Different forms of structural and chemical heterogeneity are considered including mixed-layer minerals, disordered layer structures containing rotational and translational stacking faults, interstratification of trans-vacant (tv) and cis-vacant (cv) layers in true micas, illites and illitesmectite (I-S), short-range order in isomorphous cation distribution etc.

Because determination of various structural and chemical imperfections requires elaboration of new diffraction and spectroscopic methodologies, special attention is paid to recent achievements in the development of new methodological approaches such as a multispecimen simulation of experimental X-ray diffraction (XRD) patterns from mixed-layer minerals, with account taken of layer-thickness fluctuations of the second type and possible difference between structures of outer and core layers; experimental determination of thickness distribution of illite crystals by HRTEMand the modified Bertaut-Warren-Averbach technique; XRD and thermal methods for determination of cv and tv layers in true micas, illites and I-S; generalization of Méring's rules to account for the behaviour of non-basal reflections for any defective structure in which two translations are irregularly interstratified; various ab initio calculations devoted to modelling infrared OH vibrations, octahedral cation distribution in dioctahedral 2:1 layer silicates, etc. It is shown that these recentlydeveloped methodologies have revealed new diversity in the structural and chemical heterogeneity of phyllosilicates and clay minerals, provided new insight into the structural mechanisms of their transformation in different geological environments, and discovered new natural processes.

An unusually complete sequence of pelitic rocks ranging from diagenetic to greenschist-facies metamorphic grades occurs in southern Sierra Espun˜a, Spain. Prograde and retrograde reactions have been studied by X-ray diffraction and electron microscopy (SEM, TEM and AEM). The prograde reaction series, with reactions facilitated by tectonic stress, includes: (1) R4 interstratified illite-smectite in the diagenetic Malaguide Complex that preserves the variable orientation of original smectite packets, and has 1Md polytypism; (2) chemically heterogeneous illite and Na-K dioctahedral white micas that progressively evolve toward chemical and textural equilibrium in the anchizonal Intermediate Units; and (3) thick, defect-free packets of phengite, paragonite and clinochlore which have a typical metamorphic texture, in the Alpujarride Complex. Two superimposed retrograde episodes produced: (a) sudoite at near-peak metamorphic conditions and (b) dioctahedral smectite during low-temperature retrograde diagenesis.

The main developments taking place in Nuclear Magnetic Resonance Spectroscopy (NMR) of quadrupolar (spin I > 1/2) nuclei with half integer spins in solids, particularly clays and other layered materials, have been reviewed. The advent of Multiple-Quantum (MQ) Magic-Angle Spinning (MAS) NMR spectroscopy has been a step-change development to the studies of quadrupolar nuclei in solids. It is now possible to record high-resolution spectra of important nuclei, such as 11B, 17O, 23Na, 27Al and 69,71Ga, in synthetic and natural clays. Since its introduction in 1995 MQMAS NMR has evolved considerably and, at present, a range of very useful related techniques are available and have been reviewed, as has the current situation with regard to applications to clays and other layered materials.

Deuteration (H/D substitution at 200ºC) was performed on powders of two ground talcs of different particle shapes (different basal/lateral surface ratios). Results indicate that the deuteration process is only efficient on lateral talc surfaces, and suggest that the hydrogens located in the hexagonal ring of the talc basal surfaces are not exchanged. The FTIR spectra collected from the two talc samples show that it is possible to discriminate between particles with the same specific surface area but with different basal/lateral surface ratios using the deuteration process.

