An understanding of the behaviour of contaminants in compacted clays is important in assessing the effectiveness of clay-based barrier materials used in many waste containment strategies. Here the diffusion and sorption behaviour of I− with selected compacted clays is examined (129I is a relatively long-lived radioisotope present in high-level nuclear fuel waste.) Diffusion coefficients, D, and distribution coefficients, Kd, were measured for I− with four clays: bentonite, Lake Agassiz clay (a glacial lake clay composed mainly of smectite, illite, kaolinite, and quartz), interstratified illite-smectite, and kaolinite. For the diffusion experiments the clays were compacted to a dry bulk density, ρ, of ∼1.2 Mg m−3. The mean measured D values, Dm, were as follows: bentonite, 310 µm2 s−1; Lake Agassiz clay, 0.62 µm2 s−1; illitesmectite, 190µm2s−1; and kaolinite, 74µm2s−1. The measured values were compared with those calculated, Dc, from the following model: D=Doτan/(n + ρKd), where Do is the diffusion coefficient in pure bulk water, τa the apparent tortuosity factor, and n the solution-filled porosity of the clay. Except for the Lake Agassiz clay, where Dm was about an order of magnitude lower than Dc, the Dm and Dc values agreed within a factor of about two. The Lake Agassiz clay has a markedly higher organic carbon content than the other three clays, and this could affect τa, which may be overestimated in the model.

It has been suggested that interlayer water in halloysite is due to the presence of hydrated cations that balance the negative layer charge produced by Al for Si substitution. To find evidence of 4-coordinate Al (Al(IV)), we investigated six halloysites and two kaolinites using ‘high-field’ and ‘medium-field’ solid-state 27Al MAS NMR spectroscopy. We found Al(IV) in both kaolinite and five halloysite samples, but the contents are all <1% and provide no basis for distinguishing between kaolinite and halloysite. Therefore, the presence of interlayer water in halloysite cannot be attributed to Al for Si substitution. There are, however, signals, tentatively assigned to A1(V), present in the kaolinite spectra but not in the halloysite spectra. The shapes of the low-frequency ‘tails’ of Al(VI) signals in medium-field NMR vary from sample to sample. We interpret this variation in terms of a ‘crystallinity index’. Disorder in kaolinite appears to be primarily the result of Al-vacancy displacements in the octahedral sheet. The NMR crystallinity indices correlate with those from IRS and DTA but not with those from XRD.

Eight illites were studied by Mössbauer spectroscopy at room temperature and 4.2 K. On the basis of their Fe contents, these illites can be divided into an Fe-rich (>5 wt% Fe) and an Fe-poor (<1 to ∼3 wt% Fe) group. Mössbauer spectroscopy showed that the Fe-rich illites had lower proportions of divalent Fe than their Fe-poor counterparts. Slow paramagnetic relaxation was observed in the Fe-poor illites and must be accounted for when fitting the spectra. Two samples contained iron oxides that were superparamagnetic and thus, although contributing to the Fe3+ doublets, escaped detection at room temperature. Structural Fe3+ had a temperature-independent quadrupole splitting that was lower for the Fe-rich illites (0.59 mm/s) than that of the Fe-poor illites (0.73 mm/s). The Fe2+ quadrupole splittings were higher at 4.2 K than at room temperature, but the Fe-rich illites again had lower Fe2+ quadrupole splittings both at room temperature (2.69 vs. 2.88 mm/s) and at 4.2 K (2.96 mm/s vs. 3.08 mm/s). Distinction of Fe sites in the illites with cis- and trans-OH coordination was not possible. The presence of tetrahedral Fe3+ was observed only in the most Fe-rich sample (8.4 wt% Fe).

