Porosity development in a sepiolite from Toledo, Spain, has been studied as a function of acid (0·25, 0·50, 0·75 and 1·0 m HNO3) and subsequent heat treatments (110–400°C). Adsorption isotherms of N2 at 77 K have been analysed by f-plot, t-plot, αs-plot and the Cranston & Inkley methods. Results are compared with those obtained on products heated without previous acid activation. Experimental conditions favouring porosity development are discussed.

Asulam (p-aminobenzenesulphonylmethylcarbamate) is adsorbed by alkaline and alkaline-earth cation saturated montmorillonites at room temperature either by protonation or by physical forces. On heating it decomposes to different products depending on the polarizing power of the exchange cation. On Mg–clay, sulphanilic and carbamic acid are obtained; the latter decomposes after equilibration at 100% relative humidity. In Ba– and Ca–systems, sulphanilamide and sulphanilic acid are produced with minor amounts of p–aminobenzenesulphonylcarbamate anion. Sulphanilamide is the main decomposition product in Na–, K–and Cs–systems. On Li–clay, decomposition of the Asulam results in the formation of p–aminobenzenesulphonylcarbamate anion and p–aminobenzenesulphonylcarbamic acid, probably as a consequence of the intermediate polarizing power of this cation.

Many problems—from soil research to ceramics—require a reliable characterization of the clay minerals involved. This can be done using four clay-organic reactions: (i) staining tests and dye adsorption; (ii) glycerol and glycol adsorption; (iii) intercalation; (iv) alkylammonium ion exchange. Dye adsorption (staining tests) and glycerol adsorption allow a preliminary identification of the clay mineral groups. Intercalation reactions indicate minute differences between kaolins which cannot be detected by XRD and DTA. Alkylammonium ion exchange provides the best method for characterizing smectites and is sensitive to changes in the layer charge.

Four bands occur in the Mössbauer spectra of pelletal glauconites ranging in age from Cambrian to Oligocene. Components due to Fe3+ in the cis M(2) positions are visible in all the spectra but their shapes depend on the expanding layer content. In the spectrum of the Oligocene glauconite a very well-developed shoulder due to Fe2+ in the trans M(1) positions is observed. In the younger glauconites (15–20% expanding layers) the conventional assignments of the spectra indicate that Fe2+ occupies the trans M(1) positions, the cis M(2) positions being vacant. Conversely, in the Palaeozoic samples (~5% expanding layers) the cis occupancy significantly prevails over trans occupancy.

Polarizing microscope, electron microprobe and X-ray diffraction examination of minerals in a soil profile developed on a glauconite sand indicate that destabilization of glauconite can be a progressive process which appears to be the reverse of glauconitization. Glauconite in these soils appears to be destabilized into a mixed-layer glauconite-nontronite phase, which crystallizes as a plasma mineral. This material in turn is transformed into smectite+kaolinite+oxides. Loss of K and Fe is evident in whole rock as well as microprobe analyses of the samples. Thus glauconite can lose both Fe and K to aqueous solution during weathering, leaving aluminous clay minerals in the soil.

The reactions of ten C6-alkenes over a Cu2+-montmorillonite catalyst have been examined between room temperature and 100°C. The reaction products depend upon inter alia the branching in the alkene, the position of the double bond, and the temperature. Hex-2-ene, cis and trans-4-methyl-pent-2-ene and 4-methyl-pent-1-ene do not react at all, whereas 2-methyl-pent-1-ene and 2-methyl-pent-2-ene and 3,3-dimethyl-but-1-ene form alcohols below ∼40°C and dimers at higher temperatures. Hex-1-ene forms the di-2,2′-hexyl ether.

The effects of dry grinding on the structure, morphology and properties of imogolite have been investigated by means of X-ray powder diffraction analysis, IR, DTA, and measurements of CEC and specific surface area. Imogolite is very susceptible to alteration by grinding compared to layer silicate minerals such as halloysite, kaolinite and montmorillonite. In the earlier stages of grinding, the parallel array of imogolite tubular structure units is markedly disturbed. On further grinding, disruption of Si-O-Al linkages, polymerization of the silica component released and loss of structural OH groups proceed simultaneously, these changes eventually resulting in complete breakdown of the structure. Ground imogolite is reaggregated finally into granular particles of irregular shape. All these changes are associated with a decrease in water holding capacity, CEC and specific surface area.

Nous proposons l'étude du mècanisme de gonflement d'une smectite, basée sur l'exploitation des diagrammes de diffusion aux petits angles obtenus avec la source de rayons X synchrotron de Lure (Orsay). Cette technique a permis d'étudier le gonflement, en fonction de la température, d'une montmorillonite Na, en suspension dans l'eau et concentrée à 20%.

