Ten montmorillonites of different origins with variable Fe contents were analysed using transmission IR spectroscopy. Special attention was devoted to the OH-bending region to obtain information about the distribution of octahedral cations. For low to medium Fe contents (≤0.56 per Si8 formula unit), a linear relationship between the position of the δAlFeOH band and Fe content was observed. Such correlation might be explained by changes in the cis-trans occupancy of Fe in the octahedral sheet. Deconvolution of the OH-bending domain allows us to discriminate three components (δAlAlOH, δAlMgOH and δAlFeOH) which are correlated with cation abundances derived from chemical analysis. The relative area of each band can then be compared with theoretical areas calculated assuming a fully random distribution of cations in the octahedral sheet. Using such treatment, eight of the 10 montmorillonites studied presented a nearly randomized octahedral distribution. The two samples from Wyoming were clearly different as they exhibited a strong ordering tendency.

A progressive decrease in cation exchange capcity (CEC) values was observed by treating Ca-montmorillonite with sulphuric acid solutions and this can be understood in terms of the layered structure of the clay. Elemental analysis showed that moderate activation occurred and only 25–30% of the octahedral cations were removed. At the same time the total surface area and the clay acidity increase. X-ray and FTIR data confirmed that acid activation affects both the octahedral and the tetrahedral sheets. The efficiency of acid-activated montmorillonite for the bleaching of cottonseed oil was investigated. The differences in bleaching efficiency appeared to be due to differences in the physical and chemical properties of the bleaching media. The oil acid value was not affected by the bleaching procedure but a slight shift in the absorption maximum of the bleached cottonseed oil was observed. Medium activation of the clay (treatment of Ca-montmorillonite with 4 N H2SO4) was the most effective in bleaching the cottonseed oil, resulting in the best colour index and the lowest peroxide value. A linear dependence of the bleaching efficiency on the clay surface area and acidity was observed. The role of the increased Bronsted acidity is also discussed.

Arid soils in central Iran that evolved from the weathering of post-Tethyan sediments contain palygorskite. Clay fractions of gypsiferous soils and their associated sediments from different landforms from central Iran were investigated. Palygorskite was the dominant silicate clay mineral in clay fractions of the soils, and of Oligo-Miocene limestone, a less common parent rock. The Jurassic shale and Cretaceous limestone contain illite and chlorite with a trace amount of palygorskite. Association of large amounts of palygorskite bundles with gypsum in the gypsiferous soils studied, supports the hypothesis that palygorskite was probably formed after the initial precipitation of gypsum, that created a high pH and Mg/Ca ratio. The major portion of the palygorskite present in colluvial and plateau soils was probably formed authigenically when central Iran was covered by post Tethyan shallow hyper-saline lagoons. Palygorskite in alluvial soils appeared to be essentially detrital.

To investigate the influence of carbonate on the formation of goethite and lepidocrocite, ∼200 samples were synthesized by oxidizing FeCl2 solutions with air/CO2 gas mixtures at ambient temperature and pH 6 and 7. The proportion of lepidocrocite in the lepidocrocite/goethite mixtures (Lp/(Lp + Gt)) decreased from 100 to 0% with increasing in solution and with decreasing average oxidation rate (AOR). These two parameters explained 81% of the variation of Lp/(Lp + Gt)). At a given , more goethite was formed at pH 6 than at pH 7. The Lp + Gt mixtures contained 0–8 mg g−1 carbon (Ct) which could not be removed by washing. Ct reached apparent saturation at a equilibrium concentration of ∼6–8 and 60–80 mmol l−1 at pH 6 and pH 7, respectively. In a plot of Ct vs. Lp/(Lp + Gt) all data fell on the same line irrespective of oxidation parameters (pH, AOR). IR spectra showed two broad bands at ∼1300 and 1500 cm−1 which can be assigned to distorted carbonate adsorbed at the goethite surface. Identical bands were also found in a young, poorly crystalline goethite formed from coal mine drainage in Ohio. It is suggested that carbonate anions direct the polymerization of the double bands of FeO3(OH)3 octahedra common to both minerals toward a corner sharing arrangement, and thereby to goethite, whereas chloride permits edge-sharing as in lepidocrocite.

