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.

A statistical comparison has been made of chemical data for terra rossa and carbonate-free residues of Cretaceous limestones and dolostones in Apulia in order to evaluate the hypothesis that the terra rossa is a product of weathering of the underlying carbonate rocks. It has been shown that the differences in chemical composition between the residue of the carbonate rocks and the terra rossa are consistent with the former being the parent material of the latter. The transformation from carbonate rock residue to terra rossa was governed mainly by chemical weathering which produced a marked decrease in the K2O/Al2O3 (i.e. illite/kaolinite) ratio. The geochemical pattern of the Apulian terra rossa has been influenced by sedimentary processes which have led to a characteristic distribution of mineralogical components and a moderate contamination by biogenic silica.

The controversy on the origin of the English Mesozoic fuller's earths is reviewed. Findings of (1) unaltered, devitrified, zeolitized and argillized glass, (2) fragments of holocrystalline and hypocrystalline igneous rocks, and (3) high temperature feldspar and sphene, conclusively demonstrate their volcanic origin. The bulk chemistries of the Bathonian and Lower Cretaceous earths are similar and suggest derivation from ash of trachytic composition. The two earths can be differentiated mineralogically and petrographically by: (a) the presence of sphene, and zeolites replacing glass in the Lower Cretaceous earths; (b) the presence of minor amounts of basaltic glass in the Bathonian earths.

K/Ar radioisotope dating on high-temperature alkali feldspar gives (1) 152–159 Ma for the Bathonian fuller's earth, and (2) 118–125 Ma and 142–148 Ma for the Lower Cretaceous fuller's earths. The distribution of the Mesozoic fuller's earths and the origin of their parent ash is discussed.

Concrete and bentonite are being considered as engineered barriers for the deep geological disposal of high-level radioactive waste in argillaceous rocks. Three hydrothermal laboratory experiments of different scalable complexity were performed to improve our knowledge of the formation of calcium aluminate silicate hydrates (C-A-S-H) at the interface between the two materials: concretebentonite transport columns, lime mortar-bentonite transport columns and a portlandite- (bentonite and montmorillonite) batch experiment. Precipitation of C-A-S-H was observed in all experiments. Acicular and fibrous morphologies with certain laminar characteristics were observed which had smaller Ca/Si and larger Al/Si ratios with increasing temperature and lack of accessory minerals. The compositional fields of these C-A-S-H phases formed in the experiments are consistent with Al/(Si+Al) ratios of 0.2– 0.3 described in the literature. The most representative calcium silicate hydrate (C-S-H) phase from the montmorillonite–cement interface is Al-tobermorite. Structural analyses revealed a potential intercalation or association of montmorillonite and C-A-S-H phases at the pore scale.

X-ray powder diffraction patterns of halloysite (7 Å) are characteristically altered following solvation with ethylene glycol. Some effect was first noted in the classic work of MacEwan but its value in the unequivocal identification of halloysite (7 Å ) seems to have been overlooked subsequently. The response to ethylene glycol solvation involves a decrease in the intensity (peak height) of the peak at ∼7.2 Å and an increase in the intensity (peak height) of the peak at ∼3.58 Å thus narrowing the 7.2 Å /3.58 Å peak height intensity ratio. For pure samples of halloysite, this ratio is narrowed by an average of ∼50%. This distinctive change is related to the interstratified nature of halloysite (7 Å), specifically the presence of ‘residual’ interlayer water, i.e. halloysite (10 Å), which can be replaced with ethylene glycol so forming 10.9 Å layers, a spacing that is almost exactly one and a half times the thickness of dehydrated (7.2 Å) layers which do not imbibe ethylene glycol. Thus the separation between the 001 peaks in the 7.2 Å /10.9 Å interstratification is increased and the 0027.2 (3.58 Å) and 00310.9 (3.63 Å) peaks become more or less coincident, compared to the 7.2 Å /10 Å interstratification, i.e. the partially hydrated state. The widespread use of ethylene glycol solvation in clay mineral studies makes it a particularly useful and simple test to determine the presence of halloysite. Pure halloysites should be readily identifiable and experiments indicate a ‘routine’ sensitivity of ∼20% halloysite in mixtures with kaolinite, although this will depend on factors such as ‘crystallinity’ and could be improved with careful attention to intensity measurements. It is proposed to call this phenomenon the ‘MacEwan effect’ in honour of its discoverer Douglas Maclean Clark MacEwan.

