The determination of kaolinite order-disorder by X-ray diffraction is problematic due to overlapping peaks from associated kaolin minerals and X-ray amorphous phases. This paper presents a new index (Aparicio-Gala´n-Ferrell index — AGFI), measured on 02l and 11l reflections after decomposing individual peaks in the complex diffraction band in an effort to reduce interferences. The new index was tested with three kaolins, of varying structural order, and their admixtures containing different percentages of quartz, feldspar, illite, smectite, chlorite, halloysite and Fe hydroxides and silica gels. The AGFI is highly correlated with the percentage of low-defect kaolinite and the Hinckley Index. It is not as prone to interference by associated minerals and X-ray amorphous phases as other indices. The AGFI can be used to determine kaolinite order-disorder in a wide variety of kaolins and kaolinitic rocks; the only prerequisite is that the kaolinite content should be >10 wt.% in order for the results to be reproducible.

The Fourier transform Raman spectra of the kaolinite minerals have been measured in the 50–3800 cm−1 region using near infrared spectroscopy. Kaolinites are characterized by remarkably intense bands in the 120–145 cm−1 region. These bands, attributed to the O-Si-O and O-Al-O symmetric bending modes, are both polymorph and orientation dependent. The 200–1200 cm−1 spectral range is a finger-print region for clay minerals and each kaolinite clay has its own characteristic spectrum. The structure of clays is fundamentally determined by the position of hydroxyl groups. Fourier-transform Raman spectroscopy readily enables the hydroxyl stretching region to be examined allowing identification of the component bands. The advantages of FT-Raman spectroscopy are shown to enhance the study of the kaolinite structure.

The crystal-chemical mechanisms by which transition metals are associated with goethite are fundamental to our understanding of the solubility and bioavailability of micronutrients and heavy metals in soils, and in the formation of laterite ore deposits. Transition metals such as Cr, Mn and Ni may sorb onto goethite by forming surface precipitates, surface complexes or by replacing Fe3+ in the goethite structure. In the work reported here, we investigated the local coordination environment of Cr, Mn and Ni in synthetic goethite using EXAFS spectroscopy. We demonstrate the isomorphous substitution for Fe3+ by Cr3+ (up to 8 mol.%), Mn3+ (up to 15 mol.%) and Ni2+ (up to 5 mol.%). We find, however, that the next-nearest-neighbour coordination environment changes with composition. The perturbations are likely to be responsible for limiting the accommodation of Cr3+, Mn3+ or Ni2+ in the FeOOH structure.

The smectites present in some bentonites from the Island of Milos, Greece, derived from acidic rocks, have been examined by XRD and electron microprobe. The possible influence of loss of alkalis during analysis and contamination by fine grained Si-phases on microanalyses has also been evaluated. Smectites are closely associated with opal-CT and their composition ranges from beidellite to Tatatilla-type montmorillonite. Their chemistry is dominated by negative relationships between Mg and AlVI and Si and total Al. However, it cannot be resolved whether there is a solid-solution between the smectites or whether the variation observed is due to mechanical mixing of domains with different composition.The role of Fe is of minor importance reflecting an inherited factor imposed by the parent rock, but the role of the fluid phase is also important. Beidellite occurs in the rims, while Tatatilla-type montmorillonite, often associated with free opal-CT, is formed in the interior of the altered shards, indicating a variable degree of Si release during glass dissolution. The formation of beidellite is not necessarily associated with Mg uptake.

High-resolution gas adsorption techniques were used to analyse the evolution of the aspect ratio and adsorption energy distribution on synthetic saponite samples with increasing layer charge. Using Ar as a gaseous probe, the aspect ratio of the saponite particles can be determined easily by decomposing the derivative adsorption isotherms and taking into account high-energy sites which can be assigned to talc-like ditrigonal cavities. Changes in the shape of the elementary particles are observed for layer charges above 1.30, i.e. when all the ditrigonal cavities contain at least one Al atom substituting for Si. When N2 is used as a probe, high-energy sites that could be wrongly interpreted as micropores on the basis of classical t-plot treatments are observed whatever the layer charge. Using the information obtained from both Ar and N2, schemes for describing adsorption can be proposed for all layer charges and suggest complex adsorption mechanisms for charged clay minerals.