The mineralogy, petrology and trace element geochemistry of volcanogenic glauconites and smectite-rich clays are described and related to clay assemblages in Lower and Upper Cretaceous sediments of southern England and Northern Ireland. Volcanogenic glauconite grains represent argillized lava particles of predominantly mafic composition and may have been derived from submarine basaltic magmatism; they occur in all the sediments examined (Aptian-Senonian), and are particularly abundant in the Cenomanian-Campanian Hibernian Greensand of Antrim. The smectite-rich clays in southern England have developed by the argillization of predominantly acid or alkaline ash during early diagenesis. Three types of volcanogenic deposit are recognized. Primary bentonites are thin ash-falls deposited in quiet, brackish and marine waters (Speeton Clay, Ryazanian; Weald Clay, Barremian). Secondary bentonites are local accumulations of ash transported into the Cretaceous seas by rivers draining ash-blanketed, local land areas (London Platform, Portsdown Axis). These deposits are well-developed in the Sandgate Beds, Folkestone Beds and their contiguous deposits, and the lower part of the Gault (Upper Aptian-Middle Albian). The ash originated from penecontemporaneous, subaerial vulcanism located in the southern part of the North Sea. The most conspicuous phase of activity occurred during late Aptian times and has been dated by 40Ar/39Ar isotope analysis at 112 m.y. Bentonitic clays and marls are widespread accumulations of argillized ash that occur as a fine-grained fringing facies to glauconitic quartz sand facies. They make up the upper part of the Atherfleld Clay (Aptian) and the upper part of the Gault (Upper Albian), and they are associated respectively with the Hythe Beds (Aptian) and the Upper Greensand (Upper Albian). They may also occur in the lower part of the Lower Chalk (Cenomanian). The distribution pattern of these smectite-rich clays in southern England is related to the changing palaeogeography of the area in Cretaceous times, and the general coincidence of extensive glaueonite deposits and smectite-rich clays in the Middle and Upper Cretaceous of western Europe and along the eastern seaboard of North America is briefly discussed.

FTIR spectroscopy has been applied to NH4+-exchanged dioctahedral clay minerals to determine the molecular environment of NH4+ and to quantify N concentration. FTIR under vapourpressure control, coupled with heating and freezing treatments has shown that NH4+ ion symmetry varies with the nature of clay minerals. NH4+ has a perfect tetrahedral symmetry in hydrated or dehydrated smectites and belongs to the Td symmetry group. The NH4+-bending vibration is centred at 1450 and 1425 cm–1.

The Si4+-Al3+ substitution in dioctahedral clay minerals induces the loss of symmetry elements of the NH4+ tetrahedron which acquires a C2v symmetry. As a consequence, the Td –C2v transition can be used to characterize the smectite–illite transition. Quantification of NH4+ content per half unit cell is provided by nNH4 = k[NH4]/[OH] where [NH4]/[OH] is the band area ratio of the NH4+-bending vibration to the OH-stretching vibration. k = 1.1 for hydrated smectite, 0.9 for dehydrated smectite and 0.8 for illite or tobelite. The bending vibration of NH4+ is chosen for the calculation because it is not affected by superimposed contributions.

Chemical and crystallochemical properties of natural dioctahedral Fe-rich smectites formed by weathering of basaltic rocks in the Berici-Euganei Area, northern Italy, were investigated. It was found that: (i) the b-dimension could be correlated both to Fe3+/Σ ratio (Σ = octahedral occupancy) and to dehydroxylation peak temperature in the range 400–600°C; (ii) it is not correct to impose a total octahedral occupancy strictly equal to 2 for dioctahedral smectites; (iii) in the smectite group there is not a continuous isomorphous series as would appear on the basis of chemical features only.

