The thermal stability of bentonites is of particular interest for containment barriers in nuclear waste storage facilities. The kinetics of smectite reactions have been investigated under laboratory conditions for some time. The variables of time, chemical composition and temperature have been varied in these experiments. The results of such an assessment are that there are about as many kinetic values deduced from experiments as there are experiments.

Experiments using natural bentonite to study the smectite-to-illite conversion have been interpreted as a progressive transformation of montmorillonite to illite. It is highly probable that the initial reaction product is not illite but a high-charge beidellite + saponite + quartz mineral assemblage which gives, then, beidellite-mica interstratified mixed-layer minerals. These experimental reactions are noticeably different from those of diagenesis, being closer to reactions in hydrothermal systems.

The smectite to illite transformation in early Miocene sediments was studied in wells and outcrops in the western Pannonian Basin, where magmatic activity caused very high Miocene heat flows (250-400 mW/m2). Although the thermal history is similar, large differences in smectite to illite diagenesis were observed. (1) The boundaries between early/middle and middle/late diagenesis in two wells (Pichla, Radkersburg) and the Maribor area correspond to vitrinite reflectance values of 0.4-0.8 and 1.1-1.5% Rr. Anchimetamorphism starts at ~2.1% Rr. In spite of magmatic heating, the smectite to illite transformation can be modelled using kinetic data proposed for areas with a 'normal' burial diagenesis. (2) Smectite to illite reactions are advanced compared to vitrinite reflectance in the Ribnica-Selnica Trough. This is probably related to fluids with elevated K+ concentrations. (3) Clay mineral alterations lag behind coalification significantly in the Mitterlabilt well. These different correlations indicate that clay mineral thermometry should be used with caution and that factors other than temperature and time can influence clay mineral reactions.

Cambrian and Ordovician pelites and greywakes from the Iberian Range were studied using XRD, EMPA and AEM/TEM techniques, lllite and chlorite are the main phases in Cambrian <2 µm fractions. Illites show crystallinities (IC) from 0.28 to 0.23°2θ the 2M1 is almost the only polytype; they do not have expandable layers, and they are heterogeneous in composition. Chlorites show two compositional types, both containing variable proportions of smectite-like layers. Clinochtores growing over bedding surfaces are also present. An evolutionary trend from anchizone to near the anchizone-epizone limit has been proposed for Cambrian samples.

In the Ordovician <2 µm fractions, illites are associated with minor chlorites and kaolinites. The ICs range from 0.68-0.28°2θ the 2M1 polytype dominates over 1M, and it is associated with illitesmectite mixed-layering (R3 ordering). An evolutionary trend from diagenesis to low grade anchizone has been inferred.

Chemical analyses of coarser detrital micas indicate muscovite and phengite compositions. The compositional heterogeneity exhibited by authigenic and detrital phases suggests that chemical equilibrium was not attained.

Bottom sediments from 78 sites in the Skagerrak-Kattegat area have been used to determine the semi-quantitative distribution of clay minerals, and to interpret sediment provenance and the transport pathways. The sediment reflects incorporation of four sources, affected both by earlier glacial processes and on-going marine mixing. The southern North Sea, characterized by dioctaheadral illite, kaolinite and smectite, is the dominating source. The central North Sea provides a limited, but significant, source for chlorite. The sediment from the Swedish west coast has an unweathered character, containing trioctaheadral illite, vermiculite, illite-vermiculite mixed-layer minerals and chlorite. The contribution from the southern Kattegat (southern Sweden and Denmark) is dominated by kaolinite and smectite. Correlation between clay content and the contents of smectite, kaolinite and chlorite in the Skagerrak indicate that the clay mineral distribution is influenced by sorting. This grain-size dependency must be considered when interpreting sources.

Fixed nitrogen in illite-smectites (I-S) has been measured for Miocene shales from a Gulf of Mexico oil well. Fixed N values for the <0.2 µm fraction increase with depth from 150 ppm (1000 m) to a maximum of 360 ppm (3841 m). This increase is coincident with illitization from 41% I in I-S to 75% I in I-S. Below 3841 m, fixed N values decrease to 190 ppm (4116 m) while I-S is maintained with a slight increase from 77 to 82%. The changes in fixed N with increasing illitization are consistent with the notion that illitization proceeds via both transformation and dissolution/ precipitation reaction mechanisms. The trend of decreasing fixed N in illitic I-S is compatible with surface-controlled crystal growth and Ostwald ripening mechanisms for illitization. The trend may also be linked to the timing of maximum NH] release from kerogen maturation during oil generation. The changing rate of NH+4 liberation from organic matter and multiple illitization reaction mechanisms can result in complex N geochemical cycling pathways throughout early diagenesis to metamorphism.

