The procedure for synthesizing proto-imogolite (an acid-soluble hydroxy-aluminium orthosilicate complex) and imogolite (a tubular aluminosilicate mineral) was used to produce ferruginous aluminosilicates over a range of Al/Fe ratios to determine whether Fe3+ can be incorporated in the imogolite structure. Analysis of the synthesized products by transmission electron microscopy, electron diffraction, and IR spectroscopy indicated that, while imogolite was formed in the presence of iron, increased Fe3+ in the systems caused the formation of ferrihydrite and poorly-organized aluminosilicates resembling proto-imogolite allophane. Treatment of these materials with Na-citrate/dithionite/bicarbonate dissolved the ferrihydrite and poorly-organized aluminosilicate, and concentrated products with tubular morphology. Analysis of the structural Fe3+ by ESR spectroscopy suggested that little or no Fe3+ was incorporated in the structure of imogolite, although the less crystalline proto-imogolite allophane may have accommodated some structural Fe3+. A separate iron-rich product, identified as ferrihydrite, was formed at low Al/Fe ratios. Mössbauer spectroscopic analysis of 57Fe3+ doped at very low levels into proto-imogolite and imogolite indicated that the sites of substitution were better defined in the latter. At least part of this Fe3+ may have been incorporated in the structure of boehmite, an impurity formed during synthesis.

The rate of dissolution of synthetic Al-goethites with 0–10 mole% Al in 6 m HCl at 24°C decreases markedly with increasing extent of Al substitution. Most of the dissolution-time curves are S-shaped, suggesting some increase in surface area during the initial stages of dissolution. Based on electron microscopic examination, the increase in surface area is explained by preferential dissolution of less ordered ‘interdomainic’ zones and the consequent production of isolated ‘domains’. Thin sections of crystals provided some support for defect zones oriented parallel to the crystallographic c-direction. Stepwise multiple correlation analysis employing various properties of the goethites was used to investigate the large variation in half-dissolution time (1–96 h). 94% of the variation could be explained by the variation of the mean crystallite dimension perpendicular to the planes (110) and (111) (MCD110, MCD111). Inclusion of Al-substitution increased R2 to only 97%. As shown in a separate experiment, increasing Al concentration in the system retarded crystal growth. It is therefore believed that Al affects the dissolution rate of goethites not directly, but indirectly, by influencing crystal growth rate which, in turn, affects crystal size and order as measured by MCD110 and MCD111.

Mineralogical investigations of weathered andesitic bedrock and overburden on the valley flank of the Tondano river, Manado, Sulawesi, indicated that these had been subjected to different weathering conditions. The underlying andesite was completely altered to 7Å-halloysite and allophane. In the transported latosols of the overburden, 10Å- and 7Å-halloysites were found. The ratio of 10Å- to 7Å-halloysite increased linearly with depth in the profile. It is considered that weathering of the andesitic bedrock reflects a palaeoclimate with more strongly pronounced seasonal desiccation and lower annual rainfall. Neoformations by weathering of the overburden took place under the recent tropical climate.

Des kaolinites sont exploitées dans les sédiments du Tertiaire inférieur des Charentes, constitués d'argiles, de lignites, de sables noirs à gros bois et pyrite et de sables roux. Deux niveaux de kaolinite sont exploités: (i) des kaolinites ordonnées accompagnées de muscovite, (ii) des kaolinites désordonnées toujours accompagnées d'anatase. Ces deux niveaux correspondent à deux épisodes sédimentaires successifs, datés du Sparnacien et du Cuisien par la palynologie. Le premier épisode est vraisemblablement issu de la reprise d'altérations éablies sur le socle cristallin du Massif Central, alors que le second reprendrait des matériaux issus de l'altération des roches sédimentaires des bordures du bassin. Des kaolinites très bien ordonnées et de la gibbsite résultent de transformations géochimiques au sein des dépôts. Elles sont liées aux milieux acides des dépôts ligniteux et à l'oxydation des pyrites qui amènent la mobilisation de l'aluminium.

A green smectitic mineral from Jordan with 16% Cr2O3 (almost 22% on an ignited basis) has been shown by chemical, X-ray, electronoptical, electron microprobe, infrared and thermoanalytical evidence to be an iron-free volkonskoite of composition:

The octahedral occupancy of 4·7, with only Mg2+ and Cr3+ in octahedral positions (in a ratio of 1·14), suggests that the mineral is intermediate between di- and trioctahedral—an inference supported by the uniformity of composition of individual particles (as revealed by the microprobe) and IR evidence, which indicates that it is predominantly dioctahedral but with certain trioctahedral characteristics. A volkonskoite sample from the type area in the USSR, examined for comparison, contained 23·5% Cr2O3 (about 29% on an ignited basis), but proved on electron microprobe examination to be a mixture of at least three species, one of which was aluminium-rich. IR and thermoanalytical characteristics are discussed in relation to the existence of tri-dioctahedral species.

Mineralogical properties (XRD, DTG) and chemical compositions of some chlorite-smectite interlayer minerals in alteration products of ophiolitic rocks from the Northern Apennines are presented and discussed. The presence of iron hydroxides and the continuous variation in Al2O3 content suggest that the corrensites are intermediate stages in the process of alteration of chlorite to smectite in an environment characterized by high element mobility. A thermal test to characterize the swelling component of the interlayer species is proposed.