A series of organo-silicates was obtained from synthetic Na-fluorotetrasilicic mica (Na-TSM) by ion exchange with dodecyl-, tetradecyl-, hexadecyl- and octadecyltrimethylammonium (C12, C14, C16 and C18) bromides. When the concentration of the alkylammonium ions has varied from 0.43 to 3.4 mmol/g of the silicate, the maximum amount adsorbed on the mica was 0.92 mmol/g. As shown by 23Na MAS NMR, ∼0.30 mmol/g of Na+ ions were non-exchangeable, even if a large excess of the organic ions was used. The XRD patterns of all the samples obtained show that alkylammonium ions are distributed inhomogeneously within the mica interlayers. As a consequence, the organo-micas contain several phases consisting of randomly and regularly interstratified layers differing in terms of the amount of alkylammonium ions adsorbed. These different layers swell in ethylene glycol in the same manner and form new interlayers with identical spacings. The d001 values of individual organo-TSMs after glycolation suggest that the alkylammonium ions are oriented perpendicular or almost perpendicular to the mica layers. The Na-TSM layers were not detected in organo-TSMs by XRD. The presence of organic and inorganic (Na) mixed-ion interlayers with spacings controlled by the arrangements of the organic ions is consistent with the results obtained. These data also suggest that the alkylammonium/Na+ ion ratios vary in alternate interlayers.

The crystal chemistry and mechanisms of chlorite formation (more particularly the ferrous varieties) are currently attracting growing interest in various fields of application. In oil prospection, this interest is linked to the origin and formation of diagenetic chlorite rims that preserve the quality of sandy reservoirs down to unusual depths of burial. In the nuclear industry, the use of iron containers for the storage of radioactive waste is likely to produce ferrous chlorites by interaction with the clayey barrier within which the waste is stored. The ditrioctahedral and tri-trioctahedral chlorites are also some of the best clay mineral markers of hydrothermal activity leading to the formation of uranium deposits underlying the Proterozoic unconformity that make up the largest current economic reserves.

The high chemical and structural variability of chlorite is an obstacle in understanding its mechanism of formation, transformation and alteration. This also applies to the determination of the thermodynamic parameters that are required for digital models based on thermo-kinetic laws.

A nearly monomineralic 1.5 m thick bentonite bed sampled in Melo (Uruguay) appears to be a pure high-charge montmorillonite: [Si3.94Al0.06](Al1.40Fe3+0.11Ti0.02Mg0.49Mn0.01)O10 (OH)2Na0.01K0.08Ca0.18. However, contrasting swelling behaviours have been demonstrated by fitting the experimental X-ray diffraction patterns which were recorded on oriented preparations of the same sample in different saturation states. According to the expandability of the layers in the Ca-, K- and K-Ca-saturated (i.e. saturated first with K+ and subsequently with Ca2+) states, three ‘layer types’ were defined. Low-, intermediate-, and high-charge layers are fully, partly, and not expandable, respectively, after K-saturation. Collapse of high-charge layers is not reversible after subsequent Ca-saturation, probably because of tetrahedral substitutions. These three different layer types are segregated in two distinct randomly interstratified mixed-layer phases. Total surface area and cation exchange capacity are shown to depend on the interlayer cation composition.

The mechanisms governing the adsorption of a commercial humic acid (HA) on a kaolinitic clay were determined. The amount of HA fixed increased with the ionic strength of the medium and the presence of divalent ions (Ca2+). The effect of Ca2+ is due to its ability to establish (1) intramolecular bridges causing condensation of the HA, and (2) intermolecular bridges between the clay and the HA. A stable clay-humic complex containing only the fraction of HA that withstands washing with water was prepared and characterized by high-resolution Ar adsorption volumetry. The overall results show that (1) HA interacts strongly with the basal surfaces of the clay, (2) Ca serves to establish Ca2+ bridges between HA and clay, and (3) HA and clay interact via at least two distinct adsorption mechanisms.

