Samples of the mineral pyroaurite, formed from the weathering of partially serpentinised harzburgite (olivine + pyroxene) were found in an arid region of the Sultanate of Oman. These were either golden or silver in colour depending on the horizon from which they were derived. Chemical analysis showed that the colour variation was primarily due to the differing conditions in the hydrological environment. The golden colour was attributed to small Fe(III) oxide particles detected by Mössbauer spectroscopy. In addition, the samples were examined by X-ray diffraction, scanning electron microscopy, and glycerol intercalation. These results were compared with a synthetic pyroaurite sample prepared under conditions (previously reported) similar to those in nature. These conditions are shown to approximate to those found in the hydrological environment in the zones of the natural pyroaurite formation.

Strongly held carbonates anions in the interlayer region of pyroaurite-type compounds, [MlII1−xM2IIIx(OH)2]x+[(CO3)x/2yH2O]x− (x = 0·15−0·44), can be relatively easily exchanged by other anions such as NO3−, Cl− or SO42− dissolved in heated glycerol. Some Fe(III) of pyroaurite is reduced to Fe(II) by the glycerol treatment. If pyroaurite is treated with glycerol in the absence of dissolved anions or with glycerol vapour at 120°C, glycerol becomes intercalated in the interlayers. In all examples the interlayer CO32− is partially or completely lost as CO2. In aqueous salt solutions a glycerol intercalated form allows other anions to substitute in the interlayer. Expansion in the c-axis direction following glycerol treatment depends on (1) the method of glycerol treatment, (2) the trivalent metal in the octahedral sheet, and (3) the interlayer anions. Heated glycerol causes a different expansion to that produced by its vapour phase at 120°C. The variation in d(003) follows the series: hydrotalcitevap. > pyroauritevap. > hydrotalciteliquid > pyroauriteliquid > SO42−-pyroauritevap. > SO42−-hydrotalcitevap., where liquid refers to glycerol used at 160–180°C and vap. refers to glycerol vapour at 120°C Carbonate-free, glycerol-intercalated pyroaurite compounds may serve as re-usable anion absorbers. Glycerol intercalates of pyroaurite-type compounds may also be useful to study and identify stacking sequences and interlayer compositions in these minerals.

Structural features of very highly disordered 2:1 trioctahedral phyllosilicates from the Madrid Basin have been studied. Application of Fourier methods to XRD patterns shows that these materials consist of random kerolite-stevensite mixed-layered phases (R = 0) with 50–80% kerolite layers, and have a very small particle size with a mean of three unit-cells stacked in the c* direction. IR spectrum deconvolution in OH-stretching and librational modes seems to indicate five different types of water. Specific surface measurements (N2, BET) indicate anomalous high values (240–320 m2/g). These results suggest important catalytic and surface chemistry applications.

The sequential development of kaolin by progressive alteration of smectite, involving kaolin-smectite interstratifications as a genetic link is described from a red and black complex. Mineral compositions were studied using XRD, DTA and XRF techniques. The basalt-derived soils are situated along a 600 m transect and grade in colour from dark grey (10 YR 3/1) to red (5 YR 3/3). The kaolin proportions in the interstratification increase almost linearly with increasing reddening up to ∼80%. Whole-soil chemical analyses exhibit no significant variations in the major element composition, but dithionite extractable Fe increases along the transect from 1% to 4·16%. Hematite and goethite are the only secondary iron phases. Topographic differences are slight but sub-surface bedrock contours plus appreciable variations in sand content between red and black soils could be genetically significant.

Smectites in the montmorillonite-beidellite pseudo-binary system were synthesized from glass at the hydrothermal conditions 100 MPa and 250–500°C. Products were analysed by X-ray powder diffraction for randomly oriented and glycolated samples, and also according to the Greene-Kelly test. A new mixed-layer mineral consisting of regularly interstratified montmorillonite-beidellite was found below 400°C for M50B50 composition, where M and B denote the compositions of ideal Na-montmorillonite and Na-beidellite, respectively. Montmorillonite was obtained as a single phase below 375°C in the composition range from M100B0 to M78B22. Above 375°C, montmorillonite decomposed into an assemblage of beidellite, saponite and a silica mineral. Beidellite as a single phase was synthesized below 400°C for M0B100 composition. Above 450°C, Na-rectorite (regularly interstratified paragonite-beidellite), Mg-free dioctahedral smectite and/or Mg-rich trioctahedral smectite appeared together with silica minerals. These smectites showed the same swelling behaviour as that of montmorillonite. Phase relations in the montmorillonite-beidellite pseudo-binary system indicate no complete solid-solution series.

Exchangeable cation associations in physical mixtures of bentonite and laponite have been examined using X-ray diffraction and a combination of electron microprobe analysis and multivariate statistical methods. The results suggest that cation exchange can occur in air-dry mixtures when pressure (∼64 kbar) is applied. Although no attempt is made to describe the mechanism of the reaction it appears to require the presence of adsorbed water.

X-ray diffraction methods are effective and practical for the quantitative analysis of the often forgotten amorphous material phase in sediments. The method reported is simple, employing normal oriented samples, and quartz or fluorite as a diluent. However, special care must be taken to prepare the samples properly, to ensure optimum conditions for diffraction, and to measure accurately the scattering bands.

The spatial and vertical distributions of six clay mineral assemblages were identified in the pelagic marls and marly limestones and in the Ammonitico Rosso of the Lower Jurassic in the Betic Cordilleras (SE Spain). The six assemblages contain varying proportions of illite, kaolinite, chlorite, smectite and mixed-layer illite-smectite. The clay mineralogy and the geochemical data, particularly for the rare earth elements, indicate that the sediments were deposited in a pelagic environment influenced by a nearby continent. The source areas were many and lithologically varied. Clay mineralogy and geochemistry are shown to be most useful in the reconstruction of the palaeogeography of a basin in a passive continental margin.

The nature, crystallochemistry, organization and surface properties of soil clays derived from the most representative sedimentary formations of France are described, using qualitative and quantitative mineralogical methods and high resolution transmission microscopy (HRTEM). The main results show that soil clays differ significantly from reference clays. Even when 2:1 phyllosilicates are dominant, the clays always contain multimineral phases. The smectitic component is dioctahedral and of beidellite type. Soil clay organization is specific: all the clay particles have a short lateral extension and few layers (mean 5 layers, and even mono-layers occur). The superposition of such small particles leads to the formation of what are called “texturally interstratified minerals” which are dominant in soil clays. Exchange and surface properties (high ratio of external surface to total surface area), are related to clay organization.