Des lizardites substituées (Mg,Fe,Ni)3Si2O5(OH)4 ont été étudiées par spectrométrie en réflectance diffuse de 340 à 2500 nm, de manière à déterminer l'environnement de Mg2+, Ni2+ et Fe2+, en particulier la symétrie des sites et les distances métal-ligands. L'examen de la structure de la lizardite sensu stricto ayant conduit à définir pour Mg2+ deux sites hexacoordonnés M1 et M2 non équivalents dans la couche octaédrique du feuillet serpentineux, l'interprétation, à l'aide de la théorie du champ cristallin, des spectres d'absorption de lizardites de compositions chimiques variées a fourni les résultats suivants: 1. Le site M1 a la symétrie C2v: octaèdre orthorhombique. 2. Le site M2 a la symérie C3v: octaèdre rhomboédrique. 3. Fe2+ est hexacoordonné, et occupe les deux sites M1 et M2 si sa teneur est supérieure à environ 2%. Pour une teneur inférieure, Fe2+ n'occupe que M1. 4. Dans une lizardite Mg + Fe + Ni, Ni2+ est hexacoordonné et occupe le site M2. Ce site du nickel est plus grand en présence de Fe2+, que lorsque le nickel y est seul (népouite). Ni2+ y occupe une position plus rapprochée du centre de la couche octaédrique que Mg2+, ce qui contribue à réduire la flexion du plan des cations dans cette couche. 5. Dans une lizardite proche du pôle nickelifère (népouite), il y a réorganisation de la structure autour d'un site hexacoordonné unique de symétrie C3v. Ce site est déformé, et plus petit: distance moyenne métal-ligand r = 2·06 Å dans la népouite, r = 2·11 Å dans la lizardite magnésienne sensu stricto. Cette réduction de la taille du site ne peut se concevoir que par la réduction de l'épaisseur de la couche octaédrique: 2·20 Å (pôle Mg) à 2·16 Å (pôle Ni).

Quantitative estimates were made by visually matching computer-simulated with experimental X-ray powder diffractometer patterns for two samples. One was a natural mixture of dickite and nacrite in about equal proportions. The second sample contained mostly quartz with corundum and mullite in small (0.5–1%) amounts. Percentages deduced from pattern matching agreed to within ±10% of the weight fractions of the components determined by an alternative method of analysis.

The method described uses external standard mixtures, both natural and artificial. Variations in degree of structural order between standard and analyte mineral are taken into account. ‘Amorphous’ materials in standard mixtures are simulated by alkali-boron-silicon glasses. Variations in response of the X-ray equipment with time are assessed by running a standard multiphase mixture. Strict control is exercised on particle orientation, and thermal treatments are used to separate overlapping reflections in the low-2θ region.

L'étude expérimentale de l'adsorption d'ions La3+ sur une smectite, successivement saturée en ions K+, Na+ puis Ca2+, montre que la stoechiomérie de l'échange s'effectue dans le rapport des charges des cations participant à la réaction. Dans un domaine de force ionique suffisamment restreint, l'échange a lieu dans les rapports suivants: 1 La3+ pour 3 Na+ ou 3 K+, et 1 La3+ pour 1·4 Ca2+. L'adsorption de toutes les terres rares sur la smectite et une illite s'effectue sans ségrégation notable dans cette famille d'éléments, les terres rares les plus lourdes étant un peu plus adsorbées que les terres rares légères. La désorption des terres rares est plus difficile, leur charge trivalente devant les stabiliser en position interfoliaire dans les minéraux argileux.

On étudie les équilibres de distribution de deux éléments de transition divalents A,B, entre gels silicatés inorganiques de formule SiO2-(A,B)O-nH2O et solutions aqueuses A2+,B2+ (A.B = Mg, Mn, Ni, Co, Cu, Zn). On montre que l'enthalpie libre molaire standard d'échange de deux éléments A,B entre gel et solution est une fonction linéaire de l'enthalpie libre d'échange calculée en considérant un échange théorique entre hydroxyde et solution. Les relations obtenues permettent de prévoir le comportement d'autres éléments divalents.

In a study of sandstones from the Coal Measures (Westphalian A and B) of the East Midlands, UK, fibrous illites exhibiting random core and lath structures were observed during SEM examination of material dried in air, but not during TEM examination of ultrathin sections made from the same material. Previous studies have reported random core and lath structures during TEM examination of extracted clay material and it is suggested that they are an artefact of the specimen preparation technique.

