A lamellar macroscopic Mg-vermiculite from Santa Olalla, Spain, which is accompanied by some talc and mica, shows a reversible expansion on glycolation due to the presence of layers with two different charges. The most probable origin of the vermiculite is by low-temperature hydrothermal transformation of phlogopite.

Transmission electron microscopy of ultramicrotomed thin-sections of Pleistocene and Eocene glass shards revealed the neoformation of (i) illite and (ii) halloysite at the glass periphery. According to previous experimental studies, halloysite neoformation in marine environments can occur on glass shards deposited in Si-rich sediments; an excess of Ca tends to inhibit the reaction.

The kinetics of the dehydration of ethanol on acidic Al3+-exchanged sepiolite has been studied. The product distribution indicates that the formation of diethylether and ethylene takes place by a series of consecutive and parallel reactions. A mechanism for the formation of both products has been derived by studying the influence of different operational variables on the reaction rates.

Mineralogical and chemical examination of Tertiary interbasaltic clays from the Faeroe Islands revealed three types of alteration of volcanic tufts and basalts: (1) Argillization of volcanic tuffs within a lacustrine environment resulted in the formation of aluminous beidellites and metahalloysites; these clays are associated with extensive coal layers at Suduroy. (2) Argillization of volcanic tuffs due to hydrothermal action resulted in the formation of thick layers of ferriferous beidellites associated with small amounts of hematite and zeolite (e.g. at Nolsoy). (3) Meteoric alteration of volcanic tufts and basalts, which took place under hot climatic conditions and contrasting seasons, led to the development of ferriferous and aluminous-ferriferous beidellites, metahalloysite, kaolinite, hematite and goethite in low (Høv profile) or high (Øravik profile) topographical positions. The three processes occurred individually or were superimposed.

The reductive dissolution by Na-dithionite of 28 synthetic goethites and 26 hematites having widely different crystal morphologies, specific surfaces and aluminium substitution levels has been investigated. For both minerals the initial dissolution rate per unit of surface area decreased with aluminium substitution. At similar aluminium substitution and specific surface, goethites and hematites showed similar dissolution rates. These results suggest that preferential, reductive dissolution of hematite in some natural environments, such as soils or sediments, might be due to the generally lower aluminium substitution of this mineral compared to goethite.

La décarboxylation de l'acide oxaloacétique en acide pyruvique a été étudiée en présence de montmorillonite saturée en cations différents et des mêmes cations à l'état libre (non incorporés). L'action catalytique dépend de la nature du cation, cést-à-dire de sa facilité à former des complexes avec l'acide oxaloacétique. Le réseau argileux intervient dans la catalyse essentiellement par les atomes d'aluminium (ou de fer) situés sur les bords des feuillets argileux. Il joue le rôle d'un métalloenzyme biologique. Le calcul des énergies d'activation pour les différents systèmes met bien en évidence l'activité catalytique de l'argile. En présence du phyllosilicate, la réaction a lieu avec un bon rendement jusqu'à une température de 60°C.

The dehydration of an interstratified talc-trioctahedral smectite mineral (aliettite) from Monte Chiaro, Taro Valley, Italy, saturated by Na, K, Rb, Cs, Mg, Ca, Sr, Ba and NH4 cations, was studied by X-ray powder diffraction and thermal analysis, under different conditions of temperature and relative humidity. Both in the natural and exchanged states the mineral exhibited several orders of basal reflections and, to varying relative humidity (p/po) and temperature (T) conditions, behaved like a smectite except that: (i) the c-dimension was not very sensitive to relative humidity increase; (ii) the closest packing was for T > 400°C, the collapsed state being reached gradually as T increased and an inverse trend was apparent between 20° and 300°C; (iii) the endothermic reactions at T = 400°C and T = 600°C appeared to be affected by the exchange treatment. The experimental data are consistent with a structural model in which it is possible to distinguish layers containing a patchwork of talc and smectite domains: the patchworks are superposed to make a regular alternation of the two domains.