Transmission electron microscopic (TEM) examination of the <0·1-µm fraction of montmorillonite and regularly interstratified illite-smectite (I-S) shows that these clays, when dried from suspension, consist primarily of particles 10 Å and 20 Å thick respectively. However, X-ray diffraction (XRD) examination of sedimented aggregates of montmorillonite indicate that the effective number of unit cells that are diffracting coherently is ∼9. This discrepancy can be reconciled by postulating an interparticle diffraction effect from the sedimented aggregates of oriented particles. The interfaces of these particles are capable of adsorbing water, ethylene glycol etc. so that on this basis smectite is composed of elementary silicate particles 10 Å thick, and regularly interstratified I-S is primarily composed of elementary ‘illite’ particles 20 Å thick, values which are in agreement with the TEM observations. This concept is confirmed experimentally by XRD examination of sedimented aggregates from mixed suspensions of both materials; the resulting patterns are identical to those of randomly interstratified illite and smectite layers, which indicates that the layer sequence examined by XRD has been entirely rearranged. It is concluded that the use of XRD peak breadth to determine mean crystal thickness cannot be reliably applied to these systems. Standard XRD data from sedimented aggregates may not be able to distinguish between true interstratification and interparticle diffraction effects of intimate physical mixtures.

Mössbauer spectra of Fe2+Fe3+ and Fe2+Al hydroxycarbonates taken in the paramagnetic state were fitted using two Fe2+ and Fe3+ doublets each. These spectra can be interpreted in terms of an ordered cation arrangement in the pyroaurite-type structure. The inner Fe2+ doublet seems to be the outcome of excess Fe3+ in the samples studied. The outer Fe2+ doublet (quadrupole splitting ∼2·8 mm s−1, FWHM = 0·26 mm s−1) is relatively well-defined and can, together with the fact that this doublet disappears on oxidation, possibly serve to identify these compounds in natural systems.

When describing clay minerals of natural origin one often has to take into account the possibility that they contain other phases, e.g. iron oxides and hydroxides which may form a ‘coating’ on the clay mineral itself or occur as physically inseparable ultra-fine particles. These iron oxyhydroxides are sometimes difficult to detect by XRD as a consequence of their very small ‘domain volumes’ (Kodama et al., 1977). The iron oxyhydroxides are normally removed chemically prior to detailed characterization of the clay mineral. Removal of iron oxyhydroxide coatings facilitates dispersion and subsequent particle-size fractionation of minerals and soils and also enhances the parallel orientation, and therefore the intensity, of basal X-ray reflections of layer-silicates.

Sepiolite is an Mg-silicate with fibrous morphology. Its structure consists of talc-like ribbons parallel to the fibre axis. Although the tetrahedral sheets are continuous, the apices in adjacent ribbons point in opposite directions, each ribbon alternating with an open channel along the fibre axis. Tetrahedral positions are normally filled with Si atoms. The width of the ribbons corresponds to eight octahedral positions, which are, normally occupied by Mg atoms, and the channel cross-section is 3·8 x 9·4 Å. This structure gives the mineral a high adsorptive capacity and molecular sieve capability. The ideal mineralogical formula is (OH2)4(OH)4Mg8Si12O30.8H2O (Brauner & Preisinger, 1956).

Activation of sepiolite by acid attack has been extensively studied (Fernández-Alvarez, 1972; Jiménez-López et al., 1978; Rodríguez-Reinoso et al., 1981). This note describes the changes undergone by sepiolite during dissolution of the octahedral Mg-sheet by HCl attack, silica gel being the end-product. This process was followed by IR spectroscopy, study of Mg-extraction kinetics, and morphological observations by SEM and TEM.

The amount of Fe and Al extracted from soil by 0·1 M pyrophosphate (Fep, Alp) has long been used as a criterion in the designation of spodic horizons of podzols (Avery, 1973). Although Adams et al. (1980) showed that in a group of podzolic soils developed from Lower Palaeozoic sedimentary (LPS) rocks Fep was very closely correlated with the quantities of Fe eluviated and illuviated, such evidence validating the use of pyrophosphate is not generally available. Recently there has been renewed interest in the identification of forms of Fe and Al extracted by pyrophosphate in order that greater confidence might be gained in the use of Fep and Alp in soil classification (McKeague & Schuppli, 1982).

Our interest in pyrophosphate extraction was stimulated when Nto (1981) discovered that Fep was an excellent predictor of P sorption in soils developed from LPS rocks. Indeed, the level of predictability was not improved by the inclusion, in multiple regressions, of either Fe or Al extracted by any other of the commonly used selective extractants.

The spectrum of Ni-substituted lizardite, (Mg,Ni)3Si2O5(OH)4, has been discussed by many authors, who have successively given contradictory interpretations concerning the symmetry of the Ni-sites. Lakshman & Reddy (1973) affirmed that in garnierites Ni is in a tetrahedral site. Faye (1974) rejected this conclusion and showed that Ni2+ occupies an octahedral site. However, the Ni … O distance of 2·01 Å which he deduced from the intensity of the crystal field is unacceptable. Recently, using a completely different approach to the interpretation of the spectrum, Cervelle & Maquet (1982) claimed that Ni2+ occupies a six-coordinated site of C3v symmetry, and concluded that the mean Ni … O distance is 2·06 Å.

The aim of this note is to show that it is possible to interpret the spectrum of Ni-lizardite as arising from a predominantly octahedral field with a weak C2v component which provokes a small increase in width of the absorption bands.

In recent years, the availability of high-field superconducting magnets complemented with mechanical spinning of the samples (magic-angle spinning technique) has resulted in a considerable improvement in resolution of NMR spectra and has widened the use of this technique in the structural analysis of solid samples. In particular, NMR spectroscopy can distinguish the different chemical environments of tetrahedrally coordinated silicon and also determine short-range order in the distribution of Si and Al in micas (Sanz et al., 1983) and zeolites (Fyfe et al., 1983; Klinowski et al., 1982).

For aluminium (I = 5/2), the existence of strong quadrupolar interactions means that, in general, only the m½↔m−½ transition is observed. However, the signal is broadened by second-order quadrupolar effects, dipolar interactions and chemical shift anisotropies.