X-ray photoelectron diffraction data for three cleavage surfaces from a crystal of a Norwegian lepidolite containing 2·3% Rb and 3% Mn are reported and interpreted. Rubidium is shown to occupy anhydrous interlayer sites equivalent to those of potassium, and to be distributed uniformly throughout the crystals. Cleavage occurs in Mn-rich regions in which the manganese(II) is located in octahedral sites essentially equivalent to those of lithium. The octahedral Al sites can readily be distinguished from the Li,Mn sites and it is concluded that Al occupies predominantly M(2), cis sites while Li and Mn(II) prefer M(1), trans sites. Photoelectron diffraction data also indicate that 40 ± 5% of the Al is tetrahedrally coordinated, compared with a figure of ∼34% deduced independently from a surface analysis.

La majorité des termites sont en rapport avec le sol et utilisent l'argile de ses horizons, pour édiffer leurs termitières. Pour les termites du sol, dont la plupart sont des maçons, l'argile est avant tout le matériau de base offrant les propriétés de rétention d'eau, de plasticité et d'ahésivité, leur permettant de confectionner les mortiers et les microbétons de leurs unités de constructions. Si les termites humivores absorbent toujours une quantité relativement importante d'éléments argileux avec leurs aliments, les termites lignivores n'ingérent qu'occasionnellement une fraction minime d'argile. Leur action sur l'évolution des silicates se traduit par des néoformations de minéraux argileux au sein de leurs constructions: illite ‘artificielle’, hydrobiotite, vermiculite, halloysite, montmorillonite, et parfoils sépiolite et attapulgite. Ces néoformations résultent de l'action conjugée du travail de broyage et de trituration salivaire de minéraux encore peu altérés de la profondeur du sol, et des conditions physico-chimiques de milieu particulières aux termites.

Ferrihydrite transformed to crystalline Fe(III)-oxides in salt solutions but the degree of transformation generally decreased with increasing concentration (decreasing water activity) and haematite was favoured over goethite. More transformation occurred for chloride than for nitrate or sulphate and more for calcium than for magnesium or sodium. Calcium and magnesium favoured haematite over goethite. The results support the hypothesis that haematite not only can be formed but will be favoured over goethite in natural environments with low water activity, for instance dry or saline soils or sediments. The ionic environment modifies the general water activity effect.

The point of zero charge (pzc) of synthetic Fe-oxides is well documented and usually ranges between pH 7 and 9 (Parks, 1965; Schwertmann & Taylor, 1977). In contrast, the pzc of natural Fe-oxides has only rarely been determined. Using electrophoretic mobility, Van Schuylenborgh & Arens (1950) found that a natural goethite had a much lower pzc (∼3) than synthetic goethites. They attributed this to better crystallinity of the natural goethite caused by slower crystallization. Soils dominated by Fe- (or Al-) oxides rarely have pzc values as high as those of pure oxides. This is usually attributed to the presence of negatively charged impurities such as clay silicates and organic matter (Parfitt, 1981).

Ferrihydrite, a natural, poorly-crystalline Fe-oxide mineral of bulk composition 5Fe2O3.9H2O, occurs in hydromorphic soils (Schwertmann et al., 1982) and is the main component in ochrous precipitates formed when Fe-bearing fresh waters come in contact with air (Schwertmann & Fischer, 1973; Carlson & Schwertmann, 1981). Under these conditions the ferrihydrite is reasonably free of other charge-active minerals. The aim of this study was to find out if the pzc of these natural ferrihydrites differed from those of synthetic samples.

It is usually assumed that the oxidation state of the small proportion of Mn sometimes present in micas is +2, although there is evidence from electronic spectroscopy (Burns, 1970) for at least the occasional occurrence of Mn(III) in manganophyllite. We describe here X-ray photoelectron spectroscopic (XPS) measurements on the Mn in a Norwegian lepidolite which was the subject of a concurrent structural study by X-ray photoelectron diffraction (Evans & Raftery, 1982). To establish the Mn oxidation state we have compared the Mn2p core-electron binding energies (BE), the Mn2P3/2-O ls BE differences, and the Mn2p XPS peak profiles from the four common oxides of manganese (MnO, Mn3O4, Mn2O3 and MnO2) with those from the lepidolite. A re-examination of these oxides was undertaken because the agreement between reports in the literature was unsatisfactory, and uncertainty existed concerning the integrity of some of the surfaces previously examined.

Traditionally, adsorption from binary solutions is determined as the difference between the adsorbate concentration in the solution before and after contacting with the solid adsorbent e.g. hexadecylpyridinium-clay. this is essentially a static method. The bulk composition can be measured by means of a differential liquid interferometer (Dékány et al., 1978b) or by gas chromatography (Nikulitchev et al., 1979). The specific excess adsorption of the mixtures, n1°, is then calculated as (Kipling, 1965):

1

where n0 is the total amount of the liquid mixture referred to unit mass of the adsorbent (mmol/g adsorbent), xl,0 is the mole fraction of the ith component before adsorption, and xi is the mole fraction of the ith component in the equilibrium homogeneous liquid phase.