In Argentina, the only halloysite deposits found so far are located in the southwest of the province of Rio Negro. The areas are named Mamil Choique and Buitrera, separated by 50 km in a W–E direction. The deposits are located in pyroclastic rocks, rhyolitic tuffs and ignimbrites.

The aim of this paper is to determine the origin of the alteration, any relationship between the processes in Mamil Choique and Buitrera, and the factors which controlled the halloysite formation. The work was based on field work, mineralogical and geochemical studies, form and distribution of the alteration. Fresh as well as altered rocks were collected. The first were studied by petrographic and chemical methods. In altered samples, the clay mineralogy was determined by XRD and textures by SEM and TEM. Chemical analyses on fresh and altered samples were used to characterize the alteration process and the rock composition.

It was concluded that halloysite has been formed by weathering in a time range from Middle Eocene to Middle Oligocene and extended along 50 km in a W-E direction, independent of the source rock. Particle morphology depends on the original texture of the rock. Spheroidal halloysite is related to rocks with low porosity and tubular particles are related to rocks with open spaces.

L'étude de l'adsorption des α amino-acides à leur point isoélectrique par la montmorillonite saturée en cations minéraux de valences différentes montre que la quantité fixée dépend de la réactivité de l'acide aminé pour le cation minéral saturant la smectite. La fixation des α amino-acides est plus forte avec les cations bivalents qu'avec les cations monovalents; elle s'accompagne d'une libération des cations minéraux (échange).

Nous avons pu mettre en évidence l'existence de relations linéaires entre les quantités d'amino-acide fixé et les quantités de cations incorporés dans le minéral. Ces relations permettent de connaître à priori le comportement d'un acide aminé donné vis-à-vis de la montmorillonite saturée en un cation donné. Elles permettent de classer les amino-acides en trois familles: (1) les amino-acides qui sont incorporés sous une forme cationique simple par échange d'ions; (2) les amino-acides qui forment des sels et/ou des complexes de coordination avec les cations métalliques; (3) les amino-acides qui présentent un comportement compliqué et dont l'incorporation dans la smectite se conjugue avec une polycondensation.

The geological disposal of radioactive waste, based on a multi-barrier concept wherein the first barrier consists of the metal waste container and the final barrier the host rock, is widely considered the only viable solution to this issue. The bentonite-based seal around the canister forms one of the barriers. The unique swelling and sealing capabilities of bentonite play a major role in repository safety concepts in that they allow the bentonite barrier to withstand serious mechanical damage without its function being compromised.

This paper presents experimental research focusing on the dynamics and mechanics of the sealing of cracks and joints using bentonite-based materials. Physical models were used to simulate the contact point of bentonite-based sealants with cracks in the rock mass. The models examined the ability of the tested material to fill the crack thus preventing the creation of a preferential water pathway. The results show that in most cases total bentonite advance (for the same material) into fissures is, primarily, linearly dependent on fissure width. The absolute value of advance could be related to the overall swelling ability of the material characterized by its swell index or swelling pressure.

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.

Clay mineral quantification by XRD is difficult when mixed-layer clay minerals and discrete clay types are both present. New procedures for peak decomposition and pattern simulation offer increased opportunities to obtain mineral abundance estimates. This proposed methodological sequence, for quantitative representation (QR) of complex clay samples, involves: (1) determination of layer type, mixed-layer proportion and order (R); (2) simulation of XRD patterns using MULCALC, an adaptation of NEWMOD; and (3) interpretation of the clay assemblage by fitting the whole pattern with CLAY++, a statistical program. The product is a QR of individual phases or a summation of layer types. The absence of quantitative reference standards means results cannot be checked for accuracy, but the statistical fit is highly reproducible and less prone to operator error. The QRs may be obtained with simulated or actual reference mineral patterns in the database. Results for freshwater marsh samples illustrate the approach.

Because of increased interest in mineralogical analysis of soils a rapid, generally applicable method to separate clay minerals is needed, and such a method is described here. The technique is a modification of the heavy liquid density gradient centrifugation method which is commonly applied in mineralogical practice. Modifications are:

1. (1) The use of suitable surface active agents to overcome the flocculation problem.

2. (2) The use of the ultra centrifuge at about 33,000 g to hasten sedimentation.