Molecular simulations using the Cerius2 modelling environment have been used to investigate the structure of montmorillonite (MMT) intercalated with Zn cations. Basal spacing and bonding of Zn2+ cations in the interlayer have been investigated with regard to their dependence on the water content. In the first part of the work, Zn cations in the interlayer are coordined by six water molecules and the energy of the system is discussed. The energy discussion is focused on the influence of the different starting orientations of the Zn octahedron, and different positions of Mg atoms in the octahedral sheet. An energy scan at fixed d-spacing was carried out. Later the hydration simulation giving the hydration curve (i.e. the dependence of d-spacing on the number of water molecules in the interlayer) was also carried out. Two different hydration states were found — for certain humidity ranges there is almost constant d-spacing and between these intervals there is a sharp edge. Finally the structure of totally dehydrated Zn-MMT was simulated.

Dissolution of feldspar and mica and precipitation of kaolinite require a through flow of meteoric water to remove cations such as Na+ and K+ and silica. Compaction driven pore-water flow is in most cases too slow to be significant in terms of transport of solids. The very low solubility of A1 suggests that precipitation of new authigenic clay minerals requires unstable Al-bearing precursor minerals. Chlorite may form diagenetically from smectite and from kaolinite when a source of Fe and Mg is present. In the North Sea Basin, the main phase of illite precipitation reducing the quality of Jurassic reservoirs occurs at depths close to 4 km (130-140°C) but the amount of illite depends on the presence of both kaolinite and K-feldspar. Clay mineral reactions in shales and sandstones are very important factors determining mechanical and chemical compaction and are thus critical for realistic basin modelling.

Kaolinite-ferrihydrite and montmorillonite-ferrihydrite associations were prepared following a procedure based on the Russell method for the synthesis of ferrihydrite and the texture of the clay-ferrihydrite complexes was studied using different techniques. The textural properties of kaolinite were little affected by the Fe association, showing only a slight increase in the specific surface area measured by nitrogen adsorption and a decrease in the largest pores (>10 µm), as measured by mercury porosimetry. In contrast, the nitrogen specific surface area of the montmorillonite complexes was much higher than that of the clay without Fe and the pore structure depended on the amounts of Fe in the complexes. Application of the fractal approach to nitrogen adsorption data indicated that the surface roughness (microporosity) was greater for the complexes prepared from diluted Fe(III) solutions, in agreement with the information obtained from classical interpretation of the adsorption isotherms (shape of the isotherms and t-plots).

Goethite and maghemite are the stable species of Fe oxyhydroxides-oxides formed an acidic and alkaline environments from the oxidation of Fe(II) perchlorate solutions. The influence of montmorillonite on the oxidation products of 0.02 M ferrous perchlorate at pHs of 6.0 and 8.0 was studied by X-ray diffraction, infrared and transmission electron microscopic analyses. Increased rate of OH consumption during the oxidation at constant pH indicated that the presence of montmorillonite accelerated the rate of Fe(II) oxidation. The presence of montmorillonite, with high surface reactivity, at an initial montmorillonite/Fe (w/w) ratios of 1.4 and 3.4 retarded the formation of goethite, lepidocrocite and maghemite, and maghemite and goethite, and promoted the formation of ferrihydrite and lepidocrocite at pHs of 6.0 and 8.0, respectively. Kaolinite, on the other hand, with relatively low surface reactivity had no influence on the nature of the Fe(II) oxidation products formed at either pH.

Glauconite and Ca phosphate peloids occur in Jurassic and Cretaceous bioclastic carbonate rocks from pelagic swell sequences of the Algayat-Crevillente Unit (Subbetic Zone). The size and morphology of the peloids are controlled by the bioclasts. The glauconite in both stratigraphic positions is K rich (>0.69 atoms p.f.u.) and shows well-defined 10 Å. lattice fringes. Poorly crystalline areas with a composition of Fe-smectite are found within the peloids, indicating the nature of the glauconitic precursor. This precursor would be formed in the shielded microenvironments of the bioclast and later transformed to glauconite by equilibration of peloids with sea water that culminated with the crystallization of a phosphatic phase. The greater presence of smectite areas in the Jurassic peloids and the lower K contents (0.69-0.81) of these glauconites, compared with the Cretaceous glauconites (0.81-0.89) can be explained by the calcitic early diagenetic cementation which stopped the process of glauconitization.

In the course of hydrothermal experiments with a saponitic clay, evidence for the dissolution of the accessory sepiolite and the formation of smectite has been detected above 120°C Hydrothermal reactions with a clay to water ratio of 1:3 were performed at temperatures of 60, 90, 120, 175 and 200°C with time intervals of one month to one year.

The BET surface area and cation exchange capacity (CEC) are correlated with the sepiolite and the smectite content determined from XRD data. These relations have been used to recalculate the time dependence of the mineral contents in the time/temperature conditions of the experiments. The Ea values obtained for sepiolite dissolution (7-18 kcal/mol) or smectite formation (4.8-5 kcal/mol) indicate that sepiolite dissolution controls the rate of the process. Both results fit an apparent firstorder reaction and the system seems to evolve to a stable mineral composition in a short time period, ranging from one to 10 years as temperature decreases.

