Reevesite, the Ni-Fe member of the hydrotalcite group, occurs as a secondary mineral in the serpentinized ultramafic rocks of the Kwangcheon area in Korea, replacing the pecoraitc-magnetite-millerite-polydymite assemblages in small veins lining cracks and fracture zones. Chemical analyses for the reevesites indicate the presence of the Co-analogue of reevesite and the complete solid-solution between Fe3+ and Co3+. The Co-analogue of the reevesite shows the approximate ratio of 3:1 of divalent to trivalent cations, which is in distinct contrast to the comblainite with the ratio of 2:1 reported by Piret & Deliens (1980). The presence of the complete solid-solution phase between Fe3+ and Co3+ is confirmed by synthesis at room temperature. On the basis of the results, the Co-analogue of the reevesite should be named as a new member of the hydrotalcite group, and thereby be distinguished from the comblainite. The partial substitutions of SO2-4 for CO2-3 in the interlayer region might affect the slight increase of the basal spacing in the reevesite. The synthesis performed suggests that the reevesite could be precipitated under either an alkaline or a neutral environment, if carbonate activity is high enough.

Mineralogical and geotechnical investigations on the possible use of compacted bentonite as a buffer material in nuclear waste repositories are reported. The swelling capacity is highly dependent on the density of the compacted bentonite. Swelling pressures >30 MPa were measured for dry densities of ~2.0 g/cm3. Added iron or magnetite powder up to 20 wt% had no influence on the swelling capacity. Compacted mixtures of 20 wt% ground set cement and bentonite showed higher swelling pressures but lower swelling strain capability than compacted bentonite alone. Steam lowered the swelling pressure of compacted bentonite to ~60% of the original value. The influence was, however, reversible by ultrasonic treatment. The thermal conductivity of saturated compacted bentonite at a density of 2.0-2.1 g/cm3 is ~1.35-1.45 W/m°K The volumetric heat capacity ranges from 3.1 x 106 to 3.4 x 106 j/m3°C The saturated hydraulic conductivity of the compacted bentonite is <10-12 m/s. The apparent diffusion coefficients for various ions in compacted bentonite for water contents in the range of 20 to 25 wt% are: K+: 5 x 10-11, Cs+: 6 x 10-12, Sr2+: 3 x 10-11, UO22+: <10-13, Th4+: <10-13, Fe2+: 4 x 10-11, Fe3+: 4 x 10-11, Cl-: 1 x 10-10 and I- : 1 x 10-10 m2/s. The 'breakthrough time' for an apparent diffusion coefficient of 10-11 m2/s in compacted bentonite 1 m thick was estimated to be ~3000 years. The mineralogical longevity was investigated on natural K-bentonites from Kinnekulle, Sweden, and Montana, USA. Although these materials have undergone considerable changes during diagenesis and contain various amounts of mixed-layer illite-smectite, they still have a substantial swelling and adsorption capacity. The investigations demonstrate that although the properties of bentonite are negatively influenced to a certain extent by heat, hot steam, iron and cement, compacted bentonite is still the best choice to act as a buffer material in a nuclear waste repository.

The clay mineral associations in the Cretaceous-Tertiary Boundary (KTB) and in the Danian and Maastrichtian levels of sections from the Mediterranean and the Atlantic Domains have been studied. The Mediterranean sections have a single mineral association consisting of smectiteiltite and kaolinite, whereas the Atlantic sections have several associations: illite-chlorite, illite-R1 I-S-kaolinite and illite-R 1 I-S-chlorite. Data are presented relating to the influence of K-feldspars and Fe oxide sphemles on the clay mineral associations. Study of rare-earth elements shows that regional geological factors affect the clay mineralogy of the KTB, examples showing significant anthigenesis in the Mediterranean sections, and important detrital supply in all the Atlantic sections. We propose that the KTB studied in these marine sections is equivalent to the uppermost layer of the two-layered clay unit originating in a cloud of a vapourized bolide. Regional tectonic conditions have been responsible for differences in clay sedimentation in these geological domains and among the stratigraphic sections of the Atlantic Domain.