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 effects of dry grinding on the structure of sepiolite were studied by XRD, surface area measurement, TG-DTA and IR spectroscopy techniques. The results show that the sepiolite framework is rather resistant to mechanical stress and its alteration by dry grinding occurs through three successive, overlapping processes: (i) thinning of the fibrous particles without structural alteration; (ii) distortion of the parallel units of sepiolite fibres, affecting essentially the particle surface which transforms into an amorphous protective coating; (iii) disruption of the inner structure and a prototropy effect, producing an amorphous phase with some residual order.

One of today's big challenges is to store safely the increasing amount of high-level radioactive waste (HLRW) in the world. In some of the concepts devised for this challenge, bentonite, a natural swelling clay, plays a key role in encasing the canisters containing the waste. The use of bentonite as a geotechnical barrier in HLRWrepositories is a new venture; specifications to ensure either optimum performance or that a minimum standard is reached at least do not exist yet. The present study summarizes relevant research and discusses possible HLRW-bentonite specifications. The importance of these specifications for any given repositories has to be assessed on a case by case basis, depending on the concept being employed and any special circumstances for the individual repositories.

Ten key issues were identified which were used to discuss bentonite specifications. In some of these key issues the optimum bentonite performance depended more on processing and production (compaction) than on the bentonite type (e.g. swelling pressure and thermal conductivity). In contrast, in some of the other key issues, the type of bentonite was found to influence possible specifications: the bentonite should not alter its mineral composition or its geotechnical parameters such as the swellability. Therefore, the bentonite should contain neither soluble nor reactive phases (e.g. organic matter, pyrite, gypsum). The structural Fe content of the smectites should be small because of the lesser stability and greater reactivity of the Fe-rich bentonites. Also, a large layer-charge density of the swelling clay minerals leads to less corrosion at the iron–bentonite interface (relevant if iron canisters are used). The hydraulic conductivity and swelling pressure can be tailored by compaction of the bentonite resulting in different dry densities. From an engineering point of view, a bentonite with least dependence of the hydraulic conductivity/swelling pressure on the dry density would be best. Using a bentonite which has been investigated extensively over many years means less uncertainty compared to unknown materials.

Under certain circumstances, both glycerol and ethylene glycol form interlayer complexes with vermiculites resembling those formed with the smectites. The rate and degree of complex formation are related to the surface charge on the silicate layers, to the type of interlayer cation present and to crystal size. With glycerol, but not with ethylene glycol, the hydration state of the mineral immediately prior to complex formation is of some importance.

The effect of gypsum additions on the physical performance of lime-stabilized kaolinite has been determined. Kaolinite containing different amounts of lime (i.e. 6 and 14 wt%) and gypsum (i.e. 2,4,6 and 8 wt%) was compacted into cylinders and moist cured at 30°C and 100% r.h. for periods from 2 days up to 20 weeks. Unconfined compressive strength, expansion during curing and subsequent soaking, and water absorption and swelling pressure were determined. The addition of lime and subsequent moist curing was found to reduce substantially the water absorption, linear expansion and swelling pressure of the kaolinite. Although small amounts of gypsum further reduced these parameters, higher gypsum levels (up to 8 wt%) produced substantial water absorption, extreme expansion and high swelling pressures. This excessive volume instability when in contact with water was found, for a particular lime content, to be very sensitive to both the initial moist curing time and the gypsum content. The results indicate that the overriding expansion mechanism operating is imbibition of water or transfer of water by osmosis. The question of what drives this process is the subject of Part II of this paper.

The distribution of diagenetically-formed clay minerals in Rotliegend dune sandstones of the Southern North Sea Basin is closely related to the following factors: (i) the petrography of the sandstones, (ii) the paleoburial depth and tectonic setting of the area, (iii) the thickness of gas-generating Carboniferous strata underlying the Rotliegendes, and (iv) the facies distribution of the overlying Zechstein. The diagenetic clay minerals are mainly conversion products of feldspars and, to a lesser extent, of detrital clays and micaceous lithic fragments. Sandstones containing dominant drusy illitic and chloritic clay minerals have been buried to depths > 3000 m; if kaolinite is the dominant clay mineral, burial depths were less. Sandstones containing feldspars (detrital and authigenic) up to approximately 7% of bulk volume have permeabilities that are about four times higher than sandstones with similar amounts of kaolinite, and as much as 200 times higher than sandstones with similar amounts of illite and/or chlorite.

