Par l'étude de la variation de la charge réticulaire relative et de la surface spécifique relative, accessible à l'eau, de smectites ayant subì l'effet Hofmann-Klemen à différents degrés, on montre que celles caractérisées par des substitutions tétraédriques (Beidellite B de Rupsroth et Ascan-gel) se différencient clairement de l'Ascan-argile qui ne contient que des substitutions en couche octaédrique. Dans la beidellite, la démixtion des cations Li, Na est totale pour toutes les compositions de la garniture cationique tandis que la démixtion n'apparaìt dans le cas de la montmorillonite que si (Na+/Li+) < 0·75. L'Ascan-gel occupe une position intermédiaire. Ces observations sont interprétées comme une indication que les substitutions tétraédriques empèchent l'expansion des feuillets en suspension.

Separation of the clay fraction from a calcareous soil usually involves chemical pretreatments which could affect the separated clays. A technique based on separating the flocculated particles (flocs) of a sonified soil-water suspension was used on highly calcareous samples and a highly gypseous one collected from a limestone ridge in Egypt. The relative abundance of the clay minerals in flocs (identified by XRD and electron microscopy) was found to be identical to that of < 1 μm clay separated by sedimentation. The suspensions were prepared by washing the soils repeatedly with distilled water (about 2 1/g) until spontaneous peptization.

Washing removed both 'amorphous' silica and alumina as detected in the flocs. The composition of poorly-ordered aluminosilicates (SiO2/Al2O3) of the 1μm clays was heterogeneous whereas in the flocs it was homogeneous.

Fluoride reactivity of both clay separates and prepared mixtures of CaCO3 with palygorskite (the dominant clay mineral present) was also studied.

L'action catalytique des argiles sur la polymérisation des acides aminés en solution aqueuse a été étudiée. Les résultats rapportés par Katchalsky et coll. dans le cas de la polymérisation de l'analyl adenylate en presence de montmorillonite n'ont pu ètre reproduits. D'autres types d'activation des acides aminés ont été examines.

Three soil profiles developed on a serpentinite body (La Rochel'Abeille, near Limoges) show three stages of weathering. All soils contain iron-rich smectites and secondary chlorites. The latter are very silica-rich, more so than 14 Å chlorites from crystalline rocks. In the (B)1g horizon of the hydromorphic profile, these minerals seem to give a reaction of the type:

This reaction, typical of a closed system, appears to be operative in a soil profile which is certainly, in part, open to chemical migration. The chemistry of the weathered serpentinite and the chemical composition of newly formed minerals as well as those of the serpentinite are used to indicate the chemiographic relations of clay minerals formed in the weathering profiles.

The amorphous mineral selectively dissolved from the clay (<2 μm) fractions of twenty-six ferruginous soils contains considerable iron in addition to silicon and aluminium. SiO2/Al2O3 and SiO2/R2O3 molar ratios are between 2·03-3·52 and 1·72-2·95 respectively. The model of the amorphous ferri-aluminosilicate (AFAS) consists mainly of negatively charged tetrahedrally coordinated silica-alumina phase Si3AlO6(OH)4 containing domains of neutral FeOOH, with an outer positively charged hydroxyaluminium polymeric component [Al(OH)2.5]n.The calculated hydroxyl water content of the AFAS averages 17·8%; cation exchange capacity varies from 48·6 to 112·0 mEq/100 g and shows a negative correlation with the outer hydroxyaluminium octahedral component and a positive correlation with the ratio of the tetrahedral Si-Al component to the octahedral hydroxyaluminium component. The K-fixation capacity (1·9-6·1 mEq/100 g) of the AFAS does not appear to be related to the chemical composition. The genesis of the amorphous mineral is discussed.

Palygorskite, associated with aragonite, occurs as a fracture filling in a dunite body that has been intruded by felsic pegmatites. The palygorskite formed by direct precipitation from solutions given off by the pegmatite. Aluminium, calcium and silicon were provided by the pegmatite solutions while magnesium was derived from the serpentinization of the dunite along the host fractures. No direct replacement took place.

The behaviour of iron in the natural weathering of hornblende in a sedentary soil profile has been investigated using the Mòssbauer effect. The spectra from samples at different stages of weathering show appreciable differences which can be accounted for if ferrous iron is lost preferentially from M(3) sites where it is coordinated to hydroxyl groups in a trans arrangement. Unlike biotite the proportion of ferric iron does not increase during weathering and this illustrates the difficulty with which the ferrous iron is oxidized in the amphibole structure.