The alteration and transformation behaviour of montmorillonite (bentonite from Wyoming, MX-80) in low-salinity solutions (NaCl, CaCl2) in the presence of metallic Fe (powder and 86461 mm plate) and magnetite powder was studied in batch experiments at 300ºC to simulate the mineralogical and chemical reactions of clays in contact with steel in a nuclear waste repository. The evolutions of pH and solution concentrations were measured over a period of 9 months. The mineralogical and chemical evolution of the clays was studied by XRD, SEM, Transmission Mössbauer Spectroscopy and TEM (EDS, HR imaging and EELS). Dissolution of the di-octahedral smectite of the starting bentonite was observed, in favour of newly formed clays (chlorite and saponite), quartz, feldspars and zeolite. The formation of Fe-chlorite was triggered by contact with the metallic Fe plate and Fe-Mg-chlorite at distance from the Fe plate (>2 mm).

The hydrothermal reaction of kaolinite in the system Na2O-K2O-MgO-Al2O3-SiO2- H2O-HCl, at near-neutral pH conditions, has been investigated at 200ºC, for times from 12 h to 180 days. X-ray diffraction (XRD) study of the solid products indicates that the phases formed from 15 days’ reaction time are randomly ordered mixed-layer chlorite-smectite, dioctahedral on average, with a chlorite:smectite ratio of ∼1:2. No structural evolution was observed with increasing reaction time. Investigation by transmission electron microscopy revealed, on the contrary, that very thin packets with basal spacing of either 10 Å (smectite) or 14 Å (chlorite) are dominant. In addition, packets with periodicities of 12 –13 Å are also observed. Only rarely, randomly to ordered sequences of 10 and 14 Å are observed in the packets. These observations indicate that the mixture of these phases behaves in XRD as randomly ordered mixed-layer chlorite-smectite. Chemical analysis of the solutions suggests that the neo-formed phases are metastable and would evolve, at longer reaction times, toward ordered mixed-layer chlorite-smectite or to clinochlore.

The hydrothermal reaction of kaolinite in the system Na2O-K2O-(MgO)-Al2O3-SiO2- H2O has been investigated at 200 and 300ºC. The study of the solid products indicates that, at 200ºC, disordered illite was the first phase formed, which evolved at increasing run times towards well ordered illite and analcime. After longer reaction times illite and kaolinite were the sole phases identified by XRD. The b parameter of the illites increased at longer reaction times, from 9.05 to 9.08 Å. Study by TEM/AEM revealed the presence of surface chlorite-like layers, covering the illite packets, however. The trend observed in the products of the reaction at 300ºC was similar, except for the presence of boehmite or pseudoboehmite at the higher temperature. The values of the bparameter of illite, of the order of 9.12 –9.16 Å, indicate that illites formed at 300ºC have a more trioctahedral character. The results obtained also indicate that Mg determines which product will be formed, Kand Na-zeolites and Al-rich illites being the phases formed in the absence of Mg.

Hydrotalcite-like compounds (HTlc) were found as secondary products in two corrosion experiments performed with Al plates in highly concentrated solutions (MgCl2-saturated brine) in the presence of metallic and soluble iron at 90ºC under an Ar atmosphere. The durations of the experiments were ∼10 and 13 months, respectively. To investigate the corrosion experiments, solution and gas samples were collected and the main parameters measured. The solid and liquid samples obtained were analysed by X-ray diffraction (XRD), scanning electron microscopy (SEM) and ICP-OES. The presence of the hydrotalcite-like phase was confirmed by XRD and SEM. In one experiment HTlc was found only after dialysis. Formation of the hydrotalcite-like phase was observed at lower pH than mentioned in the literature. These results are useful in understanding the long-term behaviour of corrosion products of reactor fuel elements in relation to their possible barrier function and to comprehend the long-term behaviour of this type of waste during its final disposal.