3. (3) The use of a linear density gradient which supplies in addition a quick identification of the mineral composition.

Preliminary experiments, evaluating different variables (e.g. cation form, ultrasonic treatment, influence of surfactant), reveal the scope and limitations of the procedure presented.

Clay minerals of different density can be separated. However, if the clay minerals of a mixture have overlapping densities, or if they contain a series of mixed-layer minerals, only a broad fractionation is possible. This is shown by X-ray diffraction and electron microscope studies of isolated zones (or parts of them) which develop in the centrifuged density gradient columns containing various pre-treated natural clays or clay mixtures.

Two profiles in Devonian marine deposits have been studied, consisting of pelites, subgreywackes, greywackes and quartzites. Quartz and clay minerals are major components and feldspar and calcite are minor ones. Phyllosilicates in the fine fractions are kaolinite and illite; kaolinite has a high degree of ordering; illite is predominantly of a 1Md polytype, with low Na content and poor crystallinity and has a phengitic composition in greywackes, whereas in pelites it is muscovitic in composition. Both phyllosilicates may be inherited from a source area with intensive weathering processes, although illite may also be a diagenetic phase. These mineral characteristics indicate that the Devonian rocks did not reach the anchizone boundary in their post-depositional evolution.

The chemical composition of pelites and subgreywackes reveals a high degree of chemical maturity. Chondrite-normalized REE patterns indicate a higher degree of weathering of these Devonian sediments than of Post-Archaean Australian Shales (PAAS), possibly as a consequence of sedimentary recycling processes. The REE patterns of the Devonian rocks in addition to the high Th/Sc, La/Sc and Th/Co ratios suggest a felsic composition of the primitive source area, probably a K-rich granite.

A montmorillonite mineral occurs in the marginal facies of a biotite-granite in the younger granite ring-complex at Ropp, Plateau Province, Northern Nigeria. The mineral is associated with the development of silexitic textures (pegmatitic knots and miarolitic cavities) filling the spaces between euhedral crystals of quartz, perthite and biotite protruding from the walls. Chemical, X-ray and differential thermal results suggest that the mineral belongs in the beidellite-nontronite series; it is thought to be primary in origin.

Addition of aluminium to the ferrous sulphate mother solution used to synthesize lepidocrocite, γ FeOOH, yielded a substituted, though excessively hydrous (Fe,Al)OOH structure. Apart from reducing X-crystallinity, the main effect of isomorphous incorporation of aluminium in the lepidocrocite lattice was a shift, to higher values, in the temperature at which the lepidocrocite dehydration to maghemite was complete. Increased thermal stability of this substituted lepidocrocite, deduced from measurements of magnetic attraction made after serial heating, was consistent with the presence of a boehmitic component in solid solution. Magnetic susceptibility data for an oxisol indicated that a similar ferri-aluminous complex had developed pedogenically in the upper solum. This constituent was, however, amorphous to X-rays.

Goethite, lepidocrocite, magnetite and akaganeite were synthesized in 0.8 M, 2 M and 5 M NaCl solutions at various temperatures (25, 40, 60°C) under slightly acidic to slightly alkaline pH with or without Si additions. Elevated temperatures prevent complete oxidation of initial Fe2+ solutions and magnetite and siderite precipitate, accompanied by goethite and lepidocrocite. At higher salinity, O2 solubility is reduced and its distribution is limited, leading to coprecipitation of lepidocrocite, akaganeite and goethite.

Lepidocrocite morphology changes from plates at pH 5.5 through rods at pH 7 to multi-domainic crystals at pH 8.2, due to enhanced crystal growth along the c axis. Salinity and temperature have opposite effects on lepidocrocite crystallinity.

Goethite crystals are multi-domainic and twinning appears only at elevated temperatures. Increases in temperature and salinity improve goethite crystallinity as observed by IR spectra. Addition of Si up to Si/Fe = 0.1 retards crystal growth and Si-OH-stretching bands appear. At Si/Fe = 1 most of the precipitate is short range ordered.

Platy and rod-shaped lepidocrocite from the Thetis and Atlantis II Deeps, were probably formed under the slightly acidic conditions of the hydrothermal brines. The Si concentration was greater in Atlantis II Deep than in Thetis Deep, leading to larger lepidocrocite and goethite crystals in the latter.