Aluminium-substituted goethites were synthesised in 0·3 M KOH at either 25° or 70°C. The 25°C goethites had unit-cell a and c dimensions about 0·1–0·2% larger than the 70°C goethites of comparable Al substitution. Unit-cell b dimensions were similar regardless of synthesis temperature. The 25°C goethites contained larger amounts of Al in their structures than the 70°C goethites synthesised at the same initial Al/(Al + Fe) mole ratio in solution. The 25°C goethites had lower dehydroxylation temperatures and smaller differences between the two OH-bending bands than comparable 70°C goethites. These differences between the two sets of goethites are believed to be caused by the presence of more structural defects in the 25°C goethites than in the 70°C goethites. Goethites with the lowest Al substitutions consisted of crystals with several coherently diffracting domains, while at high Al substitutions the crystals were mainly monodomainic. Surface areas of the 25°C goethites varied in a complex manner as a function of thickness along [100], character of the domains, and physical size.

A method of analysing the colloidal stability of variable-charge mineral suspensions as a function of pH is described, which combines light scattering with soluble element and surface charge measurements. The absorbance of the suspensions closely follows the A = k−m equation, where m = −dlgA/dlgλ is an aggregate light-scattering index which is inversely related to the limiting sedimentation speed v = dA600nm/dt. Usually, suspensions of variable-charge minerals scatter less near their point of zero charge where the aggregation process is a maximum, in contrast to environments where they are charged and well-dispersed. Although the morphology of goethite is that of elongated needles, kaolinite plates, and imogolite tubes, the coalesced sphere approach of the Mie theory allows rapid correlation of the absorbance at 600 nm with aggregate-size distribution, indirectly measured by the light- scattering aggregate-size index m confirmed by electron microscopy experiments. Kaolinite and goethite suspensions obey the diffraction scattering law with a variation in size of 2–20 µm from the dispersed to the aggregated particles. Imogolite and alumina gel suspensions possibly obey the anomalous diffraction scattering law and are present as 0·05–1 µm aggregates of their 50–100 Å structural units. Silica gel and the more dispersed imogolite suspensions possibly obey the anomalous Rayleigh scattering law with a variation of aggregate size from 700–3000 Å.

The dehydroxylation of kaolinite has been investigated by isothermal thermogravimetry. Kinetic analysis using the Avrami equation shows that a combination of atomic mechanisms operates throughout the temperature range 734 K to 890 K. An empirical activation energy of 222 kJ mol-1 was calculated from the Arrhenius relationship using rate constants based on diffusion and homogeneous models. The activation energy (Ea) was calculated for a series of degrees of dehydroxylation by the time to a given fraction method, showing an increase in Ea during the early stages of the reaction. The isothermal plots indicate that OH is retained in the final stages of the reaction. The observations are explained in terms of a reaction mechanism in which kaolinite grains dehydroxylate from the edges inwards, parallel to (001).

Surface excess isotherms were determined for the adsorption of 1,4-dioxan (DIOX), tetrahydropyran (THP) and tetrahydrofuran (THF) from binary solution with methanol (MeOH) on Al3+- and Cr3+-exchanged, montmorillonite containing different amounts of pre-adsorbed water. The isotherm type is dominated by the binary mixture whereas the amplitude, which follows the sequence DIOX > THP > THF, is further influenced by the exchange cation, Al3+ > Cr3+, and the water content of the clay. The sorption of DIOX and THP from MeOH exhibit a type IV isotherm in the Schay-Nagy classification on both cation-exchanged forms, whilst that of THF from MeOH is of type I and THF is the preferred adsorbate. The azeotropic point for the type IV isotherms is close to a 1:1 mole ratio for the samples dried at 120°C but when the samples are rehydrated at 12% r.h. following thermal pretreatment at 120°C the azeotropic point shifts in favour of the cyclic ethers DIOX and THP, making them the preferred component over a substantial portion of the composition range.

