The clay mineralogy of K-fixing soils from southern Germany was investigated using the alkylammonium method. Conventional X-ray diffraction chart recordings of glycerol-saturated <0·1 µm samples showed peaks at 18 Å that were initially interpreted as smectites. However, step-scanning combined with the Lorentz- and polarization-correction and curve-fitting, revealed the presence of low- and high-charge smectites and of low- and high-charge vermiculites. The severe overlap of the 001 basal reflections of the high-charge vermiculites with the broad and intensive peaks of smectites sensu stricto prevented visual evaluation at the alkylchains nc = 5–9, but this problem was overcome by curve-fitting. The 002 peaks of high-charge vermiculites (nc = 8 and nc = 10) in the fraction 0·1–0·2 µm could furthermore be clearly resolved from the 001 peaks of illite by curve-fitting. In one example, 003 (at nc = 17) could be resolved from 001 of illite.

High-resolution electron microscopy and microanalytical studies were performed on clay and bulk soil samples developed on various igneous and metamorphic rocks from Galicia in the Coruna province in NW Spain. Two mineralogical microsystems can be distinguished showing different stages of weathering. For feldspars, exsolution and mass transformation lead to the delineation of parallel domains, and to formation of gel or paracrystalline minerals. For micas, alteration starts with a physical breakdown, i.e. exfoliation and crystal microdivision; individualization of monolayers is followed by gel formation. A solid phase diffusion phenomenon can explain the formation of 1 : 1 phyllosilicates inside 2 : 1 phyllosilicates or within the continuum of the original crystals. Evidence of such weathering stages in crystalline rocks can be related to the occurrence of specific climatic conditions between temperate and tropical areas.

The soils developed from crystalline and metamorphic rocks in Galicia (NW Spain), are characterized by high concentrations of 1 : 1 phyllosilicates and gels. Thermal analyses, X-ray diffraction after formamide treatment, and IR spectroscopy in the OH vibration range have been performed on the clay fractions, but do not discriminate clearly between the different associated mineralogical phases. HRTEM studies linked with microdiffraction and microanalyses have led to the identification of several types of gel which transform into goethite, gibbsite, clay precursors, and/ or halloysite according to their composition (Fe, Al or Si-Al). Halloysite-like minerals are the main constituents and they have a great variety of morphologies: lamellar, spheroidal, tubular, platy or poikilitic. In general, halloysite and gel formation on crystalline rocks is related to the bioclimatic conditions, involving high hydrolysis in the presence of organic matter. This halloysite seems to be a metastable mineral which would evolve into kaolinite with increasing weathering time.

An XRD method for measurement of quartz content using ZnO as the internal standard was tested on different particle sizes between 0 and 20 µm. Calibration curves showed a good correlation coefficient for particle-size fractions up to 20 µm; the slope increased for the fractions from 0·7 to 5 µm and was relatively constant for coarser particle sizes. Fine quartz fractions were etched with hydrofluoric acid to remove the surface layer damaged during dry grinding. The use of such etched quartz increased the slopes of the calibration curves for small particle-size fractions and approximated the natural fine quartz fraction much better than the original dry-ground material. The mean of six measurements gave good accuracy provided that the slope of the calibration curve was adjusted for the particular particle-size fraction. This method was used on 0–2 µm, 0–0·2µm and 0·2–2 µm fractions of French silty soils and the results are in agreement with the data from chemical analysis and with the mineralogical interpretation.

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.

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 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.

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.

Optical isomers deamination of L- and D-glutamic and aspartic amino acids and of their DL racemic mixtures has been achieved in the presence of Na-montmorillonite at pH = 6 and room temperature. The adsorption curves showed that the enantiomer adsorbed depends on the type of amino acid. Nevertheless, deamination reaction kinetics brought about a stereoselectivity of the clay mineral for the L-isomer and implicitly showed an unquestionable “structural chirality character” of the clay mineral.

Depending on the preparation temperature and the origin of the raw mineral, the products obtained by thermal activation of kaolinite lead, by hydrothermal treatment with sodium hydroxide at 102°C, either to zeolite 4A, or to cubic zeolite P, or to a mixture of cubic and tetragonal zeolite P. The change in the type of zeolite obtained was explained on the basis of a radical change of the zeolite nucleation process due to either the presence of soluble potassium or high silica content resulting from dehydration of muscovite (an impurity in the raw-kaolinite), or from “segregation” associated with the first step of the metakaolinite to mullite transformation. These phenomena occur at a lower calcination temperature with poorly-crystallized kaolinite, and iron in such a sample promotes a sensitive decrease in the formation rate of zeolite crystals. Such results could be used to characterize the crystallinity of kaolinites.