Multi-domainic goethite could have precipitated throughout. Pure phase goethite might have precipitated in the less concentrated brine, whereas mixtures of goethite and lepidocrocite might have precipitated in the more concentrated brine, depending mainly on oxidation rate and oxygen mobility within the brine.

Although seldom appreciated, the hydrocyclone offers several advantages over the more commonly used sedimentation/decantation method for clay separation. Large amounts of relatively concentrated clay suspensions may be separated in a matter of minutes. In addition, no dispersing agents are required and the shear conditions encountered by the suspension during passage through the hydrocyclone lead to particle disaggregation both in the silt and sub-micron size range. Formulae relating design factors of the hydrocyclone to separation performance exist but are of limited value as they are unable to predict the maximum particle size of the clay product or percentage recovery of any particular size of particle. Such data, determined from practical trials, are given for kaolinite, smectite and mixed-assemblage clay suspensions. Design limitations prevent the effective separation of clay products with ‘cut-off’ sizes finer than 5 µm.

Current methods of quantitative whole-rock clay mineral analysis of sandstones often provide little more than an estimate of clay mineral abundances, especially where the total clay mineral content is <10 wt% of the sandstone. More accurate determinations of clay mineral abundance in the whole rock can be made by combining thermogravimetry/evolved water analysis (TG/EWA) and X-ray diffraction (XRD) data. The TGA/EWA system incorporates a purpose built thermobalance linked to a water specific infrared detector which is used to measure quantitatively the clay mineral dehydroxylation water evolved from the whole rock when heated from 250°C to 900°C. This gives a measure of the total hydroxyl content of the clay minerals in the whole rock which, when combined with XRD analysis of a separated clay size-fraction, enables individual clay mineral abundances in the whole-rock sample to be determined. Results on artificial sand/clay mineral mixtures prepared with known amounts of different clay minerals (chlorite, illite and kaolinite) show that the accuracy of the combined method is most influenced by the accuracy of the XRD data. Errors associated with TG/EWA were found to be negligible by comparison. A case study is included in which the technique has been used to determine accurately the illite abundance in the Magnus Sandstone Reservoir, Northern North Sea.

Bentonite mixed with varying quantities of sand is currently of widespread interest as engineering liners for water containment and waste disposal. However, the alternative use of pumice, widespread in many volcanic regions, has been less studied and requires further characterization of geotechnical properties and performance prior to its extensive use. The objective of this study was to determine and compare the compaction and swelling characteristics of sandbentonite and pumice-bentonite mixtures using available geo-materials from Turkey. Standard Proctor compaction tests and constant volume swell tests were carried out using mixtures with four different bentonite contents (15, 20, 25, 30%) and three different sand and pumice grain size ranges (2.00–1.00; 1.00–0.50; 0.50–0.25 mm). The results indicate that pumice-bentonite mixtures have lower maximum dry unit weights, higher optimum moisture contents and greater swelling potentials than equivalent sand-bentonite mixtures. Important differences occur in the swelling potentials of the sand and pumice mixtures with respect to grain size whereby sand-bentonite mixtures show increased swelling with coarsening grain size, in contrast to the pumice–bentonite mixtures which showed a decrease. These differences are attributed to the amount of grain-size-dependent connected pore space that can be filled during bentonite expansion and the presence of dissolvable salts in the pumice. It is concluded that locally available pumice material could be used to replace sand in engineering bentonite seals, despite some differences in their geotechnical properties.

Sandstones of the Palaeocene Montrose Group were deposited in a deepwater fan environment, and form a major oil reservoir in the North Sea. Calcite concretions occur commonly within thick-bedded and structureless sandstones. These concretions have been identified by sonic logs and well reports, and were cross-checked with available core data. Regionally, 101 wells have been examined and carbonate concretions form 0.6–7.2% of the core. Concretions are most abundant along the flank of the Fladen Ground Spur, the north Witch Ground Graben (WGG), the east south Viking Graben and East Central Graben (ECG). Concretions of the ECG formed at deep burial, with C from decarboxylation. Geochemical inheritance of Mn and Sr from Cretaceous chalk clasts may occur. Concretion growth may also have been influenced by vertical expulsion of fluids (leak-off) localized above salt tectonics. Isotopic and petrographic evidence indicates that much carbonate C in the WGG was derived from biodegradation of migrating oil in meteoric water at shallow depth. The locations of abundant carbonate with characteristic negative C isotope signatures can be used as shallow exploration guides to leak-off points located above deep overpressured structures.