L'analyse des diagrammes est éffectuée par une méhode basée sur la comparaison des courbes expérimentales à des courbes théoriques. Les modèles de structure permettant le calcul des diagrammes théoriques fait appel à la notion de particule (définie comme un empilement de feuillets parallèles) et de structure interne de la particule (définie par un ensemble de translations di affectées des probabilités pi qui déterminent la position mutuelle des feuillets).

Les résultats montrent qu'à basse température, les particules de très grande taille sont constituées par la superposition de sous empilements, comportant 4 à 5 feuillets, hydratés d'une manière homogène et se désorientant mutuellement pour former la structure du gel. On explique ainsi que la structure du gel formé est correlée avec celle du solide initial et que le phénomène de gonflement est quasi réversible.

Analysis of the distribution of gibbsite in soils and saprolites of Galicia, NW Spain, leads to the conclusion that this mineral is mostly formed by inorganic mechanisms in open, well-drained and desaturated systems in the initial stages of weathering of various aluminosilicates, especially plagioclases. No evidence has been found to support the hypothesis that it is a feature inherited from hotter climates, as it has not been identified to date either in ancient sediments or in the most evolved and weathered soils. The role of organic matter and the possibility ofa biogeochemical origin is also discussed: it is considered that this mechanism is only important during podsolic soil development.

Eleven shales and fourteen turbidite sandstones from the Mam Tor Beds were analysed chemically and by XRD. The ratio of kaolinite to illite plus mixed-layer clay was higher in the sandstones than in the shales, size fractions demonstrating that this ratio decreased as the grain size decreased. Shales more basinal in character than those of the Mam Tor Beds contain more illite and mixed-layer clay and less kaolinite and it is suggested that there was a lateral variation in clay mineralogy with distance from the shore line due to particle size sorting and that the character of the clay mineral fraction was retained as the turbidity current transported sediment from a nearshore environment deeper into the basin. Support for this model was obtained from the geochemistry which showed that the sandstone matrix differed compositionally from the shales. Systematic variations occurred in the turbidite sandstones but not in the shales which are therefore considered to be non-turbiditic. Only minor mineralogical changes appear to have occurred during diagenesis.

The processes taking place during decomposition of montmorillonite by percussive grinding have been studied by IR, X-ray and DTA methods. Destruction can be described as a series of parallel changes in the montmorillonite structure which proceed at various rates. The fastest is a change in status of the exchangeable cations followed by that of the hydroxyl groups. The slowest degradation process is the separation of tetrahedral and octahedral sheets. Degradation of the structure proceeds by delamination of particles, breaking of layers, crushing of layers by shocks perpendicular to the ab-direction, rearrangement of coordination polyhedra and transfer of protons within the structure. The relative importance of each of these processes is evaluated. It is assumed that the matrix resulting from complete breakdown of the montmorillonite structure consists mainly of the residue of tetrahedral sheets and deformed incomplete tetrahedra and octahedra, or of individual atoms held together in clumps either by Van der Waals forces or possibly, in some cases, by newly formed chemical bonds.

Une étude expérimentale des équilibres d'échange entre gel de composition (A,B)O-SiO2-nH2O et solution aqueuse (A2+, B2+) a été réalisée avec les couples (A,B) suivants: Mn-Mg, Co-Mg, Co-Mn et Cu-Ni. Les coefficients de distribution (A/B)gel/(A2+/B2+)sol obtenus sont énormes, l'élément A se fixant presque totalement dans le gel et n'apparaissant le plus souvent qu'en traces dans la solution. Le dosage de celles-ci par activation neutronique a permis d'atteindre le domaine des concentrations où ces gels présentent un comportement idéal (loi de Henry) et de mesurer avec une précision accrue les enthalpies libres standard des réactions d'échange correspondantes.

Neuf saponites synthétiques ont été utilisées pour étudier l'influence des substitutions Si-Al sur le paramètre b et la distance basale d001 des phyllosilicates 2/1 trioctaèdriques. Dans les saponites hydratées à deux couches, le paramètre best le résultat des interactions entre couches octaèdrique et tétraèdriques. Dans les saponites hydratées à une couche, il dépend non seulement des paramètres des couches octaèdrique et tétraèdriques mais encore de la nature de la couche interfoliaire. L'effet du cation interfoliaire varie avec sa taille et sa localisation dans l'espace interfoliaire.

A naturally occurring hematitic iron oxide/layer-silicate complex has been found in red mottled patches of a deeply weathered granite in north-east Scotland. X-ray diffraction shows a basal spacing of 36 Å—also observable by high resolution electron microscopy—which expands to 40 Å with glycerol and contracts to 33·5 Å on heating. Selected area electron diffraction reveals a composite hematite/layer-silicate pattern with the a-axis of hematite parallel to the b-axis of the silicate. The IR spectrum of the complex clearly shows the contribution made by each of the components. The silicate, with bands due to OH stretching at 3602 cm−1, OH deformation at 855 cm−1, and Si-O stretching at 1085, 1035, 540 and 471 cm−1 resembles ferruginous pyrophyllite, while the hematite, with a perpendicular band at 647 cm−1, in-plane bands at 519, 438, 400, 302 and 227 cm−1 and a characteristic pattern of relative band intensities, is similar to a platy form of soil hematite. Electron microprobe analysis of individual particles gives the complex an (Fe + Al): Si ratio of 6:1, which is consistent with a structure made up of twelve octahedral sheets terminated on both sides by a silicate sheet. It seems likely that the complex developed from a siliceous ferrihydrite which became progressively more organized with geological time.

