Mineralogical and chemical heterogeneity within three standard clay mineral samples have been identified by X-ray diffraction and chemical analysis of various size-fractions. This heterogeneity is partly attributed to accessory minerals, but mostly to structural and compositional variations in the 2:1 layer minerals of different particle size in the same specimen.

An unusual occurrence of palygorskite was found near Montestrutto, lower Val d'Aosta (Italy), within an eclogite-facies leucogneiss of the Sesia-Lanzo zone. Under the optical microscope the mineral appears as a felt of fibres hundreds of μm long and a few μm thick. SEM observation proved each filament to be a bundle of smaller but remarkably long fibres (length mostly 15–20 μm and thickness <1 μm). EDS and TGA showed a water content [zeolitic H2O ( ≅ 6%) + structural OH2 (≅8%) + framework OH (≅2%) ≅l6 wt.%] lower than average (≅20 wt.%). Structure refinement by the Rietveld method confirmed the clay dioctahedral character and coexistence of two polymorphs, monoclinic [77.4(4)%] and orthorhombic [22.6(6)%], the latter showing anomalous variations of some cell parameters [c shortens to 5.138(3) and a lengthens to 12.903(7) Å]. The high crystallinity of this palygorskite suggests that it formed under low-T hydrothermal conditions due to interaction between Mg-rich hydrous fluids, derived from mantle ultramafics, and Al-rich continental rocks. Predominance of unusually long fibres (>5 μm) causes the Montestrutto palygorskite to be potentially carcinogenic.

The ability of clay minerals to adsorb heavy metal cations is an important property in the context of the increasing contamination of the aquatic environment and soils by toxic waste. Laboratory studies of the sorption of heavy metals by clays have mostly been carried out using montmorillonite and illite (Chester, 1965; Egozy, 1970; Bourg & Filby, 1974; Helios-Rybicka & Schoer, 1982), less frequently for kaolinite, and only rarely for mixed-layer illite-montmorillonite (Helios-Rybicka, 1983) and the fibrous clay minerals sepiolite and palygorskite. This note describes the sorptive behaviour of sepiolite towards Ni, Zn and Cd.

The Capim Kaolin District (eastern Brazilian Amazon), is one of the largest kaolin deposits in the world; with the kaolin used mainly for paper coating. The kaolin developed at the expense of Cretaceous sandy-clayey sediments of the Ipixuna formation, through intense lateritization from the Mesozoic to Cenozoic times.

This work describes the morphological, mineralogical, crystallochemical and geochemical evolution of the Capim kaolin facies. Based on the profile analysis in the open pit fronts, it encompasses X-ray diffraction, thin-section optical analysis, EDS-assisted scanning electron microscopy and transmission electron microscopy, chemical analysis, infrared and Mössbauer spectroscopies.

Six facies were defined as different stages of the supergene process. Ferruginization led to a thick duricrust on the soft kaolin, which in turn evolved from sandy-clayey sediments of the Ipixuna Formation. A subsequent deferruginization event degraded the duricrust, resulting in the flint kaolin facies.

La prise en compte simultanée des résultats de diffraction des RX et de spectrométrie infrarouge a permis de préciser la nature des interactions entre l'eau, les cations compensateurs et les feuillets dans les phases à 14.6, 12.2 et 10.1 Å d'une saponite-Li de synthèse. En effet, la diffraction des RX renseigne sur l'espace interfeuillet disponible à l'eau et aux cations compensateurs alors que les perturbations observées sur les fréquences de vibration de valence des hydroxyles de structure apportent des indications sur la nature et la position des atomes situés à l'aplomb des cavités hexagonales. Cette étude a permis de confirmer la présence de cations hydratés (Li-H2O)+ dans les cavités ditrigonales des feuillets de la saponite-Li amené àun état d'hydratation correspondant à la phase à 10.1 Å.

The ophiolitic rocks appearing near Monte Chiaro, characterized by serpentine masses crossed by veins of talc, sometimes of remarkable thickness, are to be found on the right of the Taro river a few miles from Borgotaro. On examination three interstratified clay minerals were found: saponite-talc, chlorite-saponite, chlorite-vermiculite, which were studied optically, with X-ray, d.t.a. and chemical analysis.

