For the purpose of clay examination, a new one-step method for the removal of organic matter was developed using sodium peroxodisulphate combined with different buffers such as sodium hydrogen carbonate, disodium hydrogen phosphate and disodium tetraborate. From an early Cretaceous black shale from the Apennines with a high organic carbon content, the <2 µm clay fraction was separated and contained 10.9 wt% organic carbon. To prevent decomposition of the clay layers, the period of oxidation was short (25-60 min) and the pH was kept between 5 and 9.5. Up to 98% of carbon was removed by this method.

The characterization, pillaring ability and some catalytic properties of a saponite from the Vicálvaro deposit of Tolsa, located in Madrid have been studied. This mineral is associated with sepiolite in the same lacustrine environment. The raw saponite presents, as special characteristics, high specific surface area (>200 m2/g) and a high percentage of tetrahedral Al (90% of the total Al in the sample). Natural saponite gives rise to high conversions when used as a catalyst in the reaction of alkylation of benzene with 1-dodecene. Increased catalytic performances are obtained over the alumina-pillared saponite derivatives. These materials have also been tested in the cracking of gas-oil (microactivity test), giving good conversions and high selectivity towards the gasoline fraction, especially the pillared forms.

Stevensite has been found at Dean Quarry, The Lizard, Cornwall, associated intimately with the disordered, hydrated variety of talc known as kerolite. Both minerals occur as pseudomorphs after pectolite and as microcrystalline anhedral masses filling cavities and veins in gabbro. Associated minerals include calcite and prehnite. The XRD data show that stevensite occurs as a physical mixture with kerolite, rather than as mixed-layer stevensite-kerolite. Kerolite is unaffected by glycerol treatment whilst stevensite readily expands: this test provides the only reliable distinction between the two minerals. Differences in IR and chemical data for stevensite and kerolite are subtle; data are broadly similar to those in the literature. Both minerals are readily decomposed to amorphous silica by hot concentrated HCl. Stevensite dehydroxylated at 400°C for 1.5 h is virtually indistinguishable from kerolite. The Dean Quarry occurrence is similar to many others worldwide. Stevensite and kerolite formed after prehnite, calcite, analcime, pectolite and natrolite during a phase of decomposition and alteration of these earlier minerals. ‘Stevensite’ from the first British occurrence in the Whin Sill, North Tyne, has been re-examined and appears to be kerolite or ‘hydrated-talc’.

Palaeoweathering profiles are widely represented in the Iberian meseta. Transmission electron microscopy (TEM) and analytical electron microscopy (AEM) were used to describe the mechanisms governing the processes involved in the formation of one of these weathering profiles developed over slates of the Iberian Hercynian Massif. Three well-differentiated weathering stages were distinguished, based upon well-defined mineralogical signatures characteristic of primary or secondary minerals. The lower stage is characterized by fresh green slates composed of Fe-chlorite, micas, quartz and feldspars. The soft slates represent an intermediate weathering stage. A confined environment has permitted the development of smectite from the different primary phyllosilicates, both by neoformation of montmorillonite and transformation into beidellite. In the upper part of the profile, the red slates are the most weathered. A tropical or subtropical climate has resulted in the formation of kaolinite and abundant iron oxides over several metres. The present study demonstrates that the three identified weathering stages were formed under the same climatic conditions. Such ‘toposequences’ are developed at the present time in tropical areas (Bocquier, 1971).

A range of allophane samples (atomic AI/Si bulk ratios 1.1-1.9) from Silica Springs, New Zealand, has been characterized by X-ray photoelectron spectroscopy (XPS). Binding energies of Si 2s, Si 2p, Al 2p, O 1s, C 1s, and N 1s electrons, together with the kinetic energies of Al KL23L23 Auger electrons, at or near the surface of allophane aggregates, have been derived. The values for Al, Si and O electrons are similar to those for kaolinite but also to those for some framework silicates (feldspars) having 4-coordinate Al. Values for N electrons suggest that N occurs in organic structures. Comparison of XPS and bulk Al/Si ratios shows an enrichment of Al at or near the surface of allophane aggregates. The same is true for C and N. Extraction with citrate-dithionite-bicarbonate (CDB) reagent leaves the surfaces depleted in Al. The CDB extracts have higher Al/Si ratios than the bulk allophanes. Similarly, CDB treatment reduces the degree of surface enrichment of C and N. Small increases in the binding energies of Si electrons following CDB treatment suggest partial dissolution of the bulk structure though a concomitant removal of a separate phase or species cannot be ruled out. The results may be accounted for in terms of the structure previously suggested for the primary spherules of Silica Springs allophane (Childs et al., 1990) though the composition of the spherules at or near the surface of the allophane aggregates is different from those of the bulk.

