Complexes formed between 0·034 m dimethyltin dichloride solutions and Na-montmorillonite at pH 2·4 and 4·0 have been characterized using 119Sn Mössbauer spectroscopy, X-ray diffraction, infrared spectroscopy, thermogravimetric analysis and water sorption isotherms. The dominant exchange cation at pH 2·4 is Me2Sn2+ characterized by an isomer shift, δ, and quadrupole splitting, Δ values of 1·25 and 3·80 mm s−1, respectively. The major exchange cation at pH 4·0 is the dimer, [Me2SnOH]22+, (δ = 1·38, Δ = 3·34 mm s−1) although the monomer, Me2Sn(OH)+, (δ = 0·95, Δ = 3·29 mm s−1) is also present. These complexes have basal spacings of 15·0 and 16·0 Å, respectively, which are stable up to 200°C. Mössbauer spectroscopy has shown that these ions are converted to SnO2 via Me2SnO upon thermal degradation. The temperature at which these oxides appear coincides with the collapse of the basal spacing. Both complexes display BET Type IV isotherms for the adsorption of water, following thermal pretreatment at 150°C, but the complex prepared at pH 4·0 has a lower sorption capacity. Both complexes, contrary to the normal behaviour of layer-silicates, have a definite pore volume and no further uptake occurs when this is filled.

Mössbauer spectroscopy obtained with the sample at low temperatures and subject to large magnetic fields is discussed in terms of its use in analysing the naturally occurring polymorphs of FeOOH and the common growth precursor, ferrihydrite. Experiments on well-characterized synthetic samples provided benchmark results. A relatively simple means of quantitative analysis was used, in which sextets of Voigtian-shaped lines were summed and least-squares fitted. The results are discussed in terms of the magnetic structures of each compound.

Two samples of illite-smectite (I-S) isolated from Upper Jurassic clays in well 2/7-3 (3365 m) and well 2/11-1 (4548 m) have been investigated by scanning tunnelling microscopy (STM), and the particle shape and dimensions have been correlated to results from transmission electron microscopy (TEM) on shadowed specimens and results from atomic-force microscopy (AFM). By STM, lath-like and equant particles were observed in both samples, some of the particles having sharp edges. In the I-S from well 2/11-1, spiral-shaped particles were also seen. For both samples, the most frequent particle diameter was 100–200 Å. Particle-thickness distributions from STM and TEM were similar for the I-S from well 2/7-3, being dominated by 10 Å thick particles. For I-S from 2/11-1, the STM particle-thickness distribution has a predominance of 20 Å thick particles, but the TEM particle-thickness distribution is broad, with about equal amounts of 20, 30, 40, 50 and 60 Å thick particles. The AFM particle-thickness distribution for this I-S resembles the TEM thickness distribution. It is concluded that mainly thin (10 and 20 Å) particles are seen by STM. Failure of AFM to show sharp particle edges (seen in STM and TEM) might be attributed to the AFM tip movement or tip shape. In STM, I-S particles from well 2/7-3 have peaks along edges, whereas I-S particles from well 2/11-1 have rims. These rims are also seen in AFM and are therefore real geometrical features, probably a result of two-dimensional growth, whereas the spirals in the I-S from well 2/11-1 demonstrate three-dimensional growth. The minimum thickness of most particles is 10 Å.

Rhaetian (Upper Triassic) sandy horizons were sampled from the Paris Basin from SW to NE, crosscutting the Rhaetian at different depths from outcrop in the NE to 2700 m in the centre of the basin. The smallest day sub-fractions (<0·2 μm) from the deepest samples consist mainly of illite and chlorite having a K-Ar age of 〜190 Ma. Both minerals probably formed under specific hydrothermal conditions at high temperature, but at a burial depth of only 500 m. This thermal event could represent an echo of the "crustal" breakdown of the Northwestern European craton during the opening of the North Atlantic Ocean. Two other generations of illite-smectite mixed-layers formed in the same Rhaetian horizons at somewhat lower temperatures about 150 and 80 Ma ago. The three generations of clay minerals could be characterized and dated because of combined mineralogical, crystallographical and morphological data supporting the dating attempts.

