and illite have been recorded using TEM techniques. Complete dispersion of the clay material was achieved by saturating the clay with either Na+ or Li+, removal of excess ions by dialysis, and isolation of the <0·1 or <0·2 µm fraction by centrifugation. The samples have mean maximum dimensions of 1900 to 90 nm and the dispersed system can be considered as colloidal in nature. The mean thickness of the clays is about 1 nm for smectites, corresponding to that of elementary 2:1 silicate layers, from 1·9 to 4·9 nm for the interstratified clays, and 9 nm for illite. From these data the volume, total surface area and other parameters have been calculated and compared with independent determinations of surface area and CEC. The total surface area by TEM, assuming a density of 2·6 g/cm3, varies from ∼675 m2/g for smectites to 86 m2/g for illite, and is inversely proportional to the mean particle thickness. The charge density of monovalent cation exchange sites on the surface of the particles as determined for nine of the samples varies from 0·54 to 1·16 nm2/site. The particle-thickness distribution data can be used to calculate interstratified XRD layer-sequence probabilities and composition parameters, and agree with XRD data for interstratified clay with <60% illite layers. The thickness data also provide a rationale for the interpretation of TEM lattice-fringe images. Relationships between the particle area, length, thickness and volume are shown to be potentially useful in assessing the mechanism(s) of crystal growth of these extremely small phyllosilicate particles.

The chemical composition of the pyroaurite-type compound Fe(II)Fe(III) hydroxy-carbonate (‘green rust’) synthesised from freshly precipitated ferrihydrite and Fe(II) chloride solution at pH 7.0 (‘induced hydrolysis’) was determined. The compound was nearly stoichiometric, with the formula the Fe(II):Fe(III) ratio being independent of the Fe(II):Fe(III) ratio in the initial reaction mixture. It shows all the XRD peaks reported for this compound. Visible-near IR shows a broad peak at 650 nm which is ascribable to intervalence charge transfer and therefore the absorbance maximum decreases with increasing degree of oxidation. Wetting the material with compounds containing hydroxyl groups (such as glycerol or glucose) retards the oxidation of the otherwise very oxidation-sensitive compound. Similar polar organic compounds may stabilize the Fe(II)Fe(III) hydroxy-carbonate in wet soils. The importance of the green rust phase as an intermediate in the formation and transformation reactions of oxides and oxyhydroxides of iron in natural environments should not be ignored.

The JP (Jelšový Potok, Slovakia) and SWy-1 (Wyoming, USA) montmorillonites were treated in 6 m HCl at 95°C. The rates of dissolution of tetrahedral and octahedral Al (Altet and Aloct) are comparable. Aluminium in the tetrahedral sheets was also identified in both untreated samples by 29Si MAS NMR spectroscopy. About 12% of total Si in SWy-1 was found to be bound in quartz. Traces of undecomposed montmorillonite and a small amount of Altet in the three-dimensional SiO4 framework were found in the reaction product of montmorillonite decomposition. Three different types of structural units were identified in acid-treated samples: Q4(OAl) units of amorphous silica with three-dimensional crosslinked framework; (SiO)3SiOH units, remaining as a result of poor-ordering of the framework without the possibility of cross-linking; and Q4(1A1) units.

Hematites obtained by heating goethite gave different IR absorption spectra depending on the temperature of formation. Hematites formed between 250–600°C consisted of lath-like crystals (average size 0.4 ×0.08 µm) and showed, in accordance with theoretical predictions, very similar IR spectra whose absorption bands could all be assigned to surface mode vibrations. However, significantly different IR spectra were given by hematites formed between 700–950°C, the differences being correlated with variations in the size and shape of the particles. Differences observed in the IR spectra of powder hematite do not therefore justify new names for the mineral, as have been proposed in the literature.

A number of natural kaolinites from a variety of world sources have been investigated using electron spin resonance at both X-band and Q-band. The results show systematic differences some of which are related to the crystallinity of the material, some to the presence of transition metal ions other than Fe3+ and hole-trapping defects.

The results provide evidence for at least three sites for substitution of Fe3+ having large crystal fields : (i) a site with near maximum rhombic character having crystal field parameters D > 1·2, λ = ⅓ the occurrence of which is correlated with lack of crystalline perfection due to stacking defects or to the proximity of surfaces, (ii) a site with less rhombic character with λ = 0·234, D = 0·585 as likely parameters, and (iii) a site with parameters near the values λ = 0·207, D = 0·322.

Mössbauer and ESR evidence suggests that the principal sites of Fe3+ substitution are octahedral. The presence of Fe3+ in adjacent cation sites leads to a very broad resonance centred near geff = 2. Other resonances in the spectra are attributed to the effect of trapped holes, some at least of which are situated at oxygen sites adjacent to Al3+ ions substituting in sites normally occupied by Si4+.

Hyperfine effects due to the presence of the transition ions Mn2+ and (VO)2+ are also observed.