By means of a gold decoration transmission electron microscopy (TEM) technique, interlaced spiral steps with a 10 A step height were observed on (001) surfaces of lath-shaped or elongated hexagonal rectorite crystals, i.e. interstratihed mica-smectite minerals, from the Goto pyrophyllite deposit in Nagasaki Prefecture, Japan. This is the first direct evidence to demonstrate that the growth of interstratified mica-smectite takes place by the spiral growth mechanism. The interlacing pattern is interpreted as appearing by co-operation of two hexagonal spiral layers of mica and smectite-like layers which have originated from a single dislocation and are stacked by rotation of 60°.

A 29Si MAS-NMR one-pulse and ﹛1H﹜ CP comparative study of sepiolite, folded sepiolite, aluminated sepiolite, and palygorskite was conducted. The three main resonances of sepiolite at -92.0 ± 0.1, —94.3 + 0.1, and — 97.8 ± 0.1 p.p.m. were assigned with respect to the position of the Si atoms within the three pairs of symmetrical sites in the basal plane. The line at —97.8 p.p.m. was attributed to the central sites by comparison with the resonances and chemical environments of Si in talc and palygorskite. The line at —94.3 p.p.m. was attributed to the edge sites, because it is strongly reinforced by cross-polarization. Alumination of sepiolite led to a broadening of the peaks and new downfield resonances in line with what can be observed in other phyllosilicates and zeolites.

The pillaring process of montmorillonite and beidellite with Al and Ga polymers has been studied using XRD, IR, 27Al, 71Ga, and 29Si MAS NMR, TGA, TEM, N2 adsorption and chemical analyses. The Al adsorption maximum for montmorillonite is close to 5.5 mEq Al/g clay, whereas the maximum for Ga is higher. Basal spacings of both Ga- and Al-pillared clays vary between 16.7 and 18.8 Å. Freeze-drying of pillared products followed by calcination yielded more regular pillared structures. Pillaring montmorillonite increased the BET surface area from 35 m2/g to 350 m2/g mainly by the creation of micropores <20 Å in diameter. The Al-pillared clays are thermally stable to ∼700°C. Calcination of pillared montmorillonite liberates protons from the pillar, which diffuse into the clay sheet, lowering the thermal stability. In pillared beidellite, mainly silanol groups are formed by breaking Si-O-Al bonds. No reaction is observed between pillars and montmorillonite upon calcination, whereas in pillared beidellite a structural transformation links the pillar to inverted tetrahedra of the tetrahedral sheet. The basal spacing of Ga-pillared montmorillonite collapses to 9.5 Å at 350°C due to the Ga polymer decomposing to Ga3+ cations.

The JP (Jelšový Potok, Slovakia) and SWy-1 (Wyoming, USA) montmorillonites were treated in 6 m HCl at 95°C. The rates of dissolution of tetrahedral and octahedral Al (Altet and Aloct) are comparable. Aluminium in the tetrahedral sheets was also identified in both untreated samples by 29Si MAS NMR spectroscopy. About 12% of total Si in SWy-1 was found to be bound in quartz. Traces of undecomposed montmorillonite and a small amount of Altet in the three-dimensional SiO4 framework were found in the reaction product of montmorillonite decomposition. Three different types of structural units were identified in acid-treated samples: Q4(OAl) units of amorphous silica with three-dimensional crosslinked framework; (SiO)3SiOH units, remaining as a result of poor-ordering of the framework without the possibility of cross-linking; and Q4(1A1) units.

Six different X-ray diffraction (XRD) methods for determining the relative amounts of 2M1 and 1M white K-mica polytypes were compared. Unoriented samples of different concentrations of the two polytypes were prepared using the technique described by Handschin & Stern (19X9) and Stern (1991). An evaluation of different XRD techniques reported in the literature revealed that the method proposed by Caillére et al. (1982) is the most accurate. The main reasons are the use of the 1M 112 and 2M1 025 reflections with relatively strong intensities, showing no superposition with feldspar reflections, and being only slightly affected by sample preparation effects.