The nature and composition of the Fe minerals in the rusty ferruginous material frequently located in the zone of contact between the thallus of Stereocaulon vesuvianum and the leucite bearing rock of Mt. Vesuvius have been studied by microscopic and submicroscopic investigations (SEM, TEM with EDXRA) complemented by X-ray diffraction and chemical analysis. Ferrihydrite is likely to be the main component of the short-range order iron oxyhydroxide pool, although the possibility that significant feroxyhite may be present cannot be completely excluded. Hematite and goethite are the crystalline iron oxides. The organic acids produced by lichens play a key role in the enrichment of poorly-ordered phases at the rock-lichen interface. It is suggested that the peculiar morphology of the Stereocaulon vesuvianum thallus and the irregularity of the volcanic rock surface lead to different microsites in the rock-lichen interfaces which have their own conditions of pH, humidity and redox potential, favourable for the neoformation of crystalline phases.

Mixed hydroxy Al-Fe-montmorillonite complexes (5 mmol of Al+Fe per g of clay) were prepared at pH 5.0 by mixing different amounts of Al(NO3)3 and Fe(NO3)3 to give initial Fe/Al molar ratios (R) of 0, 0.1, 0.25, 0.5, 1.0, 4.0, 10.0 or ∞. The effect of ageing on the interlayering, mineralogy, chemical composition, CEC and external surface area of the complexes was studied. The degree of interlayering of freshly prepared and aged complexes (up to 120 days at 50°C) was always greater in the samples with R ranging from 0.5 to 4. Gibbsite and hematite (with traces of goethite) formed in the samples with R ⩽ 1 and R∞, respectively. No crystalline Fe oxide was detected by XRD analyses in the complexes with R ranging from 0.1 to 10, whereas no crystalline Al and Fe oxides were formed in the complexes at R = 4–10. However, some evidence suggests the possible formation of a microcrystalline Fe oxide/oxyhydroxide undetectable by XRD analyses in the complexes richer in Fe. The CEC and the surface area of the freshly prepared mixed hydroxy Al-Fe-montmorillonite complexes increased with increasing R. Large quantities of Al+Fe were solubilized by oxalate in the complexes with R ⩾ 0.5, even after ageing for 120 days at 50°C, indicating that interlayering of Al-Fe species at certain Fe/Al molar ratios was particularly stable.

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.

An homogenous 8.4 Å hydrate was obtained after washing intercalated KAc- nacrite. X-ray diffraction analysis based on comparison of the experimental and calculated profiles enabled the amount of water (one molecule per Si2Al2O5(OH)4) and the z coordinate along c* (6.5 Å) to be determined. The hydration state was accompanied by a decrease in the coherent domain along c* in the order nacrite > KAc-nacrite > H2O-nacrite. The IR spectrum of the hydrated nacrite showed an evolution of the structure with a shift of ν(OH) from 3702, 3649 and 3630 cm−1 to 3690, 3641 and 3620 cm−1, respectively for the nacrite hydroxyls. The ν(OH) stretching bands of the interlayer water appeared at 3602 and 3545 cm−1 and their bending band at 1655 cm−1. The TGA/DTG analysis of the air-dry hydrated nacrite showed a loss of water at 245°C the weight loss (1 molecule per Si2Al2O5(OH)4) corresponding to the interlamellar water, in agreement with that determined by XRD.

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.

Two cryptokarstic (covered karsts) halloysitic deposits were compared: the Entre Sambre et Meuse in Belgium and the Périgord in France. Both belong to a continental margin subject to marine transgression and regression cycles which enhance the karstic activity. This geodynamic context appears to be very favourable to the formation of halloysite. The similarity of the geodynamic context of both sites leads to a genetic convergence linked to specific properties of the cryptokarstic system, especially, the ability to collect the percolating waters and to juxtapose the conditions of precipitation, accumulation and maturation of the halloysite silico-aluminous gel precursors. However, each site also displays pronounced specificities relevant to: the nature of the host rock of the halloysite (a partly silicified limestone in the first, a smectite-kaolinite argilite in the second); the aluminium source (allochtonous in the first, relatively autochtonous in the second); and the halloysite content of the deposits (of ∼ 100% in the first, and 40% maximum in the second).