A selective chemical dissolution method has been worked out in order to characterize quantitatively amorphous Al, Fe and Si oxides simultaneously in soils. Based on theoretical considerations an alkaline Tiron solution was tried out. Experiments on well-characterized synthetic and natural substrates were carried out in order to find the optimum set of conditions of pH, temperature and reaction time. These are: an alkaline medium consisting of 0·1 m Tiron and buffered at pH 10·5, an extraction temperature of 80°C and an extraction time of 1 h.

The clay-mineralogical assemblages of Purbeckian carbonate sediments of the Swiss and French Jura Mountains are often composed of illite and interstratified illite-smectite. These illitic minerals occur mainly in thin layers of green marls which show evidence of subaerial exposure and mark the top of shallowing-upward sequences. X-ray diffraction, chemical and thermal analyses coupled with transmission electron microscopy suggest that the Purbeckian illitic minerals replaced smectite in intermediate continental-marine environments. The transition from smectite to illite and interstratified illite-smectite probably resulted from repeated cycles of wetting by marine waters and subsequent drying in hypersaline environments, under a hot Purbeckian climate.

Chemical and X-ray analysis of eight samples of nontronite are consistent with the Hofmann et al. montmorillonite structure. The finest fraction of some fully hydrated (15·3 Å) samples is composed of crystals all about 6 or 7 unit cells thick, and having 12 H2O per 8 Si, arranged at three levels in the interlayer, possibly as upright octahedra. During dehydration, non-integral 00l sequences, and variations in d001 result from a range of layer thickness in one crystal, caused by loss of water from the crystal periphery with consequent margin collapse. At about 50% relative humidity, the H2O content of nontronite is determined by the tetrahedral aluminium, probably because Aliv controls maximum crystal size.

Various physical and analytical techniques (XRD, XRF, SEM, FTIR and EPR) have been used to investigate the effects of chemical and/or physical modification of Ranong kaolin, which has been proposed as a potential bleaching clay for vegetable oils. Acid treatment after grinding resulted in major changes compared with acid treatment of the original mineral sample or mechanical treatment alone. Previous work has shown that the combined treatments produce increases in surface area and new porous structures, and the present measurements show reductions in Al:Si ratios. These are accompanied by a major reduction in O–H stretching vibrations as a result of grinding, although acid treatment produced little subsequent effect on the O–H bands in the FTIR spectra. However, acid treatment resulted in a reduction in the Al–OH–Al bending vibrations and the appearance of Si–O bands associated with newly synthesized material; these effects were much greater with samples that had been ground prior to the acid treatment. There were appreciable qualitative differences in the way in which the EPR spectra of Fe and Mn were affected; the Fe signal was sensitive to mechanical treatment, but little subsequent change was induced by acid extraction, whereas the Mn peaks were sensitive to the both the pH and the chemical nature of the acid used. These results therefore indicate that the Fe and Mn are in different types of site in the kaolin structure. Little change was observed in the main oxygen-based free radical centre associated with Si atoms, but that associated with Al was lost as a result of the treatments. Such mineral characterization is of fundamental importance to understanding the modification of kaolins and their uses as adsorbents in the food and environmental sciences.

Fine clay, coarse clay, and silt fractions of four Bangladesh soils were studied using XRD, ion exchange, and selective dissolution techniques. Constants for the XRD intensity-weight proportion relationships were derived using these techniques. Smectite and mica were found to be dominant in Batra and Ghior soils, whereas kaolinite and mica were dominant in the Naraibag and Ghatail soils. Other minerals present were vermiculite, vermiculite with hydroxyaluminium interlayers, chlorite, randomly interstratified chlorite-mica, feldspar, quartz, and trace amounts of amphibole. The amorphous material content varied between 1 and 11% and decreased with increasing particle size. No crystalline iron oxides were detected in untreated clay fractions.

The sorption of boron by clay minerals from natural waters has been studied experimentally. The quantity of boron sorbed per unit weight of clay mineral is dependent on both the salinity and the boron content of the solution. Previous work has shown that illite is the best clay mineral sorbent, though kaolinite and montmorillonite do sorb some boron and this is confirmed by the present work. The new experimental results demonstrate that the amount of boron sorbed by illite is not affected by the original boron content of the mineral and suggest that the process of incorporation of boron into the lattice proceeds in two stages. The bearing of these experimental results on the use of boron as a palaeosalinity indicator in sedimentary rocks is discussed and this leads to the conclusion that the rate of sedimentation may also influence the boron content of such rocks.

