It has been shown that clay minerals transform into (Mg, Al) or (Ni,Al) serpentine-like phases if treated hydrothermally in the presence of MgCO3 or NiCO3 (Shitov et al., 1974, Frank-Kamenetskii et al., 1978, 1983a,b; Kotel'nikova et al., 1976; Varela et al., 1983; Kotov et al., 1985; Ryumin et al., 1978) and that the polytypism of the initial kaolins is inherited by the products. It has also been shown that (Mg,Al) serpentine-like phases react with KC1 to form dioctahedral micas and Mg-rich serpentine phases, but the latter do not react to trioctahedral micas (Kotel'nikova et al., 1976). Consequently, it was desirable to look for mechanisms for obtaining trioctahedral micas from serpentine-like phases.

La mise en contact d'un minéral interstrafié mica-vermiculite, en paillettes de 1 à 4 mm2, avec une solution concentrée d'halogénure de métal alcalin conduit à un échange d'ions ainsi qu'à une adsorption de sel. L'échange d'ions n'est que partiel et l'adsorption de sel a lieu dans les interfeuillets vermiculite. La teneur du minéral en sel adsorbé dépend de la nature du cation et de l'anion du sel ainsi que de la température de la solution. Les complexes salins obtenus possèdent la propriété de gonfler macroscopiquement dans l'eau avec désorption du sel. Dans les cas des chlorures de lithium, de potassium et d'ammonium, les complexes salins tendent vers des compositions définies.

The nitric-acid dissolution of chrysotile was investigated with the aim of revealing the effect of pH on its structure. Chrysotile becomes more soluble at lower pH, with the octahedral, brucite sheet more readily dissolved than the tetrahedral sheet. The dissolution of the brucite sheet increases the pH of the solvent, because of the release of OH− groups along with the Mg2+ ions. At pH 2.00, the characteristic cylindrical fibre bundle structure of chrysotile is retained after dissolution, although the outside surface of each fibre becomes rough. Chrysotile remains fibrous upon dissolution at pH 1.08, but the fibres are no longer crystalline and their cylindrical structure collapses. The nitric-acid treatments result in an increase in both the specific surface area and the pore volume of chrysotile.

Non-metallic raw materials are largely unexplored in many African countries. In an attempt to reduce this knowledge gap, kaolin occurrences in three promising regions of southern Mauritania were examined. The aim of the paper is to describe the occurrences and characterize the material in terms of mineralogy and potential technical use in the ceramics industry. The kaolins are geologically associated with various sedimentary rock units in either the Coastal Basin (Kaédi), the Mauritanide Belt (Hassi Abyad) or the Taoudeni Basin (Néma). Geochemical data show Al2O3 contents of between 9% and 38% (corresponding to 23–96% kaolinite). Samples from the Hassi Abyad and Kaédi regions have greater kaolinite contents on average and were further investigated mineralogically. The kaolin from the Néma region contained less kaolinite (<50 mass%). The region is also less accessible and hence is not considered further in this study. X-ray diffraction, X-ray fluorescence and infrared spectroscopy confirmed the geochemically calculated kaolinite contents of the kaolins and identified quartz, anatase and goethite as the remaining major mineral constituents. The degree of structural disorder of the kaolinites (determined by infrared spectroscopy) is generally greater in the Kaédi occurrences than at Hassi Abyad. Ceramic tests proved that all of these kaolin raw materials might be used for the production of ceramics, and some may even be used for fine ceramics. From an economic point of view, the Hassi Abyad deposit is interesting in terms of its quality and reserves, aspects that will be addressed in detail in a follow-up study.

Portuguese gypsum deposits utilized by the cement industry were characterized mineralogically, chemically and technologically for possible application in dermocosmetics. The deposits studied (Loulé, Óbidos and Soure) correspond to small outcrops in diapiric anticline areas. In principle, they represent gypsites which are white, and generally of higher quality for traditional applications (e.g. white cement), or greyish, and generally not adequate for cements and mortars. The analytical methods used to characterize the materials were wet sieving and X-ray sedimentation, X-ray diffraction, X-ray fluorescence spectrometry and assessment of abrasiveness, plasticity, texturometrics (adhesivity and firmness), oil absorption and cooling rate. The Óbidos gypsum displayed greater mineralogical and chemical quality (almost pure calcium sulfate) and had a finer grain size (<63 μm), whereas Loulé and Soure gypsums contain mineralogical impurities (mainly quartz). The Óbidos gypsum shows good characteristics in general for application in dermocosmetics because of its absorption, plasticity, adhesivity, firmness and low abrasiveness.