Alumina-pillared fluorine micas were prepared from synthetic highly layer-charged expandable fluorine mica and polyhydroxoaluminium solutions under different solution loadings (Al mmol in solutions per 1.0 g mica) in order to clarify the effects of solution loading on thermal durability and microporous properties. The intercalated Al content of the pillared micas increased with increased solution loading. The intercalated Al content (i.e. the pillar density) of the pillared micas influenced both the thermal durability and specific surface areas of the pillared micas. The pillared micas obtained from the high solution loadings showed better thermal durability than those obtained from the low solution loadings. Through micropore formation upon heating, the pillared micas exhibit cation exchangeability due to the liberation of residual Na+ ions from steric hindrance in the interlayer pillaring space. The amount of the exchangeable ions depends on the heating temperature of the pillared micas.

Aggregation of colloidal clay particles (Na-montmorillonite) by CaCl2 and anionic polysaccharide (succinoglycan) in turbulent conditions was investigated using time-resolved size measurements by laser diffraction on diluted (50 mg l –1) and stirred suspensions. Excess of Ca2+ ions promotes coagulation of the clay, reducing interparticle repulsions, and allows adsorption of succinoglycan, inducing bridging flocculation. Growth/breakage cycles, characteristic of the turbulent conditions, cause the macromolecules to be incorporated in the innermost of the flocs, where the morphological units are shown by confocal microscopy to be the micrometric Ca-clay particles. Such incorporation results in an increased floc tensile strength, depending on the amount of macromolecules adsorbed, with a maximum at polysaccharide concentrations of 2 wt.% with respect to clay mass.

The Puncoviscana Formation, largely cropping out in NW Argentina, is mainly composed of a pelite-greywacke turbidite sequence affected by incipient regional metamorphism and polyphase deformation. Metapelites, composed mainly of quartz, albite, dioctahedral mica and chlorite, were sampled in the Lules-Puncoviscana and Choromoro belts. Lattice-fringe images, selected area electron diffraction and analytical electron microscopy analyses, coupled with previous data from white mica crystallinity index, indicate a state of reaction progress for Puncoviscana slates consistent with medium anchizone- to epizone-grade metamorphism. The 2M polytype prevails in dioctahedral micas, coexisting in a few cases with the 1Md polytype as a consequence of lack of equilibrium. The 2Mpolytype coexists with 3T in two slates and long-range four-layer and ten-layer stacking sequences were identified in another sample. Samples with 3T and long-range stacking sequences present b values characteristic of intermediate–high pressure metamorphism and ordered chlorites (1L, 2L, 3L and 7L) prevail.

Based on the Si contents of dioctahedral micas and considering peak temperatures of ∼350 –400ºC, pressures from 5 kbar and 5 –7 kbar were derived for metapelites from the Lules- Puncoviscana and Choromoro belts, respectively. These values agree with facies series derived from the b values. Micas with a wide range of phengitic substitution, as evidenced by Fe + Mg and Si contents, coexist. These variations could not arise from the disturbing effect of detrital white K-mica because TEM evidence indicates that they are absent or represent <10% of the mica population. Thus, compositional variations suggest that dioctahedral micas of individual slates crystallized at different pressure conditions in response to the P-T path of the metamorphism. Moreover, in several biotite-free slates the illite crystallinity (IC) values lead to an underestimation of the metamorphic grade attained in these rocks.

The coexistence of IC corresponding to anchizone and the occurrence of biotite in some slates and felsic metavolcanic rocks intercalated in the Puncoviscana metasediments are interpreted to be the result of a metamorphic path including a relatively high-pressure/ low-temperature (HP/LT) event, followed by a lower-pressure overprint possibly at higher temperatures than the HP/LTevent. Small micas formed during the high-pressure stage would prevail in the <2 mm fraction, producing anchizone IC.

The hydrothermal reactivity of a Mg-vermiculite from Santa Olalla (Spain) was studied experimentally at 150 and 300ºC, 75 and 100 bars, respectively, in (Na,K,Ca,Mg) chloride and sulphate solutions (liquid/solid ratio = 10). The formation of mixed-layer clays is demonstrated by FT-IR spectroscopy, X-ray diffraction (XRD) and electron microprobe. The gradual decrease of the 3220 cm–1 infrared band intensity, characteristic of free water, is interpretated as the dehydration of the interlayer Mg cation of the vermiculite. The presence of ‘brucite type’ sheets is also observed by the increase of 3555 cm–1 and 3420 cm–1 band intensities. The XRD results show that run products are able to expand when glycolated. Microprobe analyses document the decrease of IVSi content and a significant enrichment in Mg in the run products. The formation of a mixed-layer clay comprising saponite and chlorite layers is proposed.