Four methods of extracting clays from calcite-rich samples were compared: (i) sodium acetate buffer solution; (ii) lithium acetate buffer solution; (iii) hydrochloric acid; (iv) liquid cation-exchange acid. The methods were tested on mixtures consisting of 80% calcite, 10% quartz and 10% hectorite. Potassium had previously been exchanged on to the interlayer sites of the clay. XRD traces of the extracted clays appeared identical with those of the original clays except for a sharpening of the 001 reflection. The bulk chemistry of the clay was unchanged except by the HCl extraction, which appeared to cause a stripping of the octahedral cations. With all of the methods the exchangeable cation content was altered; the sodium acetate method, for example, retained only 17% of the original exchangeable K+, the other methods all retaining <10%.

The charge transfer process that occurs in two samples of Wyoming montmorillonite of different grain size pillared with Zr by interaction with perylene (Pe) and tetracyanoethylene (TCNE) has been studied. These smectites are strongly affected by the exchange of Zr and appear to be strong electron acceptors and weak donors. The ESR results show that their ability to form charge transfer radicals is related to the details of preparation, which most probably control the accessibility of Pe and TCNE to the internal surface.

Iron-rich dioctahedral 2:1 clays were sampled in metalliferous sediments from Atlantis II Deep (Red Sea), and studied by XRD, TEM, FTIR and Mössbauer spectroscopy. One sample with the structural formula (Si3·94Fe3+0·06)(Fe3+1·90Mg0·10)K0·1(Mg, Ca)0·03 is closer to the theoretical end-member ferripyrophyllite Si4Fe3+2O10(OH)2 than any other Fe-rich clay previously described. Other samples appear as mixed-layered clays: ferripyrophyllite-hydroferripyrophyllite or hydroferripyrophyllite-celadonite.

X-ray diffraction (XRD) patterns of a pore-lining diagenetic chlorite (14 Å) from a reservoir sandstone, offshore Norway, show broad odd-order and sharp even-order basal reflections indicating that it contains 7 Å layers. Using NEWMOD, simulated XRD patterns with 15% 7 Å serpentine layers and a maximum crystallite thickness of 30 layers match the natural mineral well. Microprobe analyses of the 7 Å-14 Å mineral indicate that it is Fe-rich and aluminous suggesting that it is interstratified berthierine-chamosite. Apparent octahedral vacancies, however, suggest a significant dioctahedral component, and an alternative interpretation is interstratified kaolinite-chlorite. Indeed, chemical analyses of the mineral suggest a mixture of chlorite with 15% kaolinite, precisely the proportion of 7 Å layers indicated by XRD. Two other examples from the literature, previously identified as diagenetic chlorite, are probably also 7 Å-14 Å interstratified minerals, and the proportion of 7 Å layers indicated by XRD is also correlated with their structural formulae, if the 7 Å layers are, in fact, kaolinitic. This type of interstratification could explain why Fe-rich diagenetic chlorites appear to be compositionally distinct from metamorphic chlorites. The structure and chemistry of the Norwegian chlorite tend to support the idea that pore-lining chlorites form early in the diagenetic history, inhibiting the precipitation of later diagenetic minerals, and hence preserving abnormally high porosity at greater depths.

The petrography, mineralogy and diagenesis of an overpressured Tertiary and Late Cretaceous mudrock sequence from a single well in the East Shetland Basin (North Sea) are described. The clay fraction is believed to be dominated by volcanic glass above 1400 m depth and by smectite below 1400 m. The smectite content increases with depth due to an increase in detrital smectite and to in situ alteration of volcanic glass and grains; this commenced almost immediately after deposition and is still not complete at burial depths >2300 m. There is little evidence of alteration of the smectite to illite despite a local available supply of K+ in the form of K-feldspar and the presence of K in the more deeply buried smectite. Very little leaching or precipitation of quartz has occurred. Microfossil tests filled with authigenic clays such as have been reported in other Tertiary mudrocks in the North Sea were not observed, though a few fossil cavities are filled with carbonate cements, and very rarely with microquartz. The suprisingly limited reactivity of this mudrock sequence may be due to the early development of overpressure, resulting in a “closed” system, and to the absence of interbedded higher permeability layers or open faults through which fluids could be exchanged.