The volatiles produced when various clay minerals were pyrolysed under a stream of He or in vacuo were condensed and analysed. They contain a wide range of cations and anions in concentrations ranging from ppm to hundreds of ppm. The reactivity of these volatiles was demonstrated by their reaction with calcite. It is suggested that the water liberated on heating acts as a scavenger of various impurities present in the clay.

La détermination des complexes silico-ferriques, hydroxydes métalliques ou autres matériaux amorphes ou pseudocristallins contenus dans les sédiments marins est généralement difficile. La méthode décrite utilise la diffractométrie de RX et permet l'analyse quantitative de quelques-uns de ces matériaux tels que la silice biogène ou les cendres volcaniques. Cette phase amorphe constitue une composante majeure dans certains sédiments marins, d'où la nécessité de pouvoir la quantifier. La méthode reste suffisamment simple, fiable et précise pour permettre aussi la détermination quantitative de la phase cristallisée qui lui est généralement associée.

Mixed-layer clay minerals of illite-smectite, having smectite contents ∼25% and IS ordering, kaolinite-smectite, and a three-component variety with kaolinite-smectite-chlorite-like material are described from a Carboniferous paleosol in South Wales. The clays, which are restricted to particular horizons, are unusual occurrences in a pedogenic sediment and pose particular problems as to their genesis. Positive geological evidence, principally in the form of mineralogical and organic thermal maturation data, demonstrates that the rocks of the region cannot have reached diagenetic temperatures > 100°C. This would appear to suggest a pedogenic rather than a diagenetic origin for these clays. In these sediments the only plausible mechanism of illitization to produce the illite-smectite would be by repeated wetting and drying cycles causing irreversible K fixation. Such cycles would be entirely realistic for a vertisol-like paleosol formed on overbank deposits in a floodplain environment subject to strong seasonal moisture contrasts. If this interpretation is correct, this illite-smectite represents the most illitic yet recorded from a non-burial or non-hydrothermal environment.

Silico-ferric coprecipitates, with chemical formula , were aged in suspension at 75°, 100° and 150°C and the structural evolution of solids with time studied by XRD, TEM, and IR, Mössbauer and EXAFS spectroscopy. The initial Si-Fe coprecipitate was found to be amorphous but showed local order similar to that of a smectite layer. At 75°C only a weak structural evolution of the silico-ferric product towards a smectite-like structure was observed. Experiments performed at 100° and 150°C led to synthesis of a ferric smectite with structural formula . During syntheses a highly soluble silico-ferric complex appeared; the Si/Fe atomic ratio of this complex was 3, and the apparent concentration of Fe3+ in solution reached 27 mm/l. These syntheses prove that the crystallization of a dioctahedral smectite, containing only Fe3+ atoms in the octahedral sheet, is possible under strictly oxidizing conditions. However, crystal growth of a ferric smectite under these conditions is slow and only syntheses carried out at sufficiently high temperatures give convincing results.

A method is described for IR spectroscopy whereby combining statistical analysis with the quantitative method described by Slonimskaya et al. (1986) results in more informative spectra. Application of this method to dioctahedral micaceous minerals with vacant trans octahedra has revealed a number of peculiarities in the octahedral cation distribution. This distribution is never completely disordered in the samples studied and shows maximum degree of short-range order in minerals having predominant Al.

Leaching of two palygorskites with different octahedral aluminium contents has been carried out in HCl at 20°–70°C. The acid concentration was varied in the range 0·05–6 mol. 1−1, using 4–500 g.g−1 liquid to solid ratios. Acid leaching of up to 60% of the octahedral cations did not provoke structural changes or amorphization of the samples. On the bases of the selectivity of extraction of Mg2+, Fe3+ and Al3+, it appeared that most of the octahedral edge positions are occupied by Mg2+, some by Fe3+, and practically none by Al3+. Differences in acid-leaching behaviour between the two palygorskites was better related to accessible surface than chemical composition.