Los rasgos característicos geológicos (relieve bajo o medio, clima árido-semiárido, contrastado) y mineralógicos (asociación ópalo-alunita-caolinita) de niveles continentales Prelutecienses apoyan una hipótesis genética no hidrotermal. La evaporación de aguas portadoras de sílice disuelta, procedente de una meteorización del basamento, durante una estación seca, en depresiones, conduce a una precipitación de ópalo. Por otra parte, la desecación de áreas cubiertas por vegetaciones implica una descomposición bacteriana anaeróbica de materia orgánica y génesis de sulfuros; posteriormente, la aireación oxida a estos sulfuros y se generan aguas sulfatadas ácidas. Estas se mezclan con las que contienen silice, precipitando entonces conjuntamente ópalo, alunita y, si el Al3+ está en exceso, tambien caolinita.

Two chlorites, sheridanite and clinochlore, have been examined to determine their surface charge characteristics. In order to increase their surface area and to produce a measurable surface charge, the chlorites were treated with 10−3m, 10−2m, and 10−1m HCl in 10−2m MgCl2 solutions. These treatments are shown not to alter the crystallinity of the chlorites and to produce a small pH-dependent negative charge which is not directly related to the total surface area. The clinochlore, (Si6.13Al1.84)(Al1.53FeIII0.53FeII0.18Mg9.52)O20(OH)16, is more readily attacked by the acids than the sheridanite, (Si5.43Al2.55)(Al2.90FeII0.05Mg8.86)O20(OH)16, and also produces material with greater surface area and pH-dependent, negative, surface charge. It is concluded that: (i) isomorphous substitutions in the lattice are not reflected in a permanent surface charge; (ii) the observed surface charge arises not only at the edges of the particles but also at points where the chlorite is predisposed to attack by acids; (iii) in the chlorite-acid system used, anions—probably mainly silicate—block positively charged sites.

L'examen des textures de kaolins au M.E.B. apporte des données complémentaires utiles pour la compréhension de leur genèse. Les kaolins provenant d'une altération météorique ont une texture poreuse constitutée par de gros paquets de plaquettes hexagonales empilées de façon lâche ce qui les différencie nettement des kaolins d'origine hydrothermale dont la texture est compacte et la porosité faible. Les kaolins sédimentaires montrent une association de petites plaquettes accolées les unes aux autres quand on les volt suivant les plans de clivage, ou présentent une orientation plus ou moins marquée dans la direction perpendiculaire. Enfin I'observation des textures permet de mesurer I'influence et I'importanee de la diagenèse dans les dépôts argileux.

The structure of kaolin has been examined together with aspects of dosimetry and energy loss mechanisms of radiation to explain the formation of g = 2 EPR centres. The analysis points to the formation of a trapped hole on the ‘inner layer’ oxygen atoms of kaolin located at the boundaries between divalent ion and trivalent ion ‘cells’, in particular at the boundaries with excess negative charge. Direct interaction of X-rays with atoms and the possibility of proton recoil are eliminated. The means of production appears to be by transfer of charge following ionization of atoms by secondary electrons, with transfer of vacancies ultimately to the oxygen ions. Mechanisms which result in a decrease in signal strength with increase in concentration are examined. It is concluded that the cell mechanisms discussed are consistent with the rates of production and that at 20 Mrad (air) the number of centres should be reaching saturation.

A sepiolite from Toledo, Spain, was activated with dilute solutions of HNO3 (0·25, 0·50 and 0·75 m) followed by heat treatment at 110, 200, 300, 400 and 500°C for 10 h. The retention of n-butylamine in cyclohexane solution was used to follow the variation of surface acid centres of the sepiolite as a function of the acid and heat treatments, these reaching a maximum for the product treated with 0·75 m HNO3 and heated at 300°C. Acid treatments within the range 0·50–0·75 m and subsequent heat treatment at 200–300°C for 10 h yielded products with the highest surface activity.

A method for estimating Gibbs free energies and stabilities of clay minerals is proposed for use with computer programs aimed at calculating the chemical evolution of natural water-rock systems. This is based on (i) a model for ideal solid solutions of a large number of end-member compositions and (ii) a data set of estimated solubility products from 36 end-members. The application of the method to the production of experimental or natural clay stabilities is discussed.