The major and minor elements for Pugu D, a rock containing a well-crystallised but disordered form of kaolinite are presented, The concentrations in the various fractions are discussed and reasons adduced for element behaviour. Relationships between element distribution and Sedimentation conditions are considered.

The Tremp–Isona basin (south-central Pyrenees, Lleida, Spain) shows maximum development of the Tremp Group (early Maastrichtian to late Paleocene) covering a wide geological record across the Cretaceous–Paleogene (K/Pg) boundary in continental facies. The mineralogy and geochemistry of lutites were used to assess the evolution of weathering from the Maastrichtian to the Eocene, and particularly for the red beds of the Lower Red and Upper Red Units (pre- and post-K/Pg, respectively). Chemical weathering decreased initially in the Maastrichtian (Gray Unit to Lower Red Unit), increasing subsequently from the Paleocene (Upper Red Unit) to Eocene units. ANOVA analysis of mineralogical compositions and cluster hierarchical analysis have been useful tools to select convenient lutites for assessment of weathering evolution by the chemical alteration index and by broadening of the 001 X-ray diffraction line of illite after ethylene glycol solvation. The lesser chemical weathering found in the Upper Red Unit (Danian) is interpreted in the context of relative warming and aridification, in climates of contrasting seasons and pronounced dry seasons.

The influence of excess Al2O3 on 3:2 mullite produced from α-Al2O3 and pyrophyllite powder was examined. A mixture consisting of 28 wt.% dehydroxylated pyrophyllite and 72 wt.% α-Al2O3 was milled in an attrition mill. The milled powders were sintered by spark plasma sintering (SPS) at 1600°C for 10, 20 and 30 min. Subsequently, the samples were heated at 1350°C for 2 h to determine the influence of the excess Al2O3 on the microstructure. No glassy phase was detected in the sample containing 72 wt.% Al2O3 and sintered at 1600°C for 20 min. The sample with 72 wt.% Al2O3 had greater hardness and fracture toughness compared to 3:2 mullite. The greatest hardness and fracture toughness of 12.43 GPa and 2.71 MPa m–0.5, respectively, were obtained in the sample containing 72 wt.% Al2O3 sintered at 1600°C for 20 min.

The cation exchange capacity (C.E.C.) of nontronite was determined by titrating the acid clay, prepared by the action of H-resin on nontronite, in water and some nonaqueous solvents. The base-titratable acidities of the acid nontronite, freshly prepared as well as aged, were found to be greater than the acidities extractable with 1 N NaCl; the difference was attributed to the non-exchangeable pH-dependent component of C.E.C. In the freshly prepared sample, H+ and Fe3+ ions were the only exchangeable cations. Ageing produced basic Fe ions which were exchangeable and whose amount increased at the cost of H− and Fe3+ ions. Whatever the age, the total quantity of these exchangeable cations corresponding to the total isomorphous charge, remained constant. The amount of the pH-dependent acidity also remained unchanged. A likely mechanism to account for the observed pH-dependent component of the C.E.C, of nontronite is suggested.

This study examines part of the thick palaeoweathering mantle that formed on the northern area of the Spanish Central System. The study of a compound profile indicates that despite weathering processes, the primary structure of the metamorphic rocks is preserved, and is only partially lost in some intervals of the upper part of the compound profile. Macro/micromorphology, mineralogy and geochemical changes within the profiles revealed two weathering paths. In the first path, Fe-chlorite weathered to chlorite-smectite mixed-layer/smectite/kaolinite+ iron oxides. In the second path, biotite and/or muscovite weathered to kaolinite + iron oxides. The profiles show a progressive decrease, from base to top, in mica and mixed-layers and an increase in smectite and kaolinite. Thus, the profiles only comprise the lower or intermediate zones of the weathering mantle. The weathering occurred under humid climates; the lower zones of the profiles were poorly drained, whereas the topmost zones were better drained and more oxidizing. The results obtained indicate that detailed mineralogical studies are very useful to reconstruct the characteristics of the weathering mantles, and as palaeogeographic and palaeoclimatic indicators.

A clayey soil from southern Tunisia has been studied in order to assess its possible use as an anti-pollution barrier. The soil has been characterized from both geotechnical and physicochemical points of view. The physicochemical characterization was carried out using X-ray diffraction, thermogravimetric and differential thermal analyses, specific surface area and cation exchange capacity measurements. Hydration tests were carried out using oedometers to determine the swelling behaviour of compacted specimens under different stress conditions. The large swelling pressure (420 kPa) is in agreement with the mainly smectitic character of the soil highlighted by the physicochemical tests. The changes in the fabric of the soil and in its porosity after the hydration tests were investigated using the same physicochemical methods, and related to the macroscopic hydro-mechanical properties. X-ray diffraction and mercury intrusion measurements show that there are changes in the interlayer spacing, particle size and inter-aggregate pore distribution. Comparing the properties of the soil with the Belgian guidelines for the construction of anti-pollution barriers showed that the material was suitable for that use.

