Celadonite, glauconite and Fe-illite samples were studied by XRD, EXAFS, IR and Mössbauer spectroscopy. The samples were monomineralic and corresponded to 1M polytype. In the OH-stretching region of the IR spectra the content of each definite pair of cations bonded to OH groups was determined. The number of heavy (Fe) and light (Al, Mg) octahedral cations nearest to Fe was found by the EXAFS technique. The predicted quadrupole splitting values for each definite arrangement of cations nearest to Fe3+ were used to interpret the Mössbauer spectra. After the fitting procedure, the intensity of each doublet corresponded to a definite set of local cation arrangements around Fe3+ and to a definite occurrence probability of these arrangements. Computer simulation and the experimental data obtained were used to reconstruct the distribution of isomorphous octahedral cations in the 2:1 layers. For all samples, R2+ cations prefer to occupy one of the two symmetrically independent cis-sites and R2+-R2+ and/or Al-Fe3+ were prohibited in the directions forming ± 120° with the b axis. Therefore, octahedral sheets of the samples revealed domain structure, in which domains differ in size, in the nature of predominant cation and/or by cation ordering.

Fundamental particles of illite-smectite from bentonites were separated into classes by high-speed centrifugation after infinite osmotic swelling of mixed-layer crystals, achieved by Na-exchange and dispersion in distilled water. In samples free of detrital contamination, the thinnest fundamental particles yield older K-Ar ages than the thicker fundamental particles. This implies that they do not preferentially lose radiogenic 40Ar due to size, and that the illitization process is a crystal growth mechanism (not nucleation plus growth). As a result, any K-Ar age of fundamental illite particles from bentonites is an integral over longer or shorter periods of time, depending on the thermal history of the rocks. In thick bentonite beds, the measured age difference between the beginning of the illitization process at the contact with the host rocks and the end in the centre of the bed records extremely slow K diffusion in these well compacted rocks. These data explain why measured K-Ar ages of illite-smectite from bentonites are younger than the corresponding age of shale illitization, inferred from the burial history of the basin. The finest technically separable size-fractions of associated shales (<0.02 μm) yield K-Ar dates* greater than the stratigraphic age. This observation points to incomplete recrystallization of detrital illite during burial diagenesis.

The use of field emission electron microscopy for the study of clay mineral petrography in mudrocks and sandstones is discussed. The methodology including sample preparation is outlined and three examples of the application of the technique are described: the formation of authigenic illite in mudrocks, the multiple generation of authigenic illites in sandstones and the effects of KCl drilling muds on shale fabrics. In the study of authigenic illite formation in Palaeocene mudrocks from the North Sea, the FESEM analyses have demonstrated the formation of illites with increasing burial depth that conventional SEM and XRD analyses had failed to show. The FESEM analyses of the authigenic illites in Carboniferous sandstones from the southern North Sea revealed at least three different habits representing different generation episodes rather than one illite formation event.This has important repercussions with regard to the interpretation of stable isotope and dating data for the illites. Significant petrographic changes in shales after treatment with KCl drilling muds have been observed from FESEM analyses, suggesting reactivity between the shales and the KCl muds.

X-ray diffraction chlorite crystallinity (ChC) indices and major element chemical compositions of chlorites and bulk rocks were determined and correlated in meta-igneous rocks from different Mesozoic formations in various tectonic units of the Bükk Mountains, NE Hungary. The rocks, of basic to acidic compositions, range from ocean-floor metamorphic prehnite-pumpellyite facies (diagenetic zone) through regional metamorphic prehnite-pumpellyite facies (anchizone) up to the regional metamorphic pumpellyite-actinolite and greenschist facies (epizone). As in the case of meta-sedimentary rocks, chlorite crystallinity can be applied as an empirical, complementary petrogenetic tool to determine relative differences in grades of low-temperature meta-igneous rocks. Electron microprobe and XRD data show that ChC is controlled mainly by the decreasing amounts of contaminants (mixed-layered components or discrete, intergrown phases of mostly smectitic composition) in chlorite with advancing metamorphic grade, up to the epizone. The apparent increase in calculated Aliv content of chlorite with increasing temperature is related to the decrease of these contaminants, as stated earlier by Jiang et al. (1994). On the basis of the significant correlations found between ChC and temperatures, derived by the chlorite-Aliv geothermometer of Cathelineau (1988), both methods may be used for estimating the approximate temperatures of metamorphism, in spite of the contrasting interpretation of chemical data from chlorites obtained by electron microprobe analyses. After determining the effects of changing bulk chemistry on chlorite composition and ChC, the chlorite crystallinity method may complement the correlation of the illite crystallinity-based zonal classification of meta-sediments and the mineral facies classification of meta-igneous rocks.