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.

The exchange of Ca2+ and Na+ by tanthanide ions (Ln3+ = Pr3+, Gd3+, Er3+) in montmorillonite was investigated at two different ionic strengths (0·01 and 0·1 mol/kg). Preferential sorption of Ln3+ was observed and variable selectivity coefficients were found depending upon the lanthanide concentration in the solid, and ionic strength. The highest exchange extent of Ln3+ always occurred for the system Na+/Ln3+, but the exchange selectivities of Ln3+ were generally higher in the exchange system Ca2+/Ln3+. Although the relative affinity of montmorillonite for the three lanthanide ions was similar, distinctive behaviour between Pr3+ and the heavier lanthanides, Gd3+ and Er3+, was noted. The study of Ln3+ adsorption in trace amounts showed specific adsorption of lanthanides at high concentrations of Na+ in the external solution and that the exchange stoichiometries in the interlayer regions were 3 : 1 at equilibrium pH = 4.

Adsorption of Cs on two samples of marl with different calcite, quartz and clay contents and surface areas of ∼10 m2/g was followed using the batch sorption technique. The experiments were carried out in an atmosphere of N2/1% CO2 using a synthetic groundwater of pH 7·3 and an ionic strength of 0·1 m. The initial Cs concentration ranged from 10−8 to 10−4m. Sorption of Cs was non-linear, fast and reversible. At equilibrium Cs concentrations <10−8m, an isotope exchange mechanism appeared to operate, whereas at higher concentrations, sorption involved ion exchange. The distribution coefficients ranged from 23–995 ml/g. They varied markedly with rock/water ratio; this variation could be explained in terms of the non-linear isotherm displayed by Cs. Although the two samples of marl differed in composition, uptake of Cs on both samples was very similar because the content of the principal sorbing component (illite) was the same in both samples.

The adsorption of methylene blue (MB) from aqueous solution on Laponite (a synthetic 2:1 layer-silicate) exchanged with Na+, Cs+ and tetraethylammonium ions has been studied. At low loadings the monomer form predominates, but as the loading increases dimers and trimers (or higher aggregates) are formed, the latter being the most important species when the loading is equal to, or larger than, the Na+-CEC. The distribution of the molecules over the surface is time-dependent due to changes in the aggregation of the clay particles. On Cs+-Laponite, and to a lesser extent on Na+-Laponite, a small amount of MB is protonated by cation-coordinated water molecules. In similar work where adsorption was from ethanolic solution, MB is adsorbed as monomers and dimers only at small loadings and precipitates on the surface of the clay aggregates at high loadings. These precipitates can be broken up by sonication.

The adsorption isotherm of methylene blue (MB) on sepiolite gels is of the Langmuir type, indicating a great affinity of the MB towards the sepiolite; the adsorption is quantitative up to adsorption of MB of 0·1 mmol/g sepiolite. The differences observed in the absorption spectrum in the region 500–650 nm for different MB loadings are interpreted in terms of aggregation of MB molecules on the sepiolite surface with the formation of MB dimers, trimers and higher aggregates. The progressive coverage of the sepiolite surface by MB produces a sharp decrease in the viscosity of the gels, and the suspension becomes peptized for an MB loading of 0·21 mmol/g which corresponds to the adsorption of MB dimers. These results indicate that MB avoids particle-to-particle interactions. The viscosity decrease is parallel to the perturbation of the Si-OH groups on the sepiolite surface as observed by IR spectroscopy, suggesting that these silanol groups can also play an important role in the rheological behaviour of aqueous sepiolite dispersions.

The term ferrihydrite, originally defined as a poorly ordered hydrous ferric oxide exhibiting five or more distinctive X-ray diffraction maxima, has come to be extended to include species that exhibit only 4, 3 or 2 of these maxima. It is pointed out here that hisingerites may exhibit a similar 2-line diffraction pattern. The laboratory synthesis of hisingerites show that they can form under soil conditions. Acid oxalate buffer of pH 3, a reagent considered a fairly specific solvent for ferrihydrite, extracts the iron content of these synthetic hisingerites quantitatively.