The interplay of overpressure and diagenesis is an important factor in developing reservoir quality and controlling the movement of pore-fluids and hydrocarbons within sedimentary basins. The first part of this introduction deals with petrographic features by which certain highly overpressured reservoirs can be recognised. The second part considers the 20 papers, presented at the 1993 Cambridge Conference, published in this special issue of Clay Minerals and how they fit into the broader context of the subject.

The hydroxyl stretching region of two montmorillonites (SWy-1, Crook County, Wyoming, USA, and JP, Jelšový Potok, Slovakia) and a ferruginous smectite (SWa-1, Grant County, Washington, USA) have been analysed using decomposition and curve-fitting. The positions and integrated intensities of component bands were correlated with the nearest cationic environment of the OH groups, and the populations of the octahedral sheet were calculated. The differences between the octahedral coefficients obtained and those from chemical analyses were 0.07 or less. This method provides reasonable contents of octahedral cations for the smectites with various iron contents, containing no other admixtures absorbing in the 3700-3500 cm–1 region. The cation distribution was found to be not completely disordered. Though some preferential pairing has been observed, no general rule could be established.

The nature and characteristics of 1:1 phyllosilicates (crystallinity, order and proportion of kaolinite, disordered kaolinite and 7 Å-halloysite) in deep palaeoweathering profiles developed on granitic rocks in Central Spain have been studied. The profile displays a lower part of non-silicified saprolite and an upper part of silicified saprolite, with or without a sedimentary silicified cover. The <2 µm fraction was studied using XRD, TG-DTA and IR techniques. Towards the top of the profile there is a progressive decrease in both 1:1 phyllosilicates and kaolinite contents and an increase in opal. In the upper part (silicified saprolite) two weathering fades can be distinguished: the 'white fades', in which the crystallinity of kaolinite is greater than in the 'red facies. The kaolinite in the upper parts of the profile is probably a remnant of that existing in the non-silicified saprolite, whereas disordered kaolinite and 7Å-halloysite are, at least in part, a product of transformation and/or neoformation under more surface-like conditions.

‘Epoxyphilic’ montmorillonites were prepared containing various functional groups which can react with one or both of the components in amine-cured epoxy formulations. The preparation involves the ion exchange of six organic ammonium compounds containing either carboxylic acid, anhydride, phenolic hydroxyl, alcoholic hydroxyl, amide or amine functional groups. X-ray diffraction, elemental analysis and FTIR studies confirm the intercalation of the organic cations to form epoxyphilic montmorillonites. The adsorption behaviour of the modified montmorillonites are compared with the unmodified montmorillonite in an epoxy resin before and after the curing process.

INTERSTRAT is designed to help clay mineralogists identify clay minerals from powder X-ray diffraction data and is especially useful as an aid in the interpretation of interstratified clay minerals. This paper illustrates the use of the INTERSTRAT program in the identification of a set of interstratified clay minerals. One of the examples has been matched against the PDF and the answers compared with those obtained by INTERSTRAT. The most useful features of the program are the knowledge bases of diffraction parameters for the interstratified clay minerals, the ability of the program to compare the clays identified in more than one state and the hierarchical clay text file.

Fluid pressures in argillaceous sediments depend on, inter alia, mechanical stresses, temperature, diagenetic volume changes and permeability. However, the relative influence of the pressuring mechanisms depends critically upon the long time-scale compliance, C, of the overpressured layer.

In sediments undergoing first-time burial and currently exposed to their historically maximum applied stresses, C can be relatively large. Here, fluid pressure increases are principally due to mechanical causes, and overpressuring will be associated with undercompaction. The tendency for undercompaction (compaction disequilibrium) depends on the sediment thickness, burial rate and vertical permeability. In other cases, when applied stresses have been reduced by uplift, or when impermeable hard caps or seals have been formed, C may be substantially smaller. Here pore pressures may be predominantly controlled by diagenetic and aquathermal processes, with mechanical (undercompaction) phenomena being relatively less significant.