A binary ion exchange of cationic pairs involving K, Na, Ba and Ca in clinoptilolite was investigated calorimetrically. The selected cations included two pairs equal in charge and two pairs similar in size. The heats of ion exchange and the degrees of exchange were determined at 30 and 95°C. The data obtained are discussed with respect to cationic interactions in the clinoptilolite structure and hydration characteristics (heats of hydration and hydration numbers) of competing cations in the solution. No correlation was found between the heat effects and the degree of exchange. The heat of exchange depends mainly on the difference between the hydration heats of the cations in the solution, whereas the degree of exchange depends on their positioning over the extraframework sites in the clinoptilolite structure. The heats of ion exchange are lower at 95°C than those measured at 30°C, which is due to a decrease of the hydration number with increasing temperature. In all cases the degree of exchange increases with increasing temperature.

Fine-grained sediments are a major parent material in Ebro Valley soils. During the 1960s, the area was transformed by irrigation and soils underwent serious degradation processes. The sediments have a fine texture, high CaCO3 contents and high salinity-sodicity levels, and contain illite as the most important phyllosilicate in the fine fractions. Two groups of finely laminated organization have been recognized in terms of layer thickness and grain-size distribution. Based on texture, the microstructure of the silty and clayey layers is different, irrespective of their mode of formation; porosity characteristics also differ between layers, leading exclusively to a capillary evaporation flow that results in salinization of the plough horizons. The sediments are formed by two different mechanisms, each layer resulting from uninterrupted sedimentation of a suspension or from deposition of suspended particles by water discharge.

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.

X-ray analyses of argillaceous rocks from the Skipton anticline show that their average composition is quartz 30%, calcite 27%, feldspar 5%, illite 15%, kaolin 17%. Pyrite and other constituents account for the remaining 6 %. The quartz, calcite and feldspar are considered to be detrital, the variations in their quantity and size reflecting changes in the source of the sediments. The evidence indicates the general predominance of kaolin over illite, which is contrary to other published results on Carboniferous marine shales. A kaolin-rich source is necessary to account for the composition of the sediments. An occurrence of unusually pure and well-crystalline melanterite is recorded from the Draughton Limestone.

The fine structure of the X-ray absorption spectrum observed on the high energy side of the iron K edge has been recorded with a modified commercial single-crystal X-ray spectrometer. Metallic iron samples exhibited no detectable fine structure, whereas Fe3+ in ferric oxide and Fe2+ in ferrous ammonium sulphate and ferrous sulphate produced significant fine-structure peaks due, presumably, to 1s→4p and 1s→5p electron transitions. The apex of the 1s→4p peak and the minimum between the two peaks were shifted to lower energies by the Fe2+ samples, and the corresponding apices and minima on fine-structure spectra of samples containing Fe3+ and Fe2+ were shifted by an amount which was approximately a linear function of composition. These observations have led to the development of an analytical procedure for the determination of ferric (or ferrous) iron in dry, ground or sedimented clay mineral samples. The lower limit of total iron concentration in the clay sample to which the method is applicable is approximately 1 per cent. The procedure appears promising as an indicator of the oxidation-reduction potential of the geological environment with which the clay mineral may have been in equilibrium, and as a tool of more general geochemical utility.

La nacrite constitue le plus rare polytype des kaolinites. Traditionnellement, on attribue á ce minéral une genèse hydrothermale (Ross & Kerr, 1931; Hanson et al., 1981 ), cependant Btihmann (1988) décrit sa formation en condition normale de température et de pression dans des milieux de solution et de force ionique élevées. Dans ce travail, la nacrite est mise en évidence dans deux anciens gisements de Pb-Zn situés dans les massifs de Jbel Slata et Garn Halfaya au nord de la Tunisie (Fig.l).

Pure samples of the micaceous component of Yorkshire fireclays have been separated. Examination of these by chemical, optical, X-ray, and thermal methods indicates that the material is in all likelihood an intimate mixture (but not an interstratification) of mica and a kaolintype mineral—probably the b/3 disordered kaolinite commonly found in fireclays. The evidence also suggests that this kaolin-type mineral is the end-product of weathering of the mica, and a possible mechanism is suggested.