Fe substitution for Al in kaolinite (from Venezuelan laterites) is proved by infrared spectroscopy and chemical techniques. The location of Fe in the octahedral sheet is characterized by two absorption bands, at 865–875 and 3607 cm−1, assigned as δ Al-OH-Fe and v OH respectively. The detection of the 865 cm−1 band requires the use of CsCl for the preparation of the disks heated to 270°C so that the clay delaminates as a result of a kaolinite-CsCl-H2O complex formation. The 3607 cm−1 absorption is detected when KI disks are prepared. These two characteristic frequencies persist after either thermal decomposition or selective chemical dissolution of free iron and aluminium hydroxides.

Selective chemical dissolutions by consecutive treatments for; (1) the removal of free iron oxides, (2) gibbsite removal and (3) extractions of Fe and Al from kaolinite, give additional evidence about the occurrence of this solid solution.

A natural saponite was acid activated at room temperature or 90°C with different acid/clay ratios and the products were characterized by powder X-ray diffraction, infrared spectroscopy and thermogravimetry. The leaching of Mg from the octahedral sheets is enhanced by an increase in the acid/clay ratio and by an increase in temperature of activation. Textural properties are reported, and it appears that they are strongly correlated to the presence of a noncrystalline silica phase which is formed during the acid activation process. The desorption of cyclohexylamine indicates that for samples activated at 90°C the number of acid sites in the acidactivated saponites decreases following severe acid treatment. Infrared spectroscopy of adsorbed pyridine on samples after calcination at 500°C suggests that acid activation at 90°C produces a single type of Bronsted site but two types of Lewis sites whereas activation at room temperature results in only one type of Lewis site in addition to a Brønsted site. The two Lewis sites are suggested to originate from residual Al in the clay structure and to AI exsolved from the layers during activation. The dehydration of pentan-1-ol has been used as a further probe to measure acidity by monitoring the degree of conversion and selectivity for the different samples.

The mineralogy and geochemistry of the clay fraction of Victoria Land Basin (Ross Sea, Antarctica) sediments was investigated, to determine the origin of clay minerals and the features of authigenic smectite. The investigated core (CRP-3) is ~800 m long, mostly of Oligocene age. The clay fraction of the upper sequence consists of mica, chlorite and detrital smectite, while that of the central and lower part is largely made up of authigenic smectite. Authigenic smectites are ditrioctahedral, with a composition close to saponite, while detrital smectites such as Al-Fe beidellites are dioctahedral. Authigenic smectites have no illite mixed layers, show a higher degree of crystallization, higher MgO, Fe2O3, V, Cr, Co, Ni and Sc contents and lower SiO2, Al2O3, K2O, TiO2, Ba, Rb and Zr contents with respect to detrital clay minerals, and a clear depletion of LREE with respect to HREE. Authigenic smectite formed from the alteration of volcanic materials and clay minerals.

The present study investigated the removal of methylene blue (MB) and orange II (OII) dyes from synthetic wastewater by means of adsorption and photocatalysis using natural kaolins. ForMB adsorption, the raw kaolinite-rich samples showed the greatest adsorption capacity, with rapid uptake (90% after 20 min). The experimental results were fitted better using the Langmuir isotherm model parameters compared to the Freundlich model, suggesting that the adsorption corresponds to monolayer coverage of MB molecules over the kaolinite surface. For OII, neither the Langmuir nor the Freundlich model gave reliable results, because the adsorption of anionic dye molecules by the clayey particles is not favoured.

Mixtures of kaolinite/Degussa TiO2 were also prepared, and their photocatalytic properties under UVlight exposure were investigated. Decolourization of MB solutions was observed, even in a mixture with low TiO2 content. This is related to the combined effect of adsorption and photocatalysis and, unlike the pure clay samples, the efficiency of such mixtures against OII was only slightly weaker (80–94%).

For TiO2-impregnated clays, with the kaolinite layers separated by sol-gel TiO2 particles, the MB removal was slow and effective only after >24 h due to the complexity of the bonding of MB molecules. On the other hand, the removal performance against OII solutions was very efficient (nearly 100%) within only 2 h. This excellent performance was attributed to morphological changes in clay particles.