Intercalation of the tris(2,2'-bipyridine)ruthenium(II) complex cation into a layered silicate (magadiite; the ideal formula for which is Na2Si14O29.nH2O) was investigated. Since the complex cation did not intercalate by a direct ion exchange reaction with the interlayer Na ions in an aqueous medium, dodecyltrimethylammonium exchanged magadiite was used as a precursor. Depending on the loading of the complex cations and the solvents employed for the reactions, two types of tris(2,2'-bipyridine)mthenium(II)-magadiite intercalation compounds formed. One has a basal spacing of ~2.9 nm due to a large amount of the dodecyltrimethylammonium ions which remained in the interlayer space of magadiite and coexisted with the intercalated tris(2,2'- bipyridine)ruthenium(II) complex cations. The other has a basal spacing of '2.0 nm because the intercalated tris(2,2'-bipyridine)ruthenium(II) complex ions form a monolayer in the interlayer space of magadiite.

The SEM, XRD, FFIR and DTA analyses of different size-fractions of clay material from sandstone reservoirs which have experienced a large range of burial conditions have been used to examine the different steps of the depth-related kaolinite-dickite reaction. Dickite progressively replaced kaolinite within a range of burial depths estimated between about 2500 m and 5000 m. The kaolinite-to-dickite reaction proceeds by gradual structural changes concomitant to crystal coarsening and change from booklet to blocky morphology. The crystallization of dickite proceeds by two distinct paths: (1) Accretion of new material from either dissolution of smaller unstable kaolinite crystals and/or detrital minerals (chiefly feldspars), on early-formed coarser metastable kaolinite crystals which exert extended morphological control on the growing crystals. (2) Neoformation of ordered dickite which will continue to grow by a dissolution-crystallization process. The kaolinite-todickite reaction is kinetically controlled and anomalies in the kaolinite/dickite ratio observed in certain sandstone reservoirs may be used to assess the timing of invasion by hydrocarbons.

The swelling behaviour and hydraulic conductivity of Na-bentonite powder and bentonite-sand mixtures (10 and 20% of bentonite by dry weight) have been measured with distilled water and various salt solutions (0.01, 0.1 and 1 mol/l concentrations), It was found that in dilute solutions, the bentonite in mixtures subjected to small confining stresses swells sufficiently to separate the sand particles and reach a clay void ratio similar to that achieved by bentonite alone. At high stresses, or in strong solutions, the bentonite in a mixture has insufficient swelling capacity to force the sand particles apart and swelling is limited by the sand pore volume. The hydraulic conductivity of a mixture depends on the bentonite void ratio, and the porosity and tortuosity of the sand matrix. A design model is proposed to predict the engineering properties of a mixture over a range of confining stresses from the properties of its constituents and the permeant.

The claystones and mudstones investigated are London Clay, Fullers Earth, Oxford Clay and Kimmeridge Clay. The thermal conductivities were measured using a divided bar apparatus and the values measured perpendicular to layering ranged from 0.68 to 0.97 W/mK. Comparative measurements of thermal conductivities were carried out by the needle probe method and Middleton's method. Deviations of up to 50% were obtained between the needle probe and the divided bar method. The thermal conductivities estimated from the geometric mean model based on mineralogy and water content ranged from 0.87 to 2.01 W/mK, considerably higher than the measured values. A correlation was found between the grain size distributions of the samples and the measured thermal conductivities. This textural effect on the thermal conductivity is assumed to be the main reason for the low measured values and the lack of correlation between the measured and the calculated values.

A mineralogical and microstructural study of bentonites from 'Cerro del Aguila' located in the Tagus Basin (Toledo, Spain) was carried out using XRD, SEM and TEM observations and chemical data obtained by TEM-EDX. The bentonites are mainly composed of trioctahedral smectite with a unit-cell formula (Si3.76Al0.24)(Al0.44Fe3+0.26Mg1.81)Ca0.05K0.19O10(OH)2 and small amounts of illite. The relationships between illite and smectite particles observed by SEM and TEM allowed the study of the weathering process of illite to form smectite through possible intermediate stages. The transformation begins as an exfoliation normal to the stacking direction and develops by opening of the interlayer spacing, the replacement of K+ by hydrated interlayer cations and slight reorganization of the 2:1 layer structure. The so-called solid-state transformation process then proceeds by further dissolution and the formation of a colloidal phase acting as raw material in the growth of new phyllosilicates such as smectite.