Dehydration and rehydration of a phlogopitic vermiculite saturated with either Sr2+ or Ba2+ were studied by thermoanalysis (TG, DSC, DTA) and by in situ X-ray powder diffractometry. A detailed analysis of the relationship between temperature and state of hydration of the vermiculites close to equilibrium at atmospheric pressure was rendered possible by use of low heating rates in combination with a fast recording position-sensitive detector. For Sr-vermiculite, the results reveal the existence of a number of distinct states of hydration characterized by basal spacings of 1.54, 1.51, 1.49, 1.23, 1.21 and 0.96 nm. The Ba-vermiculite failed to develop a two-layer hydrate even in contact with water. Its one-layer hydrate (1.24 nm) was reversibly transformed into an incompletely dehydrated ordered ‘mica-structure’ (1.00-0.99 urn), before an irreversibly dehydrated structure was attained at temperatures exceeding 700°C The formation of regular 1:1 interstratification was confirmed for all transformations characterized by a change in the number of water layers. The concluding compilation summarizes basal spacings, stacking orders, layer translations and specific water contents for all discrete states of hydration observed in our experiments at saturation of vermiculite with different alkaline earth cations.

An ion-exchange adduct of saponite with tetra-n-decylammonium cation ((n-decyl)4N+) was prepared and dispersed in chloroform. The material was spread on a water surface to form a thin film at the air-water interface. From the measurements of surface pressure vs. area, it was concluded that the film consisted of the very thin layer which was a mixture of single and double layers of saponite adduct. A clay-modified electrode was prepared by depositing the film on a glassy carbon substrate using the Langmuir-Blodgett method. When the electrode was soaked in an aqueous solution of [Fe(phen)3](ClO4)2 (phen = 1,10-phenanthroline), (n-decyl)4N+ cations were replaced by [Fe(phen)3]2+ ions from solution. This process was followed by measuring the cyclic voltammogram.

The chemical and mineralogical properties of the A1 and B horizons of an acid brown soil (typic Dystrochrept) were compared before and after a 20 month column leaching experiment. At the end of the leaching period, the organic carbon content of the A1 horizon had decreased by ∼20% compared with the original horizon. In the leached A1 horizon, the vermiculite layers of both the vermiculite and interstratified minerals were transformed into high-charge expanding layers, with the charge mainly located in the octahedral sheet. The inhibition of expansion properties by pretreatment with dithionite-citrate-bicarbonate (DCB) was explained by an increase in total layer charge induced by Fe reduction. In the B horizon, leaching resulted in a loss of sesquioxide materials, especially in the interlayers of vermiculites. In both horizons, the amounts of Al extracted by Na citrate and the amounts of Fe extracted by DCB decreased. Analysis of solution composition showed that Al mobilization was closely associated with NO3− leaching.

Raman microscopy of the kaolinite polymorphs was used to study single crystals and bundles of aligned crystals of kaolinite. The spectra of the hydroxyl stretching region were both sample and orientation dependent. Kaolinites can be classified into two groups according to the ratio of the intensities of the 3685 and 3695 cm−1 bands. No relationship was found between the d-spacing and the crystal domain size measurement from the 001 reflection and the Raman spectral intensities indicating the Raman spectra are independent of d-spacing and crystallinity. However, a relationship of the crystallinity in the a-b direction and intensities of the 3685 and 3695 cm−1 bands indicate that the relative position of one layer to the other determines the position of the inner surface hydroxyl groups and the hydrogen bonding with the oxygen of the opposite layer. A new hypothesis based on symmetric and non-symmetric hydrogen bonding of the inner surface hydroxyl groups is proposed to explain the two inner surface hydroxyl bands centred at 3685 and 3695 cm−1. The bands at 3670 and 3650 cm−1 are described in terms of the out-of-phase vibrations of the in-phase vibrations at 3695 and 3685 cm−1.

Changes in the molecular structure of a highly ordered kaolinite, intercalated with urea and potassium acetate, have been studied using Raman microscopy. A new Raman band, attributed to the inner surface hydroxyl groups strongly hydrogen bound to the acetate, is observed at 3605 cm-1 for the potassium acetate intercalate with the consequential loss of intensity in the bands at 3652, 3670, 3684 and 3693 cm-1. Remarkable changes in intensity of the Raman spectral bands of the low-frequency region of the kaolinite occurred upon intercalation. In particular, the 144 and 935 cm-1 bands increased by an order of magnitude and were found to be polarized. These spectroscopic changes provide evidence for the inner surface hydroxyl group-acetate bond being at an angle approaching 90° to the 001 face. Decreases in intensity of the bands at 243, 271 and 336 cm-1 were observed. The urea intercalate shows additional Raman bands at 3387, 3408 and 3500 cm-1 which are attributed to N-H vibrations after formation of the urea-kaolinite complex. Changes in the spectra of the inserting molecules were also observed.

