Huit saponites de synthèse à charge croissante ont été étudiées à l'état ‘zéro couche’ (vide, P2O5) avec différents cations interfoliaires.

On observe—avec le cation K—une croissance linéaire du paramètre b en fonction du taux de substitution Si-Al, mais cette croissance est bien plus faible que celle observée avec les mêmes minéraux hydratés à ‘deux couches’.

On observe par ailleurs pour les cations K, Ba, Rb et Cs des relations linéaires entre les valeurs du paramètre b et de d001 d'une part et le rayon du cation d'autre part. Les cations plus petits (Na et Li) conduisent à des solutions structurales différentes. Avec le sodium la structure 1 M n'est plus conservée, les feuillets sont décalés les uns par rapport aux autres. Avec le lithium, le type d'empilement 1M est conservé mais c'est l'entité Li+-H2O qui est piégée dans les cavités ditrigonales placées face à face.

L'ensemble de ces résultats permet de montrer que les interactions entre touche interfoliaire et feuillets sont beaucoup plus fortes dans l'état ‘zéro couche’ que dans les états hydratés à une ou deux couches d'eau. Ces interactions sont fonction du rayon des cations interfolaires et peuvent entraîner une rotation des tétraèdres, ce qui conduit à discuter de la validité des équations donnant l'angle de rotation α en fonction de la composition des phyllosilicates.

Statistical (Q-mode) analysis of the chemical compositions of 122 dioctahedral smectites obtained from the literature have enabled the main solid solution ranges (montmorillonite, beidellite and nontronite) to be distinguished on a triangular diagram. In the montmorillonite area, the Wyoming, Tatatilla, Otay, Chambers and Non-Ideal montmorillonite compositional ranges are identified. Certain crystallochemical features of the smectites may be correlated with different genetic environments.

The morphology and fabric of kaolin weathered from a non-feldspathic mica-phyllite in Znojmo, Moravia, are illustrated in scanning electron micrographs. In contrast to accordion-like kaolinite books, or elongate halloysite(?) crystals, commonly developed from feldspar, the kaolinite from the mica-phyllite tends to be individually platy, with highly irregular edges resulting from multiple adjacent crystal comers and re-entrants. Irregular or impaired crystal growth of kaolinite from mica may be due to ‘poisoning’ of kaolinite crystals by vanadium that is common in mica.

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 method is described for calculating, and then subtracting, the background from X-ray diffraction patterns of oriented clay mineral samples. Ti-Kα radiation is used and, to minimize the absorption of this radiation by air, a vacuum and helium-flushed device has been developed. This device can be used with other X-ray sources, offering a considerable increase of intensity—e.g. Co-Kα radiation is increased by 125%. With the background-eliminated patterns a better semi-quantitative estimate of the composition of clay mineral mixtures is possible. Small differences in composition of two samples can be identified by subtracting one of the background-eliminated patterns from the other. Using this method, peak maxima of smectite-group minerals can also be accurately determined.

XRD, DTA, IR, chemical and electron microscopic data are presented for three specimens of regularly interstratified 25 Å minerals from Roseki deposits in Japan. The layer charges of these dioctahedral mica-smectites originate mainly from tetrahedral Al-for-Si substitution. The non-expandable interlayers are occupied by relatively large amounts of Ca and K as well as Na. The expansion characteristics of the expandable interlayers are montmorillonitic in spite of the beidellitic composition of the 2:1 layers.

Interstratified kaolinite-smectite minerals form the major part of a pedon transitional between ‘red’ kaolinitic and black smectitic soils in a soil toposequence derived from basalt in Burundi, Central Africa. These interstratified kaolinite-smectites have many X-ray, morphological and chemical features in common with ‘tabular halloysite’. Modifications to the 00l spacings of these minerals brought about by thermal and chemical treatments, however, show that they are not halloysite but consist of a mixture of ∼70% kaolinite and 30% smectite randomly interstratified. Field evidence indicates that these clays can be considered as a transient stage in the overall reaction smectite→kaolinite + iron oxides, which takes place in this landscape whenever the slope gradients give rise to better drainage conditions.

Proportions of the component layers in randomly interstratified illite-smectites can be determined precisely in the presence of discrete illite if two reflections (at 15–16 and 31–32°2θ Cu Kα) are strong enough to be measured accurately. The method takes into account the variable thickness of the smectite-ethylene glycol complex and involves a correction for the interference from illite reflections.

Well-crystallized kaolinite was found to exhibit marked proton conductivity at pre-dehydration temperatures. Direct current conductivity and thermopotential measurements were performed. By comparing the results obtained with Au electrodes (proton blocking) to those obtained with Pd electrodes (proton injecting), non-protonic contributions could be separated from proton conductivity. The majority charge carriers appeared to be defect proton (negative sign), corresponding chemically to an O2− replacing OH− in the lattice, but excess protons were also noted (positive sign). Below 275°C the activation energy was of the order of 7–8 kcal mole−1; between 275–450°C it was about 18–20 kcal mole−1. This indicated a two-step defect proton-excess proton conductivity mechanism involving, in succession, the two kinds of OH− in the kaolinite structure: (i) OH− located within the six-membered ring formed by the SiO4 tetrahedral sheet, (ii) the close OH− array formed by the octahedral sheet. The conductivity results were supported by IR spectroscopic measurements.

A swelling chlorite from unweathered basalt has been characterized by XRD, IR, DTA and chemical analysis. Its 001 spacing of 14.9 Å increases to 18.0 Å on treatment with glycerol and decreases to 13.9 Å (with a broad diffraction effect at 14-10Å) after heating to 550°C. The mineral has an 060 spacing of 1.537Å consistent with a trioctahedral composition, but its OH stretching band at 3638 cm−1 does not show the expected pleochroism, suggesting an anomalous hydroxide sheet which chemical analysis shows is incomplete. Although the mineral has a CEC of ∼50 mEq/100 g, smectite is not present, but all the techniques show that the mineral has some saponitic character, indicating that it probably has a structure intermediate between swelling chlorite and saponite.

Una sepiolita procedente de Toledo (España) se ha activado por tratamiento con disoluciones 0·25, 0·50 y 0·75 n de NO3H, seguido de calentamiento durante 10 h a 110, 200, 300, 400 y 500°C. Para cualquier concentración ácida, un calentamiento de 110, 200 y, a veces, 300°C, produce un aumento del número de poros mientras que las temperaturas superiores significan un cierre de los mismos tanto mayor cuanto más elevada es la temperatura de tratamiento. El máximo desarrollo de la superficie accesible al N2 tiene lugar con una disolución 0·50 n de NO3Hy 200°C (449 m2/g, método de BET); la superficie específica de la sepiolita natural es de 321 m2/g.

In the three previous papers of this series the role played by the exchangeable cations in the adsorption of methanol by homoionic montmorillonites has been investigated by different techniques, namely desorption isotherms, X-ray determination of the basal spacings and IR spectroscopy. In this last paper, models based on the previous discussions are presented. In the adsorbed state, methanol shows a strong tendency to form on the clay surface, hydrogen-bonded zig-zag chains analogous to those in crystalline CH3OH. The exchangeable cations perturb this arrangement to different extents. For Li+ the perturbation is negligible, for Ca2+ a complete rearrangement is observed and for Na+ and Ba2+ the chains are broken into smaller fragments.