The mineralogy of the clay fractions of two soil profiles representing the end-members of a catena developed on a glacial till derived from basic lavas has been determined. Particular attention has been given to the assessment of the nature of the amorphous inorganic material in the clay fraction of these soils. Chemical dissolution techniques were used and their effects on the clay fraction were followed by X-ray diffraction, differential thermal, infrared absorption, electron-optical and surface area measurements. The principal conclusion is that the soil clays are a continuum from completely disordered, through poorly ordered to well crystallized material.

Certain minerals in soil clays may be directly estimated from diffraction line intensity and the mass absorption coefficient of the sample to the radiation used. The main advantages of the method described are that no internal standards or calibration charts are required, and any diffraction line of the component to be estimated may be chosen. The results obtained compare favourably with chemical determinations. Orientation of particles during sample preparation makes the estimation of some minerals very difficult. The uncertainty involved in choosing the background level under a diffraction line is the chief source of error in estimating small amounts of very fine-grained minerals.

Four 15 g samples of an unsedimented Wyoming bentonite were treated with 200 cm3 of 0·025, 0·050, 0·100 and 0·250 mol dm−3 H2SO4 for 1 h at room temperature (samples I–IV, respectively). Three further 15 g samples were treated with 200 cm3 of 50% (v/v) H2SO4 for 1 h at 20°C (sample V), and 1 and 2 h under reflux (samples VI and VII, respectively). X-ray fluorescence and diffraction studies revealed that only samples VI and VII suffered any substantial structural attack. The resulting acidity of the clays, determined by cyclohexylamine desorption, indicated that sample V contained the largest number of protons at 0·59 mmol H+ (g clay)−1. Sample V was also the most efficient catalyst for the dehydration and etherification of hexan-1-ol, giving a combined product yield of 17·0% after 2 h reflux in neat reactant. The parent bentonite and samples I and II showed no discernible catalytic activity despite measured acidities of 0·1, 0·24 and 0·34 mmol H+ (g clay)−1. In contrast samples III and IV gave combined product yields of 4·5 and 11·0%, respectively, which correlated well with the measured acidities of 0·38 and 0·48 mmol H+ (g clay)−1. Samples VI and VII, prepared by reflux in acid, contained 0·3 and 0·1 mmol H+ (g clay)−1, respectively, and gave combined product yields of 13·0 and 6·0%.

In the soil-clays of the north-west of England there occurs a mineral with a 14 Å basal spacing which resembles vermiculite but differs in important ways. The mineral is generally more abundant in the surface soils than in the lower layers but only in rare cases does it form more than a minor part of the clay fraction. The other minerals present in these clays are almost invariably mica, kaolin, quartz, chlorite and often hydrous iron oxides. In one soil however, from Moses Cocker's Farm, Rivington, Lancashire, the 14 Å mineral forms a major part of the clay fraction of the surface soil. This soil has the laboratory number La 55/4 and will be referred to as such. The mineral also occurs in smaller amounts in the clay fractions of the deeper horizons of this soil and as the surface soil is at least in part derived from the underlying material, two other samples La 55/6 and La 55/9 were also included in the study.

Poorly crystalline inorganic materials were removed from soil clays of different origin and mineral composition by successive treatments with sodium dithionite, 2 % Na2CO3 and 0·5 N NaOH. Techniques are described whereby difference spectra representing the infrared absorption of the materials removed by the treatments can be obtained. These spectra indicate that the materials dissolved are related to the mineral composition of the soil clay. Dissolution of allophanes of different composition was only proved for two volcanic ash soil clays in which these dominated. Layer silicates, probably including kaolin, were dissolved from the clay fractions of the red-brown earth and krasnozem studied. In addition, alumina-rich gel-like material and gibbsite, but no allophanes, were dissolved from another volcanic ash soil clay in which gibbsite and layer silicates are present in considerable amounts.