Arid soils in central Iran that evolved from the weathering of post-Tethyan sediments contain palygorskite. Clay fractions of gypsiferous soils and their associated sediments from different landforms from central Iran were investigated. Palygorskite was the dominant silicate clay mineral in clay fractions of the soils, and of Oligo-Miocene limestone, a less common parent rock. The Jurassic shale and Cretaceous limestone contain illite and chlorite with a trace amount of palygorskite. Association of large amounts of palygorskite bundles with gypsum in the gypsiferous soils studied, supports the hypothesis that palygorskite was probably formed after the initial precipitation of gypsum, that created a high pH and Mg/Ca ratio. The major portion of the palygorskite present in colluvial and plateau soils was probably formed authigenically when central Iran was covered by post Tethyan shallow hyper-saline lagoons. Palygorskite in alluvial soils appeared to be essentially detrital.

Cationic binuclear oxo- and carboxylate bridged Fe(III) complexes of N-alkyl-N,Nbis(2-pyridylmethyl)amines are introduced into the interlayer region of smectites by cation exchange reactions. The reaction can be divided into three steps. After a predominant loading of external surfaces, cation exchange sites at internal surfaces react, accompanied by considerable stmctaral expansion. Both reaction stages belong to the steep section of the exchange isotherms. At a certain loading, which depends on the complex substituents and the layer charge of the smectite, the isotherms become flat. With bulky shaped complex cations and highly charged smectites, exchange is not quantitative; vermiculite-type minerals show a very small complex uptake. For this behaviour, both thermodynamic and kinetic reasons must be considered. From these results the number of cation exchange positions at external surfaces of smectite crystals may be estimated.

A high-iron content saponite, of the variety griffithite, was reduced using sodium dithionite and hydrazonium sulphate solutions. An important amount of Fe3+ was reduced during the treatments. When using sodium dithionite as reducing agent, the reduction was accompanied by the substitution of Ca2+ by Na+ as the exchangeable cation. When using hydrazonium sulphate, the reduction was accompanied by acid activation of the clay, the protons being released in the oxidation reaction of the hydrazonium cation. The charge balance in the clay layers is affected by the reduction processes. These structural changes do not significantly affect the pillaring ability of the clay, which is similar in the reduced solids and in the natural griffithite.

From mineralogical and chemical characterization of two waste-rock piles from Vigonzano and Libiola sulphide mines in the northern Apennines, two types of fine-grained weathering products were recognized, One, from the Vigonzano mine, has a typical blue colour and consists of the Cu-Al sulphate of the woodwardite group. Its formation is related to an alkaline geochemical barrier, it is rather stable in acidic solution, and it controls Cu and Zn dispersion in the surrounding environment. The other group of sediments has a typical ochreous colour. Vigonzano samples consist mostly of goethite, whereas ferrihydrite is abundant in the Libiola sample. The ochreous sediments are enriched in Co, As, Mo, Se and Mn, thus significantly reducing the potential environmental hazard of the site, whereas solubility tests indicated possible problems due to AI dissolution at Libiola.

A new method of measuring the layer charge of smectites by IR spectroscopy is proposed based on the integrated intensity measurement of the band assigned to the ammonium fundamental ν4 mode of NH+4-exchanged clays. Measurements are made before and after Li fixation (Hofman & Klemen effect). These allow the quantitative determination of the total charge, equivalent to CEC, and tetrahedral and octahedral charge distribution in both dioctahedral and trioctahedral smectites.

The evaluation of clays as barriers to fluid movement can be improved by geochemical methods that provide ways to examine the reactivity and weathering of minerals in soils and sediments. X-ray radiography, X-ray powder diffraction, and stable isotope geochemistry provide new data from field locations in the Mississippi River Delta of Louisiana indicating that the clays are not effective barriers to the vertical migration of fluids in the shallow subsurface. Systematic changes in the mineral assemblages, the soil structure and the δD and δ18O values of time clay fractions can best be explained by an alteration sequence produced as the originally smectiterich clay mineral assemblage was kaolinized by percolating groundwater.

Low molecular weight polycyclic aromatic hydrocarbons can intercalate from the solid phase into montmorillonite (Mt) saturated with quaternary alkylammonium ions. However, the interaction and relationship between guest and host organic molecules in the interlayer space of the clay are not well understood. We have intercalated phenanthrene into tetradecyltrimethylammonium (TDTMA)-montmorillonite by a solid-solid reaction. The basal spacing of the original TDTMA-Mt complex is close to 1.8 nm, indicating the presence in the interlayer space of a double layer of TDTMA ions with the alkyl (polymethylene) chains lying parallel to the silicate layers, and the carbon zig-zags adopting an all-trans conformation. After intercalation of phenanthrene the basal spacing increases to about 3.4 nm, indicating a change in orientation of the alkyl chains with respect to the silicate layers. 13C-NMR spectroscopy shows that adding phenanthrene to TDTMA-Mt leads to a displacement by -3 ppm of the -(CH2)n- signal for TDTMA. This signal and that for interlayer phenantbrene are also broadened relative to the respective pure compounds. These observations, together with measurements of nuclear spin relaxation time constants, strongly suggest that in the complex with phenanthrene the polymethylene chains of TDTMA extend away from the silicate layers, and no longer assume a rigid all-trans carbon zig-zag conformation. Rather, the TDTMA chains become relatively disordered and intimately mixed with phenanthrene.