Ni-Fe pyroaurite-type hydroxychlorides were prepared by aerial oxidation of Ni(II)-Fe(II) hydroxides precipitated in aqueous solution with various P = Fe/Ni ratios. When P≥ 1/3, Ni(II)-Fe(II)-Fe(III) hydroxychlorides characterized by a specific Fe(IIl)/[Fe(II)+Ni(II)] ratio of 1/3, corresponding to the idealized formula of NiII3-xFeIIxFeIII(OH)8Cl.nH2O (with 0 ≤ x ≤ 3), were obtained at the end of the first stage of oxidation, In a second reaction stage, these hydroxychlorides oxidize with deprotonation of hydroxyl ions into O2- ions, i.e. the remaining FeII(OH)2 groups are transformed into FeIIIOOH groups. Along with the Ni(II)-Fe(III) hydroxychloride which contains a part of the FeIIIOOH groups a second phase is obtained. It is an amorphous Fe(III) or Ni(II)-Fe(III) oxyhydroxide when 1/3<P≤3/2, and a ferric oxyhydroxide identified as γ-FeOOH (lepidocrocite) when P>3/2. On the other side of the domain, when P<1/3, the Fe(III)/[Fe(II)+Ni(II)] ratio cannot reach the specific value of 1/3; this gives rise to a pyroaurite-type Ni(II)-Fe(III) hydroxychloride with a lower chloride content, that is with an average composition of NiII3+yFeIII1-y(OH)8Cl1-y.nH2O where y = {[4/(1+P)] - 3}, down to minimum Fe(III) and Cl contents corresponding to y = 1/3 (P = 1/5). The in situ mechanisms of oxidation of Ni(II)-Fe(II) hydroxides into Ni(H)-Fe(II)-Fe(III) hydroxychlorides are discussed.

The ease with which sorbed Cr(III) is released to solution via oxidation or cation exchange depends on its mode of complexation and the broader chemical environment in which it is held. This study was conducted to elucidate the chemical environment of hydrolysed Cr(III) sorbed onto montmorillonite, both alone and coprecipitated with Al. The 27Al MAS NMR revealed that Cr in the coprecipitated hydroxy interlayer created a second environment for AI, slightly more shielded than that of Al in the montmorillonite 2:1 layer. This observation is evidence for the presence of numerous Al-O-Cr linkages, thus revealing a uniform distribution of Al and Cr within the hydroxy polymers. The 29Si MAS NMR revealed a second environment for Si in the tetrahedral sheet of the Cr clays. This environment could be created only through a redistribution of electrons at the siloxane surface caused by inner-sphere complexation of Cr(III). Due to this inner-sphere complexation, sorbed Cr(III) is difficult to displace via simple exchange reactions and, consequently, its bioavailability is decreased.

By means of transmission electron microscopy (TEM) and the gold decoration technique, the surface microtopography of rectorite (allevardite) crystals collected from Allevard, France was investigated. On the crystal surfaces, gold decoration successfully revealed growth steps of ∼20 Å in height. Only closed polygonal steps 20 Å high occur on the 001 surface of a thick platy crystal, although spiral growth patterns were not observed. Thick platy particles were easily split into thinner plates 20 Å thick in distilled water by means of ultrasonic vibration. No step patterns were observed on the surfaces of these thin plates which correspond to the cleavage plane. The growth of this rectorite, therefore, seems to be controlled by a layer nucleation mechanism.

The layered double hydroxide (LDH) Al2Li(OH)6[NiCl4]1/2 was synthesized by anion exchange of Al2Li(OH)6(NO3) and the coordination structure of [NiCl4]2− in the interlayers was investigated. The LDH had a hexagonal cell with a = 5.06 and c = 23.06 Å. Since the basal spacing of one layer was 7.7 Å, the interlayer distance was calculated to be 2.9 Å and was too narrow for the usual tetrahedral NiCl4 structure. From extended X-ray absorption fine structure (EXAFS) measurements, the first neighbour Ni-Cl distances in the LDH were 2.10 Å which was compatible with four-fold coordination. Since the electron spin resonance (ESR) spectrum of the LDH showed no absorption, the [NiCl4]2− structure was considered to be square planar, rather than the usual tetrahedral structure in the interlayers of LDH.