The diagenesis of Brent Group sandstones at Heather Field was studied to reconstruct time-dependent variations in reservoir quality, hydrodynamic history, and oil emplacement. Depositional facies, isotopic and trace-element composition of authigenic minerals, and present-day formation-water chemistry indicate several major changes in porewater chemistry related to both gravitational and compactional flow systems that significantly impacted diagenesis. Early cementation by calcite was related to influx of meteoric water and completely occludes porosity in certain areas of the Field, especially in lower reservoir zones. Geochemical, petrographic, and structural evidence indicate that average calcite precipitated at low to moderate temperature from reducing isotopically-depleted water having high levels of radiogenic Sr (40°–50°C, δ18O = −4 to −6‰, 87Sr/86Sr > 0·71). A major period of kaolinite precipitation and feldspar dissolution followed calcite cementation. The isotopic composition of pore-filling kaolinite shows Field-wide uniformity (δ18O average 13·8‰, δD average −53·2‰), suggesting thorough flushing of the reservoir by meteoric water and precipitation at low to moderate temperature (45°–60°C). Tectonic, burial, and thermal histories suggest that meteoric flushing occurred during the late-Cimmerian sea-level low, possibly in response to gravitational flow of meteoric water from exposed parts of the adjacent East Shetlands Platform. Illite and quartz diagenesis post-date kaolinite cementation, with illite K-Ar ages indicating precipitation through much of the Paleogene (55–27 Ma), coincident with migration of hydrocarbons from neighbouring sub-basins of the East Shetlands Basin. Illite stable isotopic data indicate precipitation in a system resulting from partial mixing of trapped meteoric pore-fluids with saline compaction water. The intensity of sandstone diagenesis is influenced by differences in the fluid migration history, content of detrital K-feldspar, and the time of hydrocarbon emplacement and results in spatial and temporal variations in reservoir quality.

Clay minerals find a wide range of application in composites, paints, drilling liquids, cosmetics, and medicine. This article reviews chemical and physical properties of natural and organically modified clay minerals to understand the nanometre-scale structure, surface characteristics, and application in functional materials. The relation between fundamental properties and materials design is emphasized and illustrated by examples. The discussion comprises the following: an overview; surface structure and cation density; solubility and solubility reversal by surface modification; the degree of covalent and ionic bonding represented by atomic charges; the distribution of metal substitution sites; measurements and simulations of interfacial properties at the nanometre scale; self-assembly, packing density, and orientation of alkylammonium surfactants on the clay mineral surface; the density and chain conformation of surfactants in organic interlayer spaces; the free energy of exfoliation in polymer matrices and modifications by tuning the cleavage energy; thermal transitions, diffusion, and optical responses of surfactants on the mineral surface; elastic moduli and bending stability of clay layers; and the adsorption mechanism of peptides onto clay mineral surfaces in aqueous solution. Potential applications in biotechnology and other future uses are described.

For mixed-layer clay fractions from the North Sea and Denmark, X-ray diffractograms have been recorded for specimens saturated with Mg, Ca, Na and NH4, both airdry and intercalated with ethylene glycol, and the patterns have been computer-simulated with a multicomponent program. The mixed-layer fractions consist of an illite-smectite-vermiculite (I-S-V) phase constituting ~90% of the fraction and a kaolinite-illite-vermiculite (K-I-V) phase. For each I-S-V, the degree of swelling in swelling interlayers depends on both interlayer cation and glycolation, whereas the amount of non-swelling illite and swelling interlayers and the interstratification parameters are constant. Based on structural characteristics and the degree of diagenetic transformation, the samples investigated can be divided into three groups. The I-S-V of group one is predominantly detrital and has 0.69-0.73 illite, 0.26-0.20 smectite and 0.04-0.07 vermiculite interlayers, the illite, smectite and vermiculite interlayers being segregated. The I-S-V of group two has been diagenetically transformed and has 0.80 illite, 0.12 smectite and 0.08 vermiculite interlayers, the vermiculite interlayers being segregated whereas the illite and smectite have the maximum ordering possible for R = 1. The I-S-V of group three has been further transformed during diagenesis and has 0.84 illite, 0.08 smectite and 0.08 vermiculite interlayers. Statistical calculations demonstrate that the I-S-V transformation can be described as a single interlayer transformation (SIT) within the crystallites.