Pillared smectites in which the pillars consist of iron oxide are expected to have interesting and unusual magnetic properties. Several possible routes by which such materials might be made have been investigated, namely intercalation of hydroxy-Fe(III) polycations, mixed hydroxy-Fe(III)/Al polycations, phenanthroline-Fe(II) cations, and trinuclear Fe(III) acetato cations into Na-montmorillonite. Only the last of these yielded a pillared clay (PILC) on calcination. The products have been characterized using X-ray powder diffraction and 57Fe Mössbauer spectroscopy. The precursor Fe-PILC has a d-spacing of 21 Å and expands to 23 Å on solvation with glycol. The calcined Fe-PILC has a d-spacing of 19 Å (gallery height 9·4 Å) and does not expand with glycol, confirming cross-linking of the layers. From Mössbauer spectra at 4·2 K it is estimated that there are of the order of some hundred Fe atoms per pillar.

The Kämpf & Schwertmann (1982) procedure for concentrating iron oxides in soil clays by dissolution of kaolin and gibbsite by boiling for 1 h in 5 m NaOH may not dissolve all kaolin, and also results in the precipitation of sodalite. For the complete dissolution of kaolin in kaolin-rich soil clays a boiling time of 2 h in 5 m NaOH was required. The large amounts of sodalite produced were not removed by the prescribed single wash in 0·5 m HCl. Oxalate soluble Al contents of iron oxide concentrates were sometimes very high and dithionite Fe contents were very low both in concentrates containing sodalite, and in those for which sodalite was not detected by XRD, but where a previously unsuspected amorphous sodalite-like phase may have been present. Complete removal of precipitated sodalite was achieved by two extractions with 0·5 m HCl at 25°C for 20 min. This modified procedure does not alter the Al-substitution and crystal size of goethite, hematite and maghemite as determined by XRD measurements.

The fluorine route is thoroughly investigated for the hydrothermal synthesis of montmorillonite in the Na2O-MgO-Al2O3-SiO2-H2O system. Using the optimal conditions suggested by Reinholdt et al. (2001) for the crystallization of pure montmorillonites with the formula Na2x(Al2(1-x)Mg2x☐)Si4O10(OH)2, several parameters (x, Mg content, duration of crystallization, F/Si atomic ratio, pH, nature of counterbalance cation) are varied independently from their ideal values. The products are analysed by various techniques (X-ray diffraction, thermogravimetric analysis-differential thermal analysis, 29Si, 27Al and 19F magic angle spinning-nuclear magnetic resonance). It appears that a pure montmorillonite can only be obtained within a narrow x range (0.10 ≤ x ≤ 0.20). The presence of F in the starting hydrogel and the crystallization time also have significant effects on the purity of the final products. It is shown that a small amount of fluorine is needed for the crystallization of pure montmorillonite phyllosilicates.

The thermal transformations of kaolinite have been studied using 27Al and 29Si magic angle spinning nuclear magnetic resonance (MAS NMR), X-ray diffraction (XRD), differential thermal analysis (DTA) and thermogravimetric analysis (TG). The experimental results show that this sample is a pure kaolinite which contains ∼3% carbonaceous material as impurity. 27Al and 29Si MAS NMR spectra show that the microstructural evolution of the thermal transformation of kaolinite at 450 –1050ºC is similar to that of the kaolinite– mullite reaction series previously published by many authors. 29Si MAS NMR spectra of mullite at 1190 and 1250ºC display two resonances at ∼ – 87 and –92 ppm, corresponding to sillimanite-type geometry around Si and the typical Si environment of mullite, respectively. At 1350ºC, the splitting of (hk0) and (kh0) of mullite occurs, indicating that the primary mullite transforms into orthorhombic mullite. Simultaneously, the resonance at ∼ – 92 ppm splits into two signals at ∼ –90 and –94 ppm. 27Al MAS NMR spectra of the mullite consist of three signals centred at ∼ –4, 45 and 60 ppm, corresponding to octahedral, distorted tetrahedral and tetrahedral Al sites, respectively.