Thirty clay samples collected from three hills in Koutaba (west Cameroon) were characterized in order to evaluate their potential use as raw materials for ceramics. After preliminary mineralogical identification by X-ray diffraction, three representative samples from the three different hills, referred to hereafter as K1M, K2M and K3M, were selected for further investigation by X-ray fluorescence, plasticity, granularity and thermogravimetric analysis. The main clay minerals are kaolinite (32–51%) and illite (up to 12%). Additional major phases are quartz (32–52%), goethite (6–7%) and feldspars (0–4%). The chemical composition showed variable amounts of SiO2 (60–72%), Al2O3 (15–20%) and Fe2O3 (1–9%), in accordance with the quartz abundance in all of the samples studied. The particle-size distribution showed a large proportion of silty fraction (64–88%) with moderate sandy (9–19%) and clayey fractions ( < 5% for K2M, 12% for K1M and 20% for K3M). All of the clays showed moderate plasticity-index values (8–11%). Because of these characteristics, K1M and K3M may be suitable for use in common bricks and hollow ceramic products. Sieving or the addition of ball clays is recommended to increase the plasticity of sample K2M for use in common bricks.

This study examines the leaching of aluminium from calcined kaolin (metakaolin) with citric, oxalic and lactic acid and binary combinations of these organic acids. The investigated parameters were the pulp density, acid concentration, pH, agitation speed, temperature and contact time. The kinetics of aluminium dissolution from metakaolin in binary organic acid mixtures were determined and scanning electron microscopy examination and energy-dispersive spectrometry mapping of Si and Al of untreated kaolin and residual solids were also carried out. Aluminium dissolution increased with temperature, time, acid concentration, pulp density and acidity. At maximum dissolution, citric–oxalic (1:4 w/w) and lactic–oxalic (1:4 w/w) mixtures dissolved 77% and 78% aluminium, respectively, from metakaolin in 11 h. The activation energy ranged from 57.8 to 74.6 kJ/mol. The most effective parameter in the dissolution of aluminium was the temperature, indicating that the reaction was not diffusion-controlled. It was concluded on the basis of the activation energy values and the sensitivity of the reaction to temperature that the dissolution was under chemical-reaction control.

Conductimetric curves based on dissolution in dilute HF of a series of metakaolinite samples prepared by thermal dehydroxylation of kaolinite in air from 500°C to 987°C, have been interpreted in terms of (i) short-time conductivity variation, and (ii) specific initial dissolution rate (rsi). The first parameter has been correlated with a disorganization factor (Fd, equal to unity for an entirely amorphous silica-alumina mix) which does not exceed 0·52 for metakaolinite and, like rsi, reaches a maximum for samples prepared at about 720–750°C. These results, which agree with other data obtained by calorimetry and infrared spectroscopy, show that metakaolinite is never entirely amorphous and has a degree of disorder which varies with the conditions of preparation, and increases for samples which have been mechanically treated by grinding.

Bentonite deposits in Basoren Kutahya, West Anatolia, Turkey formed from alteration of perlite and pyroclastic rocks of Pliocene age. The distribution of bentonite deposits along faults in the study area indicates that the alteration solutions were hydrothermal. Although lateral zonation is observed in bentonite deposits in some regions (i.e. Demirli, Akyokus, Seklice- Sarıokuz, etc.), alteration zones are extremely irregular in the Cayırlık bentonite deposit.

X-ray diffraction studies have shown that Basoren bentonites contain dioctahedral Ca-smectite.

The Greene-Kelly test (Li-saturation and heating) showed that the Demirli and Akyokus bentonites consist of montmorillonite and that the Cayırlık bentonite consists of montmorillonite and/or beidellite. Spherulitic or hemispherical ‘crystals’ of opal-CT minerals formed from hydrothermal alteration of volcanic glass. The MgO, CaO and total Fe2O3 enrichment in bentonites, compared to parent rocks, is related to the chemical composition of hydrothermal solutions that passed through the ophiolitic rocks such as serpentinite.