Three-dimensionally sealed overpressured zones may exhibit vertical fluid pressure discontinuities. Within a sealed aquifer, fluid pressures may rise to almost lithostatic values, relieved by episodic fracturing of the seal.

Methylene blue (MB) has been used as a probe molecule to examine how the uptake dynamics and the equilibria between this dye and the surface of Na+- and H+- montmorillonite were affected by the presence of a second dye. To prevent spectral interference, the yellow dyes thioflavin T, TFT, proflavine, PFH, and acridine yellow, ACY, were chosen to compete with MB for the exchange sites. The MB was initially adsorbed as trimer (MB+)3 and then redistributed via collisions between clay particles until equilibrium was reached. At equilibrium in the clay/MB systems, the protonated species (MBH2+) predominated at low loadings (1–5% CEC), whereas at higher loadings the trimer (MB+)3 was the major species. The presence of the second, competing dye slowed the approach to equilibrium, significantly reduced the amount of MBH2+ formed and provided evidence for the monomeric MB+, dimeric (MB+)2, and trimeric (MB+)3 forms of MB. Moreover, the presence of PFH and ACY, which are structurally similar to MB, resulted in more dimeric character in the aggregated form of MB compared to the aggregate formed in the presence of the structurally dissimilar TFT.

An expert system has been designed for the structural characterization of disordered phyllosilicates as well as the mixed-layer minerals and the minerals containing stacking faults or isomorphic substitutions. X-ray diffraction being the simplest tool for structural characterization, the expert system uses such data but can account for data provided by any characterization technique (infrared spectroscopy, EXAFS, etc.). The expert system aims to summarize in a unique set the different approaches of the structural characterization using diagrams or tables which have been proposed, avoiding the requirement of the knowledge of their application field and excluding calculations. It runs in an interactive way, to mimic the way a human expert would draw its conclusions.

Dehydration and rehydration of a phlogopitic Mg-vermiculite were studied by thermo-analysis (TG, DSC) and in situ X-ray powder diffractometry. A detailed analysis of the relation between temperature and state of hydration of the vermiculite was rendered possible by use of slow heating rates in combination with a fast-recording position-sensitive detector. The results confirm the existence of a number of definite states of hydration characterized by basal spacings of 1.441,1.429,1.376, 1.165, 1.151,1.002 and 0.926 nm, all of which have already been described in the literature. In addition, so far undetected regular 1 : 1 interstratifications have been found. They formed during those transformations which lead to a change in the number of sheets of interlayer water molecules. These structures exhibit integral series of 00l reflections and d(001) spacings of 2.54 and 2.15 nm for the 2 : 1 and 1 : 0 transitions in the ratio of molecular layers of water, respectively.

Pore-fluid pressure gradients in Lower Tertiary to Upper Jurassic mudrocks in the Gullfaks area show significant variations, both between and within individual structural compartments. Detection and quantification of abnormal pore-pressures, based on drilling parameters and wire line logs, are significantly influenced by petrological variations in the mudrocks and do not provide adequate direct pressure indicators. Empirical models show that the degree of overpressuring in mudrocks above the Gullfaks reservoirs cannot be fully explained by processes such as shale dehydration, compaction disequilibrium, aquathermal effects or in situ hydrocarbon generation, and other processes, such as caprock failure and subsequent migration of hydrocarbons (mainly gas) into the overlying mudrocks, appear to be influential in generating the observed pattern of overpressure.

Vermicular glaucony grains exhibit external features which include longitudinal grooves, ridges and striations, and transverse partings, the last defining packets of green minerals oriented at right angles to the longitudinal features. The grooves and striations are reflected in internal structures, as are transverse partings in the form of transverse voids and micaceous cleavage. There are some gross similarities in the morphological features of vermicular glaucony with those described for decapod faecal pellets. However, reanalysis of the original evidence used to support formation of some vermicular glaucony from decapod faecal pellets highlights discrepancies and alternative explanations. This suggests that vermicular glaucony grains do not form by glauconization of decapod faecal pellets.