Nous décrivons un montage expérimental permettant d'obtenir, avec une bonne précision, des diagrammes de diffraction X de poudre d'argile dans un état hydraté homogène et de suivre la cinétique du processus d'hydratation. Nous avons pu déterminer, dans le cas de l'état homogène à une couche d'eau d'une beidellite sodique, la position des cations échangeables, le nombre total de molécules d'eau par maille et leurs coordonnées z, en utilisant une méthode de modèle basée sur la comparaison des profils expérimentaux et calculés des réflexions 00l. De plus nous avons comparé les caractéristiques structurales et en particulier le mode d'empilement des feuillets de l'état monocouche avec celui des états anhydre et bicouche. L'analyse du profil des réflexions 00l montre l'existence de fluctuations autour d'un espacement basal moyen . Ces fluctuations sont interprétées comme provenant de l'hétérogénéité de la distribution des charges dans le feuillet. Enfin, un processus théorique d'hydratation et de déshydratation est proposé.

K-depleted phlogopite mica was used as a topotactic precursor and treated with alkali (Li+, K+, , Rb+, Cs+), alkaline-earth (Mg2+, Ca2+, Sr2+, Ba2+) and trivalent (Al3+) cations under hydrothermal conditions of 200°C and 30 MPa pressure. K-, NH4-, Rb- and Cs-aluminosilicate micas were synthesised at 200°C in one day. The synthesis of Cs-aluminosilicate mica, with potential applications in the management of nuclear wastes, has been achieved for the first time by this approach. Ion exchange by Li+, Na+ and alkaline-earth cations under hydrothermal conditions did not produce anhydrous mica phases but resulted in hydrous phases with one or two layers of water molecules between the clay layers. The formation of hydrous phases may be attributed to the high hydration energy of the above cations compared to K+, , RB+ and Cs+. Ion exchange with Al3+ produced a chlorite-like phase because of the hydrolysis of Al3+ under these hydrothermal conditions. These studies are of relevance in the immobilization of wastes where hazardous ions can be fixed in highly stable insoluble phases like mica or chlorite.

L'étude concerne 225 échantillons d'âge Jurassique à actuel de l'Océan Atlantique. La comparaison de l'abondance des smectites lattées, estimée sur micrographies électroniques, et de divers paramètres des sédiments, apporte des informations nouvelles sur la genèse et la signification de ces minéraux dans des sédiments non volcanoclastiques et non remainés. L'élaboration des smectites lattées par recristallisation diagénétique de smectites détritiques est favorisée lorsque la microperméabilité du sédiment est réduite (argiles fibreuses et particules calcaires, surtout les foraminifères, peu abondantes); lorsque les échanges chimiques à bilan constant avec le milieu interstitiel sont facilités (smectites abondantes) et lorsque la vitesse de dépôt est faible à moyenne. Dans les autres cas, la recristallisation de smectites lattées est très réduite ou absente. L'identification de ces minéraux dans les sédiments permet de préciser les conditions paléogéographiques de dépôt. La rareté des smectites lattées au Plio—Quaternaire pourrait être liée à l'extension des grands refroidissements mondiaux du climat.

Biotites from three peaty gleyed podzol soil profiles on ranite bedrock were examined to investigate the oxidation of the octahedral Fe during weathering. Oxidation of these biotites as determined by Mössbauer spectroscopy shows a good correlation with the in situ measured soil Eh values of the sampled horizons. In every soil profile the highest Eh measured is in the A horizon and the lowest in the C horizon. Similarly, biotites from the A horizons are the most oxidized compared with those from the lower horizons. In most of the samples the oxidation is accompanied by loss of K+ from the lattice, as demonstrated by a moderate degree of vermiculitization.