Illite/smectite mixed-layered minerals (I/S) occurring in hydrothermally altered dacite from the island of Martinique were studied using X-ray powder diffraction, electron microprobe and CEC measurement techniques. Microprobe analyses and X-ray identification of high- and low-charge smectite layers indicate that the I/S hydrothermal alteration operates from wall-rock to hydrothermal veins, with conversion of high-charge to low-charge smectite and formation of illite layers. The overall alteration reaction can be expressed as: mixed-layered high-charge smectite + Si4+ + K+ → mixed-layered low-charge smectite + illite. This reaction requires Si and K from external sources and differs from the currently invoked mechanisms for diagenetic illitization where Si is released. The proposed reaction appears, therefore, to be the first step of I/S alteration under low-temperature hydrothermal conditions, prior to the crystallization of newly-formed I/S.

The excavation of a tunnel through a mudstone formation provides an opportunity to examine the effects of the modification of the physical and chemical environment on the rock. The mineralogical and chemical consequences of hydration-dehydration cycles and of oxidation have been evaluated in the case of the Toarcian mudstone formation at the Tournemire experimental site (France). Studies by X-ray diffraction and tansmission electron microscopy of both altered and preserved samples show that the introduction of air and condensed water causes the oxidation of pyrite and the subsequent generation of acid and sulphate-rich waters at the micron scale, in the local environment of pyrites. The fluid-clay particle interactions around the oxidized pyrites induce: (1) a statistical enrichment in Si of the I-S clay minerals; (2) an increase in the Fe(III)/Fe total ratio in some of the I-S particles; and (3) the dissolution of illite layers in mixed-layer I-S. These evolutions are consistent with the results of numerical modelling which reproduced the interaction between the clay particles and the acid water.

The construction is described of a heating stage for examining oriented clay specimens in a Philips' diffractometer. The stage, which is simple and cheap to make, is interchangeable with standard specimen holders, operates over the range 20—265°C and does not require the diffractometer to be specially modified. The maximum temperature attained at the surface of the specimen collapses freely-expanding Mgsaturatcd vermiculite and prevents re-expansion of layer silicate minerals once they have been collapsed.

Glauconite and Fe-smectite, which can be distinguished by their peculiar morphology and stacking sequences, coexist in the Galapagos Spreading Centre hydrothermal mounds. Analytical electron microscopy (AEM) data suggest that Fe is entirely in octahedral sites in Fe-smectite whereas glauconite is K-rich with Fe in tetrahedral and octahedral sites. However, the Mossbauer spectra, recorded at various temperatures for samples containing both smectite and glauconite, were satisfactorily analysed with three overlapping doublets corresponding to Fe in octahedral sites. The contradictory results obtained with the two methods are explained by the presence of small particles of iron oxide intimately associated with glauconite. These particles were detected in Mossbauer spectra obtained at 77 K and 4·2 K and were observed by transmission electron microscopy. Iron oxide is a secondary phase formed by alteration of smectite. These data are in good agreement with the hypothesis that the smectite-glauconite reaction, which occurs at 30 m and low temperature in the Galapagos hydrothermal mounds, is a dissolution-precipitation process, dissolution of Fe-rich smectite being followed by precipitation of glauconite and iron oxides.

ESR has been used to obtain information on the octahedral or interlamellar position of Cu(II) in natural smectites from Burkina Faso (West Africa). On the basis of 060 XRD reflections and chemical data, these smectites were found to be Al-rich and dioctahedral. After both Mehra & Jackson, and De Endredy deferrification treatments, the Cu contents remained high (4500 and 22000 p.p.m., respectively). The Cu(II) ESR spectra of these deferrated smectites were compared to those of two reference smectites for which the structural position of Cu(II) was precisely known. The interlayer Cu(II) signal was obtained on a Cu-saturated Camp Berteau montmorillonite, while the octahedral Cu(II) signal was obtained on a synthetic Cu-rich smectite. For this latter reference sample, EXAFS spectroscopy provided evidence that Cu was in six-fold coordination in the octahedral sheet only, and was not exchangeable. In agreement with the experiments by Clementz, Pinnavaia and Mortland, a shift in the g⊥ ESR signal was observed when the air-dried Cu-saturated Camp Berteau montmorillonite (g⊥ = 2·05) was soaked in water for 48 h (g⊥ = 2·13). A small shift in the opposite sense was observed for the synthetic Cu smectite (g⊥ = 2·05 for the air-dried sample, g⊥ = 2·02 for the water-soaked sample). For the two natural smectites a small shift similar to that for the synthetic Cu-smectite was observed. These results indicate that up to 10% of the Cu atoms substitute for Al-Mg-Fe atoms in the octahedral sheets of the smectites studied.