Unique phyllosilicate assemblages occur in quartz-calcite veins cutting altered andesite at Builth Wells, UK. These veins show: (1) a central talc-dominated assemblage, grading into a saponite/corrensite/chlorite-dominated assemblage near their walls; (2) talc replacing calcite and quartz; and (3) closely associated blocky and fibrous saponite. Fibrous saponite occurs in narrow (<1 cm) fractures that extend for up to 1 m from major (⩽ 50 cm wide) talc-bearing veins. A thin halo (<5 mm) of wall-rock alteration, containing blocky saponite and minor corrensite, borders the fibrous saponite. These minerals may have originated when cooling, low-pH, quartz-saturated, Mg-rich fluid invaded pre-existing quartz/calcite veins. Quartz is replaced by talc where calcite dissolution caused the pH to increase. Wallrock alteration liberated Fe and Al to the fluid causing blocky saponite and corrensite to form. Fibrous saponite formed where lithostatic pressures were directed across hydraulic fractures.

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’.

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.

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).

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.

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.

The interaction of aminotriazole (AMT) at pH 4 on Wyoming montmorillonite (mainly with Na ions) and Mg-vermiculite has been studied by X-ray diffraction and infrared spectroscopy. The AMT is adsorbed on montmorillonite in the cationic form by cation exchange. The amount of pesticide adsorbed was 71 mEq/100 g, which comprises ∼91% of the CEC of this sample (78.2 mEq/100 g). Saturation was reached in 24 h, giving rise to a complex with basal spacing 12.5 Å. Vermiculite adsorbs 167 mEq/100 g, almost 20% greater than the CEC (141 mEq/100 g), and the basal spacing was stabilized at 13.68 Å after five weeks of treatment with AMT. A part of the AMT is adsorbed in cationic form, displacing a great part of the exchangeable Mg2+ cations. The rest is adsorbed in molecular form by coordination to the Mg2+ cations which remain in the interlamellar space, removing a great amount of water, and remaining in the interlamellar space of vermiculite after washing with water, probably because of a steric hindrance from the AMT cations adsorbed.

Soil clays contain a relatively large amount of disordered inorganic material. Chemical dissolution has been used for the removal of this material (Jackson, 1956; Hashimoto & Jackson, 1960; Follett et al, 1965 a,b; Wada & Greenland, 1970). On the other hand, Farmer et al. (1977) claimed that dissolution of allophane and imogolite with hot 5% Na2CO3 for periods of 2-100 h led to new phases which could be distinguished from the starting material by infrared spectroscopy. It is clear, therefore, that chemical dissolution can alter soil clays. This note suggests an electron microscopy specimen preparation technique to study the morphological changes. Collodion films containing densely arranged minute hollows are used for specimen supports.

One of the most important properties of zeolitized volcaniclastic materials is the cation exchange capacity (CEC). Numerous techniques are available for measuring CEC and the methylene blue absorption (MBA) (Nevins & Weintritt, 1967) and the ammonium acetate saturation (AMAS) (Chapman, 1965) methods are two of those commonly used. The aim of this study was to compare the results obtained by the two methods when used with zeolite-rich materials.

Palygorskite and sepiolite occurrences and deposits are frequent in the Iberian Peninsula, mainly in the Tagus basin, in central Spain and Portugal (Galán & Castillo, 1984).

In Portugal, the first palygorskite occurrence was reported by Carvalho (1964) in the region of Ponte do Sor, Tagus basin, in Paleogene sediments. Later, Carvalho (1968) reported palygorskite occurrences in coarse detrital sediments that were occasionally carbonatic or hardened by silicification, and built up the Cenozoic base for the Tagus and Sado basins. Carvalho referred to such formations as the 'Attapulgite complex'. Overlying this attapulgite complex is the 'Montmorillonite complex', probably of Miocene age.