The nature and mineralogy of mixed Al and Fe(III) gels (initial Fe/Al molar ratios (Ri) of 1.0 and 2.5) formed at pH values ranging from 4.0 to 10.0, both in the presence and absence of citric acid (citrate/Fe + AI molar ratio (Rcit) of 0.01 and 0.1) and aged for a long period at room temperature and at 50°C have been studied. The complexes showed considerable differences in the mineralogy of the precipitation products. The samples formed at Ri = 1.0 contained ferrihydrite at pH 4.0, ferrihydrite + gibbsite at pH 5.0-7.0, and hematite + Al(OH)3 polymorphs + ferrihydrite at pH 9.0-10.0. The samples formed at Ri = 2.5 had greater quantities of poorly crystallized ferrihydrite. Large amounts of Fe+Al (25-82%) were solubilized from the samples aged for 60 days at 50°C by ammonium oxalate. The addition of increasing concentrations of citrate to the gel suspensions containing equimolar amounts of Fe and AI strongly inhibited the formation of Al(OH)3 polymorphs both at pH 5.0 or 8.5 by promoting the formation of short-range ordered materials. Citrate added initially to Fe-Al solutions (R*cit = 0.1) completely inhibited formation of crystals even after 135 days at 50°C.

Sandstones and shales from the southern flank of the Bilbao anticline in the Gordexola and Orozko valleys, northwestern Spain, have been subjected to a detailed sedimentological, mineralogical and geochemical study. They are composed of proximal frontdeltaic sandbars interbedded with silty-clayey sediments and correspond to a prograding sequence of a deltaic system. The clay mineral assemblages of doublets of sandstones and shales are composed mainly of illite, chlorite and illite-smectite mixed-layers R3 (ISII). The clay diagenesis consists of the transformation of smectite and I-S mixed-layers to illite, the precipitation of Fe-rich chlorite in the pore spaces, and the alteration of micas to chlorite. According to petrographic, mineralogical and geochemical analyses, chlorite is more abundant in sandstones than in shales in both the <2 µm and coarser fractions. The relative abundance of chlorite increases in the sandstones located at the top of coarsening upward sandbars. Furthermore, chlorite formation occurs preferentially in the coarser grained sansdstones previously cemented by ankerite. The geochemical and petrological investigations suggest that chlorite formed during burial diagenesis in a relatively closed system.

Experimental illitization of smectite has been simulated by means of hydrous pyrolysis, using a smectite-rich starting material from a Lower Tertiary claystone from the Oseberg field, North Sea. The <2 µm fraction was subjected to hydrous pyrolysis using KCL solutions at concentrations of 1.0 and 0.01 N, temperatures from 180-350°C and reaction times from 24-72 h. The conversion of smectite into illite has been identified by XRD analysis of the pyrolysed products. It was clearly demonslrated that the K+ concentration ([K+]) and temperature are the major factors controlling the rate and extent of illitization. The distributions of activation energies around 33 kcal/mol and frequency factors in the range of 10+8 to 10+9 S-1 have been calculated by applying a parallel reaction model. However, geological modelling and comparison with buried smectite clays indicates that both 1.0 n and 0.01 N KCL are too high in K+ content compared to the pore-water. The results suggest that dilute KCL solution close to pore-water should be used in hydrous pyrolysis to obtain proper kinetic models.

Mineralogical trends have been investigated on a detailed scale in two Westphalian fluvial sandstone sequences in contact with either a coal seam or shales. The evolution in an authigenic clay mineral assemblage can be related to changes in the pH of the pore-water. Firstly, kaolinite formed early in diagenesis, mainly as a result of K-feldspar dissolution and alteration. This process, which took place under acidic conditions, consumed protons. Subsequently the pH of the pore-water increased and after compaction, illitization of kaolinite occurred under near neutral conditions. Deep burial is marked by dickite formation, which again reflects acidic conditions. Distribution of clay minerals is related to meteoric water flux through the sediments and the release of organic acids and CO2 from coalification of organic matter in the underlying strata.