The adsorption of the anionic dye Congo Red (CR) by Na-, Cs-, Mg-, Cu-, Al- and Fe-montmorillonite was investigated by electronic spectroscopy. Spectra of the aqueous suspensions of different clay concentrations and different loadings were recorded. The spectra were curve fitted and were compared to spectra of aqueous solutions of CR at different pH values. Four bands representing anionic varieties and two representing protonated varieties of CR were identified in spectra of CR-treated Na-, Cs-, Mg-, Cu- and Al-montmorillonite. Spectra of CR-treated Femontmorillonite were slightly different, showing three bands representing anionic varieties and four representing protonated CR. The ratio between areas of bands, which represent the protonated varieties, and those representing the anionic varieties were determined showing correlation with the acidity of the metallic cation. The clay in the aqueous suspensions became coloured after the adsorption of CR. The following stainings were obtained, Na- and Mg-clay, red, Cs-clay, violet-red, Cu-clay, reddish-violet, Al-clay, dark violet and Fe-clay, dark blue. It appears that CR can serve as an indicator of the surface acidity of montmorillonite. The effects of aging on the sorption of the dye and on the surface-acidity of the CR-clay complex were investigated.

The alkali leaching of two aluminosilicates, kaolinite and metakaolinite, with aqueous NaOH has been studied. Both silicates gave hydroxysodalite (HS) with or without the evanescent zeolite NaA. X-ray diffraction and high-resolution 29Si and 27Al MAS-NMR spectroscopy provide information about the reaction sequence of the clay. The conversion starts with the formation of an amorphous gel precursor at a rate which depends on the alkali concentration but not on the choice of kaolinite or metakaolinite as starting material. The rate of zeolitization of this gel is much faster when it is obtained from kaolinite, probably because it is more homogeneous.

Scanning electronic microscopy (SEM) and transmission electronic microscopy (TEM) investigations of chlorite grain coatings from four different sandstone reservoirs indicate a progressive change in both texture and arrangement of chlorite particles from the contact with the detrital substratum to the centre of the pore. Such spatial distribution results from growth by geometrical selection. Geometrical selection of chlorite crystals proceeded during a single event of continuous growth which began before the consolidation of the sandy sediments, lasted through part of the subsequent mechanical compaction and ceased before the occurrence of quartz cement. Nanopetrographic investigations near the detrital quartz-chlorite coating interface demonstrate that inhibition of the quartz cement is due to the limitation of the epitaxial growth of quartz to the interparticular space at the base of the chlorite coating and not an absence of nucleation. It is suggested that these results can be applied to most of the sandstones which contain Fe-rich chlorite grain coatings.

The mineralogical and geochemical characteristics of Eocene continental sediments in south central Tunisia (Chebket Bouloufa and Jebel Hamri) and in north central Tunisia (Jebel Lessouda and Jebel Rhéouis), which contain considerable amounts of palygorskite, were studied. The clay fraction of the sediments also comprises illite, kaolinite, Mg smectite and Al smectite, together with carbonates (calcite and/or dolomite), quartz, gypsum and feldspars, all of which are present in extremely variable proportions.

The textural characteristics of the samples containing most palygorskite, as well as the chemical composition of the fibres and the contents of certain trace and rare earth elements suggest that the genesis of this fibrous clay is intimately linked to the diagenetic transformation of illite, mixedlayered minerals and/or Al smectite, as has also been observed in contemporaneous deposits in Morocco.