A compilation of Fe3+-bearing chlorite analyses is used: (1) to investigate the Alfree di-trioctahedral (AFDT) substitution 2Fe3+ +□= 3(Mg,Fe2+) in chlorite; and (2) to estimate the composition of a ferri-sudoite end-member (Si3Al)[(Fe2+,Mg)2□Al]O10(OH)8 within the chlorite solid-solution domain. According to our observations, up to two Fe3+ cations might be allocated in the M2-M3 chlorite sites by the substitution AFDT, which does not involve Al. These unexpected observations were made possible by the development of μXANES techniques allowing in situ measurements of XFe3+ (Fe3+/(Fe2+ + Fe3+)) in heterogeneous chlorite. Although further studies are required to confirm the crystallographic position of Fe3+ and refine its ionic/ magnetic behaviour in chlorite, this development creates opportunities for developing new geothermometers.

Since digestion of imogolite or allophane in 5% Na2CO3 at 95°C for periods of 2–100 h leads to solid products different in structure and composition from the starting materials, the procedure is unsuitable for selective dissolution of reactive silica-alumina gels from complex clay mixtures. Cold 5% Na2CO3 does not alter these clays, but effects partial solution, the extent of which depends on the degree of dispersion of the clay and the ratio of solids to solution.

Upper Cenomanian–Turonian clay mineral assemblages of sediments cropping out in the Western High Atlas basin are studied in four sections. Smectite and mixed-layer illite-smectite (I-S) have been identified as major constituents of the deposits. The composition of clay associations in black shales and associated sediments varies considerably according to age, but usually depends either on the general lithology, the abundance of organic matter, or the depth of burial. A distinct correlation is evident between clay mineral distribution and sea-level. Smectite and mixed-layer I-S with greater percentages of smectite layers increase in sediments deposited during transgressive periods, whereas they decrease progressively in the shallower facies deposited during regression in favour of illite and mixed-layer I-S with a greater percentage of illite. The vertical evolution and lateral distribution of clay assemblages and their relationships with sea-level as well as the palaeogeographic conditions prevailing during the Late Cenomanian–Turonian period (flattened topography and arid climate), indicate a detrital origin of the smectite minerals and a distribution pattern controlled by differential settling processes.

A number of different commercial vermiculites have been examined and it has been found that the exfoliable minerals can be divided into three main groups; those which give a 26 Å spacing on an X-ray diffraction pattern, those which give a 14 Å spacing, and those which give various spacings and are partially dehydrated. All are mixed-layer minerals of one type or another, but all have some vermiculite layers in their constitution. The expansibility of these minerals is not linked with their structural types, but there is an indication that the largest expansion is associated with the occurrence of hydrobiotite and the lowest expansion with the occurrence of true vermiculite.

Twenty nine Fe oxide concentrates of Thai soils formed on basalt, sandstone, shale/limestone and granite were investigated. Goethite and hematite are relatively more abundant in granitic and basaltic soils, respectively. Values of Feo/Fed range from 0.01 to 0.28 indicating that free Fe oxides are mostly crystalline.

There are no systematic differences in unit-cell dimensions for goethite and hematite in soils on different parent materials. Mean crystallite dimensions calculated from the 110 reflections are greater for hematite than for goethite. Aluminium substitution varies from 8 to 24 mole% for goethite and from 4 to 17 mole% for hematite. The dehydroxylation temperature for goethite ranges from 285ºC to 320ºC. The goethite in basaltic soils has a smaller crystal size and Al substitution, as well as a lower dehydroxylation temperature, compared to soils on other parent materials. The dehydroxylation temperature of goethite is positively related to Al substitution (R = +0.58), MCD110 (R = +0.49) and Ald (R = +0.53). The Mn, Ni, Cr, V and P in these soils occur in Fe oxides rather than as discrete minerals.

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.

An exact knowledge of the chemical constitution of the surfaces of clay minerals is necessary to understand their properties. For this purpose as many methods as possible should be applied. One of these methods is the preparation and characterization of organic derivatives of clay minerals. Thus for example it ought to be possible to determine surface Si—OH groups.

The preparation of organic derivatives of clay minerals is not astonishing at all. Already in the last century Ebelman in 1846, Friedel and Crafts in 1863 and others prepared molecules containing carbon and silicon. Most outstanding is the work of Kipping accomplished between 1899 and 1944.

The volatiles produced when various clay minerals were pyrolysed under a stream of He or in vacuo were condensed and analysed. They contain a wide range of cations and anions in concentrations ranging from ppm to hundreds of ppm. The reactivity of these volatiles was demonstrated by their reaction with calcite. It is suggested that the water liberated on heating acts as a scavenger of various impurities present in the clay.

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.