The Danian of the Provence basin (SE France) is characterized by a strong subatmospheric alteration processes throughout the basin. A wide range of facies, depositional and weathering, were recognized in an interval of 5 Myr and include floodplain fine-grained alluvium, palustrine limestones, playa dolostones, mottled palaeosols, vadose and phreatic calcretes, and phreatic dolocretes. Palygorskite was invariably found in laminar, massive and honeycomb vadose calcretes and phreatic dolocretes, as well as in playa dolostones, and only exceptionally in floodplain siltstones, nodular calcretes and palustrine limestones. Regardless of the facies, palygorskite is associated with smectites. An authigenic origin, based on the morphology of the fibres and the vertical distribution of clay minerals, is proposed for palygorskites in all of the facies. The simultaneous occurrence of palygorskite in these environments is interpreted as a sign of a stable seasonal semi-arid climate and low detrital input during a period of low lake level (-0.5 Myr). The occurrence of authigenic palygorskite in siltstones around the lake was attributed to the rising of the water table, retention of the already saturated water in the pore spaces of the sediment, and its subsequent evaporation in a confined environment. A hydrogeological model is proposed for the distribution of palygorskite. The elongated shape of the Provence basin enhanced the influence of lateral inflow of freshwater during the low lake period which explains the distribution of palygorskite in different environments of the Provence basin as well as spatial distribution of phreatic calcretes and dolocretes. During the high lake level, palygorskite formed in the few remaining ephemeral ponds and on the floodplain along the lake margin.

Talc [ideal formula: Mg3Si4O10(OH)2] is one of the classical 2:1 trioctahedral phyllosilicates that has been analysed intensively with respect to its formation, thermal stability, chemical properties and structure (e.g. Wilkins & Ito, 1967; Rayner &Brown, 1973; Ward, 1975; Martin et al., 1996). Single crystal structure refinement by X-ray diffraction (XRD) showed clearly that talc is triclinic and crystallizes in the space group CI (e.g. Rayner & Brown, 1973; Perdikatsis & Burzlaff, 1981). Based on this structural model, two coordination arrangements of OH groups to Mg in the octahedral sheets can be deduced. Since XRD data give only an insight into the average structure, local information on the ligand field of the cations is limited. Currently, no analytical tool is known to determine directly the ligand field of Mg in the octahedral sites.

In the North Sea Tertiary section, wellbore instability problems are frequently reported in Palaeocene-Early Oligocene smectite-rich mudrocks. Analysis of the mechanical properties of these Tertiary mudrocks is generally hampered by the lack of suitable core material. This study represents an attempt to study the geomechanical behaviour of mudrocks by triaxial tests of side-wall cores obtained from the borehole wall. The tests performed include measuring the changes in pore pressure during shearing and undrained shear strength in specimens initially consolidated to in situ effective stress levels. The coefficients of permeability (kf), estimated from the consolidation time behaviour range from 2.6 x 10-11 to 2.4 x 10-12 m/s. The tested cores behaved like slightly overconsolidated to normally consolidated materials with an initial near constant volume (elastic behaviour) for low deviatoric load followed by an increasingly contractant behaviour approaching failure. Compared with results from onshore analogues, the strength properties of the investigated mudrocks appear to be related to their content of expandable clay minerals. A wellbore stability chart to forecast adequate drilling fluid pressures for future wells has been developed by the use of linear (Mohr-Coulomb) failure criteria based on the peak strength data. It is demonstrated that side-walt cores can provide satisfactory test materials for rock mechanical analysis, and their use may serve to improve our knowledge of the rock mechanical behaviour of typically troublesome mudrocks for which no conventional cores are available.

Nearly pure sepiolite clay crops out in a playa-like depression near El Bur, Central Plateau region of Somalia. The deposit is associated with the Lower to Mid-Eocene Taleh Formation that includes, besides limestone, dolomite and gypsiferous marls, extensive anhydrite and various evaporites, primarily gypsum. The material was examined by XRD, DTA, IR and EM. The XRD and DTA analyses indicated that from 40 cm down to a depth of 300 cm, the material consists of well crystallized sepiolite, accompanied in some layers by minor calcite and traces of quartz and halite. The chemical composition, determined by XRF, indicated a low-Fe mineral, with the formula: (Si11.888Al0.l12)(Mg7.313Al0.154Fe0.084)O30(OH2)4(OH2)4.x8H2O.

The fibres, arranged in the form of interwoven mats, are straight and have lengths varying between 2-6 µm and widths of 20-40 nm. Commonly, they are aggregated into units of two parallel-lying fibres, with a random orientation against each other, creating a dense network of pores. The high viscosity and external surface area (306-346 m2g-1) of the material, compared to those of the Spanish Vallecas sepiolite, suggest the high industrial suitability of this clay. The extent of the deposit is not known. Lithology and geomorphology indicate a lacustrine, closed basin evaporative environment of formation for this deposit. In contrast to the palaeolacustrine environments of formation of Spanish and Turkish sepiolite deposits, the E1 Bur sepiolite apparently is more recent.