Aluminous hydroxy 2/1 clay minerals (‘intergrades’) can form in two ways: Al can either come externally from the solution, or internally from the mineral itself (octahedral or tetrahedral layers). This study is more concerned with the latter which seems the main source in acid soils.

The most common organic acids present under natural conditions were used on two trioctahedral micas (phlogopite and biotite), and their chemical and mineralogical weathering effects were determined.

Different kinds of mineralogical evolution are possible: mineral destruction by a few very complexing acids, possibly with intermediate stages (interstratified minerals); formation of ‘transformation’ smectite by acids derived from sugars; Al-hydroxy vermiculite formation (the 14 Å diffraction peak of which is stable under K saturation and vacuum). This evolution is the most frequent and is produced by the most numerous organic acids (even CO2), the acidity of which results in Al migration in the interlayer, but the complexing ability of which is not sufficient for Al removal.

Les hydrates à deux couches et à une couche d'une saponite calcique ont été étudiés par infrarouge. Les résultats expérimentaux indiquent que dans l'hydrate à deux couches les molécules d'eau directement coordinées à l'ion calcium sont liées par liaisons hydrogène fortes d'une part aux atomes d'oxygène de surface chargés négativement et d'autre part à d'autres molécules d'eau. En revanche, dans l'hydrate à une couche où chaque ion calcium est solvaté par un minimum de 2 molécules d'eau, il n'y a pas de liaison hydrogène intermoléculaire. La plupart des molécules d'eau donnent une seule liaison hydrogène forte. Chacune des molécules d'eau forme un pont hydrogène, l'une avec le feuillet supérieur et l'autre avec le feuillet inférieur.

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.

This paper reports an electric birefringence study on dilute solutions of the clay mineral, celadonite. The results show that celadonite particles in solution have the form of rectangular platelets and are characterized by a permanent dipole-moment perpendicular to the plane of the platelet and an induced dipole-moment in the plane of the platelet along the longest dimension. It is suggested that the permanent dipole-moment is an inherent property of the atomic structure of the platelet, whilst the induced dipole-moment is a consequence of the ion-atmosphere surrounding the celadonite particles. The paper also demonstrates the usefulness of electro-optic techniques in studying problems in clay minerology.

Alunite-kaolinite deposits occur in argillized zones in acid volcanic rocks at Tolfa. Over seventy samples were examined by one or more of the following methods: thin section, DTA, X-ray, IR, TG, SEM, chemical, and hydrogen and oxygen isotope analysis. Kaolinite and dickite, of high structural order, are the dominant clays with micamontmorillonite and halloysite subordinate. In many samples kaolinite and dickite coexist; their relative amounts are variable but the dickite content tends to increase with depth. SEM studies show that dickite crystals—up to 30 µm—are bigger than kaolinite.

The D/H and 18O/16O ratios of six clays, a whole rock, two biotites from fresh host rock, two chalcedony veins and three local meteoric waters indicate that the clays formed in a meteoric-hydrothermal environment of acid hot spring type at about 80°C. The fresh volcanic host rocks are strongly enriched in 18O relative to ‘normal’ igneous rocks due to derivation from or exchange with 18O-rich sedimentary rocks at depth prior to hydrothermal and solfataric activity. In the absence of present-day geothermal activity the life of this hot spring system was less than 4 m.y.

On effectue la synthèse hydrothermale d'une phengite à partir d'un gel de composition 90% muscovite-10% céladonite, en présence de chlorure de lithium, à 500°C, 1000 bars. Le produit final obtenu contient environ 2000 ppm de Li. L'étude de la cinétique de la dissolution, en milieu HCl 2·4 n, des éléments majeurs et du lithium montre que 33% du lithium sont très faiblement liés au mica et vraisemblablement adsorbés. Ce qui reste de Li entre en substitution isomorphique dans le réseau de la phengite, sans doute en sites octaédriques.

Examination of the infrared spectra of synthetic kaolinites, with various degrees of substitution of hydroxyls by OD groups, allows a more detailed attribution of the stretching bands due to inner-surface hydroxyls of kaolinite. Among the three innersurface OH groups of the unit cell, two are nearly perpendicular to the sheet and give coupled vibrations which are responsible for the bands around 3695 and 3670 cm−1; the band near 3655 cm−1 is due to the other hydroxyl, which is lying close to the sheet.