Nous présentons une méthode de calcul de l'énergie totale d'une structure cristallińe (énergie électrostatique, énergie de van der Waals, et énergie de répulsion). Nous utilisons ensuite cette méthode pour étudier une série de phyllosilicates (talc, pyrophyllite, muscovite et phlogopite), pour comparer et discuter leurs propriétés physico-chimiques en particulier l'énergi de liaison interfoliaire et la distance interlamellaire. Nous constatons que des méthodes de calcul, basées sur des expressions empiriques différentes de l'énergie de répulsion, conduisent à des résultats relatifs analogues. Nous vérifions que la distance interlamellaire calculée approche la distance mesurée à 0·1 Å près. Nous calculons des énergies de liaison interfoliaire du talc et de la pyrophyllite (de l'ordre de 30 Kcal/mol) plus faibles que celles de la muscovite et de la phlogopite (de l'ordre de 60 Kcal/mol).

Different types of chlorite and related mixed-layer minerals have been found in profiles developed on ultrabasic rocks of Ivory Coast and Brazil. Normal chlorites, with low K and Ni contents, are of hydrothermal origin and are situated at the bottom of the alteration profile. At the top of the profile the mixed-layer minerals are Ni-chlorite/vermiculite, which formed under weathering conditions, or K-chlorite/vermiculite, resulting from the alteration of mica. The Ni-content of the chlorite/vermiculite can reach 10% NiO.

The clay fraction from the B horizon of a Psammentic Hapludalf containing vermiculite and smectite was treated with n-alkylammonium chloride ranging from nc = 6 to nc = 16. X-ray data were obtained by step-scanning, stored in a computer, corrected with the Lorentz and polarization factors and then fitted with a deconvolution programme. The layer charge was determined from the uncorrected, the corrected and the fitted diagrams, respectively. The evaluated confidence limits of ξ decreased in the same order revealing the vermiculite to be a high-charge type. The computed charge distribution of the smectite changed from an unlikely bimodal to a nearly symmetrical distribution.

A detailed study of the IR spectrum of goethite is given with the aim of relating variations to crystalline order and particle size. The OH stretching vibrations are split into two active components at high frequency, plus two inactive ones at low frequency. Two different bending modes exist from site group splitting. Their active modes from factor group splitting are at lower frequencies than the uncoupled ones. The lattice bands at 630 and 400 cm−1 correspond to Fe-O or Fe-OH stretching, approximately parallel to a and c, and thus respectively sensitive and not sensitive to the particle shape, as long as it remains elongated along c.

Low crystallinity of unsubstituted goethites is characterized by a small coherently diffracting domain, which may be accompanied by another type of disorder revealed by broader IR half-absorbance band widths. Inside the unit cell, the H-bond is weaker, which increases the OH stretching frequency and lowers the bending frequencies. Also, an increase in parameter a and a change in Fe-O bonds, which might correspond to a minor tilting of octahedra, occur along with a lowering of the frequency of the 630 cm−1 band.

Detailed clay mineralogical and chemical analyses of well cuttings of Tertiary sediments from two wells, Uruan-1 and Uda-1, on the eastern flank of the Niger delta, have been made in an attempt to investigate clay mineral burial diagenesis. The clay mineralogy indicates a transformation of smectite to an interstratified illite-smectite (I/S) phase. The relationship between ordered and random interlayering, however, is nonsystematic. The chemistry of the <0·1 µm size fraction shows some tendency towards a net gain in K2O and Al2O3 and a net loss in SiO2 with depth, but the relationship does not correlate well with the thermal gradient. The distribution of kaolinite and chlorite in both wells appears to be unrelated in any regular way to smectite transformation and these two minerals are considered to be either the products of other diagenetic reactions affecting various stratigraphic levels, or the result of primary sediment deposition. In well Uda-1, kaolinite decreases in relative abundance with depth. This trend has been interpreted elsewhere as indicative of a transition from nonmarine to marine facies. The results of this study indicate that lack of ion mobility, rather than availability, is a significant factor in retarding the formation of ordered I/S with depth, and that lithology, overpressuring, carbonate cement, and original smectite layer charge may be controlling factors in the smectite → illite transformation.