During weathering of forsterite and tephroite under the humid, tropical conditions of the Ivory Coast, intermediate Mg,Fe3+− or Mn2+-smectites appear prior to widespread development of iron or manganese oxyhydroxides. In forsterite, the smectite varies from an Mg-trioctahedral variety near the centres of altered crystals to an Mg,Fe3+,Al-di/trioctahedral variety near the edges. The Al-content of this latter smectite is supplied by dissolution of adjacent minerals, notably enstatite. An Mn2+-trioctahedral smectite forms in cracks in tephroite crystals.

Middle Jurassic sandstones contain a variety of authigenic clay materials. The origin and distribution of these are related to the influence of several factors, principally depositional pore-fluid chemistry, sandbody geometry and the migration of aggressive fluids. Pore-lining illite, pore-lining chlorite and pore-filling vermiform kaolinite in the Ravenscar Group mutually exclude each other because of depositional pore-water chemistry; seawater in the case of the illite, anoxic freshwater in the case of chlorite and oxygenated freshwater in the case of the kaolinite. Blocky pore-filling dickite occurs ubiquitously within the large connected sandbodies. Its origin may be related to the migration of aggressive fluids and its distribution to depositional sandbody geometry. Mixed-layer chlorite-vermiculite also occurs, and is believed to have formed from chlorite during Recent weathering. Similar patterns occur in Ninian Field Brent Group sandstones, although the situation is more complicated (in these sandstones the kandite subgroup minerals are undifferentiated in this paper). Here too, pore-lining illite occurs in marine sandstones and pore-filling vermiform kandites in non-marine sandstones. However, vermiform kandites also occur in the marine sandstones, perhaps due to freshwater-table development following progradation. The more blocky kandites occur in large connected sandbodies. Finally, a second phase of illitization occurs, postdating blocky kandites, perhaps caused by alkaline formation waters. The occurrence of chlorite in the Broom Formation is anomalous and its possible origin is discussed.

Methylene blue (MB) has been used as a probe molecule to examine how the uptake dynamics and the equilibria between this dye and the surface of Na+- and H+- montmorillonite were affected by the presence of a second dye. To prevent spectral interference, the yellow dyes thioflavin T, TFT, proflavine, PFH, and acridine yellow, ACY, were chosen to compete with MB for the exchange sites. The MB was initially adsorbed as trimer (MB+)3 and then redistributed via collisions between clay particles until equilibrium was reached. At equilibrium in the clay/MB systems, the protonated species (MBH2+) predominated at low loadings (1–5% CEC), whereas at higher loadings the trimer (MB+)3 was the major species. The presence of the second, competing dye slowed the approach to equilibrium, significantly reduced the amount of MBH2+ formed and provided evidence for the monomeric MB+, dimeric (MB+)2, and trimeric (MB+)3 forms of MB. Moreover, the presence of PFH and ACY, which are structurally similar to MB, resulted in more dimeric character in the aggregated form of MB compared to the aggregate formed in the presence of the structurally dissimilar TFT.

Lower Palaeozoic shales and slates in the External Domain of the southern Canadian Appalachians are composed predominantly of illite and chlorite with minor occurrences of I-S mixed-layer minerals (restricted to samples with illite crystallinity, IC > 0·62°Δ2Θ) and paragonite (restricted to samples with IC < 0·42°Δ2Θ). Inverted diagenesis has occurred in the NW part of the Chaudière Nappe, indicating pre-orogenic deep burial diagenesis at the original depositional site, whereas to the SE, the diagenetic pattern was affected by synorogenic heating. Within the east-dipping thrust-fold belt and the St Lawrence Lowlands, increasing grade towards the S suggests a gradual southward increase in post-tectonic burial depth. Narrow (3–5 km) thermal haloes around the Cretaceous Monteregian intrusions show limited effects on the country rocks.

The percentage of 2M1 mica polytypes and bo increase with decreasing IC. Chlorite crystallinity (CC) increases with increasing IC. Good correlations between IC, %2M1, CC and bo of micas indicate that these parameters are reliable monitors of high-grade diagenesis and low-grade metamorphism in clay-rich sedimentary rocks. IC and CC improve with increasing grain size, illustrating the effect of grain size on IC and CC. Organic material affects IC more strongly in strata with lower permeability than in those with higher permeability. In the diagenetic zone, glycolation does not uniformly produce a narrowing of the 10 Å illite peak, but may also broaden it by up to 15%, probably due to the presence of Kalkberg-type mixed-layers.