The nature and origin of authigenic clay minerals and silicate cements in the Jurassic and Cretaceous sediments of England and the North Sea are discussed in relation to penecontemporaneous volcanism in and around the North Sea Basin. Evidence, including new REE data, suggests that the authigenic clay minerals represent the argillization of volcanic ash under varying diagenetic conditions, and that volcanic ash is a likely source for at least the early silicate cements in many sandstones. The nature and origin of smectite-rich, glauconite-rich, berthierine-rich and kaolin-rich volcanogenic clay mineral deposits are discussed. Two patterns of volcanogenic clay minerals facies are described. Pattern A is related to ash argillization in the non-marine and marine environments. Pattern B is developed by the argillization of ash concentrated in the sand and silt facies belts in the seas bordering ash-covered islands and massifs. It is associated with regression/ transgression cycles which may be related to thermal doming and associated volcanism, including the submarine release of hydrothermal fluids rich in Fe. The apparent paucity of volcanogenic clay deposits in the Jurasssic and Early Cretaceous sediments of the North Sea is discussed.

Temperature-programmed desorption (TPD) and IR spectroscopy were used to characterize the number and strength of acid sites in Al3+-, Cr3+- and Fe3+-exchanged montmorillonite. The bases pyridine and n-butylamine occupied three different sites in the interlamellar space: (i) physisorbed base, (ii) base bound to Lewis acid sites, and (iii) protonated base. TPD profiles for pyridine were characterized by maxima at 40°, 150° and 340°C, whilst those for n-butylamine occurred at 30°, 200° and 410°C. The Al3+- and Cr3+-exchanged forms were stable up to pretreatment temperatures of 300°C, but the Fe3+-form required > 3 day exposure to base vapour to re-establish the high-temperature desorption peak. Variable-temperature IR studies showed that the number of Brönsted-bound pyridine molecules increased with increased outgassing temperature.

The surface free energy of Ca-montmorillonite has been determined from measurement of the contact angle of water on an oriented deposit immersed in n-alkanes; the oriented deposits were equilibrated at different relative humidities. These experiments provided the polar, γSP, and dispersive, γSD, components of the surface free energy γs. The results indicate that: (1) γs is mainly due to dispersive forces (γSD ≃ 145 mJ.m−2) and, to a lesser degree, polar forces (γSP ≃ 35 mJ.m−2). (2) The value of the apparent surface free energy of the clay, γα, tends towards that measured for water when the solid water content exceeds 50 wt%. (3) The sorption of water molecules substantially modifies the surface free energy of the solid; γs decreases from 180 mJ.m−2 to 60 mJ.m−2 when the water content increases from 0 to 50%. These results agree with other observations made on solids whose surface characteristics are similar to montmorillonite, i.e. micas, silicates and glasses. In addition, variations of γs with water content can be related to the hydration processes of smectites. Finally, these results show that the silicate no longer influences the surface properties when the montmorillonite water content exceeds 60%.

Experiments on the conversion of feldspar to illite reveal that variation of the fluid/rock ratio (flow rate) has a significant effect on the kinetics of feldspar dissolution and illite formation. Dissolution of albite in near-neutral KCl solution at 200°C and 500 bar shows that the Si and Na release rate per unit surface area of albite is faster with higher fluid/rock ratio. These results have application to the interpretation of secondary porosity formation. The experiments also reveal a special set of conditions under which rapid illite formation can occur. In contrast to the slow process of illite formation in neutral solution, there can be mass nucleation and growth of illite platelets on albite surfaces in an initially acidic solution with low fluid/rock ratio. In similar experiments with higher fluid/rock ratio, kaolinite forms instead of illite because there is insufficient solid to titrate the large volume of solution. This study suggests that illitization of feldspar or kaolinite may be triggered by a decreased rate of acidic fluid-influx during burial diagenesis.

This article reviews that clay literature from the last ten years, which is devoted to the applications of clay minerals in the interpretation of geological processes in sedimentary basins. The results, selected by the author as being of particular interest, are presented, arranged according to the successive phases of the rock cycle.