The products of air oxidation of mixed Fe(II)-Fe(III) chloride solutions at pH 6 and 7, at 20 and 60°C and at normal pressure contain green rust, maghemite, lepidocrocite, goethite and a paracrystalline ferric hydroxide (ferrihydrite). Among these maghemite, a cubic ferromagnetic iron oxide (Fe2O3) found in many soils, is favoured by slow oxidation rate, high total Fe concentration, the presence of small amounts of Fe(III) in the original predominantly Fe(II) solution, higher temperature and at pH 7 rather than pH 6. The green rust is believed to be an essential precursor of maghemite. On slow oxidation it will form maghemite probably via magnetite. Fast oxidation prevents the cubic phase from being formed and lepidocrocite is the end product. At higher Fe(III) proportions ferrihydrite can be formed which under certain influences converts to goethite and/or hematite. The common iron oxides are seen to form from the same system from small variations in environment which is to be expected from their common associations in soils

Six samples of ‘Vermiculite’ have been studied to investigate the mechanism of its well known but poorly understood property to exfoliate. The samples were analysed quantitatively by XRD to determine their precise mineralogical composition. Electron microprobe methods, including elemental mapping of native potassium and of caesium (introduced by cation exchange) were used to examine variation in the chemical composition of the particles. Most of the samples examined show heterogeneous mineralogical compositions which occur as distinct zones within the volume of individual particles, presenting a mosaic texture. Exfoliation is related to this mosaic distribution of the different mineral phases within the particles. Lateral phase boundaries between vermiculite and mica layers, or vermiculite and chlorite layers are postulated to prevent or impede the escape of gas from a particle, resulting in exfoliation when the pressure exceeds the interlayer bonding forces that hold the layers together. This mechanism provides a common explanation for the exfoliation of ‘Vermiculite’ by thermal methods or by treatment with H2O2. Paradoxically, one sample which consists of pure vermiculite, in the mineralogical sense of the term, demonstrates that pure vermiculite does not and should not exhibit the property of exfoliation. Our explanation of the mechanism of exfoliation explains the commonly observed particle size dependence of exfoliation and the tendency for obviously poly-phase ‘Vermiculite’ samples to show the largest coefficients of expansion.

Most of the important properties of montmorillonite arise because of the high specific surface of this mineral. The way this surface changes with preheating is therefore of considerable importance to those who use this mineral in industry and in research.

The composition, texture and genetic evolution of kerolite and related Mg clays belonging to the Intermediate Unit of the Miocene from the Madrid Basin have been studied. About 400 samples from the Esquivias deposit were analysed by several mineralogical and/or chemical techniques. Two genetic pathways for the development of Mg clays during early diagenesis have been observed: (1) mudflat environment: Al-smectite (beidellite) → Mg-smectite (saponite); and (2) palustrine environment: Si-Mg gel → kerolite → kerolite-stevensite → stevensite.

In the mudflat deposit the transformation processes predominate, whilst in the palustrine environment, kerolite is neoformed, probably from a gel-like medium. Stevensite seems to have originated from the transformation of mixed-layer kerolite-Mg-smectite, but also through neoformation at a later stage.

The textural features, isotopic data and sedimentary evolution within each lithofacies are indicative of shallowing-upward sequences with development of palaeosols. A post-sedimentary origin for sepiolite, calcite, authigenic quartz, zeolites and baryte is inferred.

Among the arsenic species in the environment, arsenate predominates under oxidizing conditions in soils and waters. The adsorption of As(V) by some natural Fe oxyhydroxides and clay minerals and synthetic Al(OH)3 and FeOOH has been studied. The results show a very high As(V) adsorption capacity on both synthetic Al hydroxide (122 mg/g at pH: 5) and Fe oxyhydroxide (76 mg/g at pH 5). This adsorption capacity is pH dependent and shows maximum values at ∼pH 4. The application of these adsorbents to arsenic immobilization in the remediation process of two polluted mining soils with some differences in their physicochemical characteristics has also been studied. The best results were obtained when synthetic Al(OH)3 and FeOOH were used; these materials decreased the water-extractable fraction of As by 55–79% for one soil and by nearly 100% for the other soil, the latter with higher pH and Eh values.