This paper describes the ways in which the major rock-forming primary minerals (olivine, pyroxenes, amphiboles, feldspars, micas and chlorites) break down during weathering, the products that develop during this breakdown and the rates at which this breakdown occurs. The perspective chosen to illustrate this vast topic is that of the residual soil weathering profile. Different physical and chemical conditions characterize the various parts of such a profile. Thus, in the slightly weathered rock at the base of the profile, mineral weathering will take place in microfissures and narrow solution channels and the capillary water in such spatially restricted volumes may be expected to be close to equilibrium with the primary mineral. In these circumstances, the weathering product formed may be closely related to the primary mineral both compositionally and structurally. The saprolite higher up in the weathering profile may or may not retain the fabric and structure of the original parent rock, but in either case the close relationship observed between primary mineral and weathering product in the slightly weathered rock may be lost. This part of the profile will usually be affected by freely flowing drainage waters, the composition of which will be far from equilibrium with specific primary minerals. Weathering products which do form are likely to reflect the interaction between bulk water and bulk parent material. In the soil profile, the situation will be further complicated by organic ligands derived from decomposing organic matter or from the direct activities of soil microbes or plant roots. Thus, biological weathering will assume a much greater significance in this part of the profile compared with the mainly inorganic processes dominating in the saprolite and the slightly weathered rock. The general nature of any particular weathering profile will reflect the interactions between climate, topography, parent material, soil biota and time and superimposed upon this complexity, when considering how individual primary minerals break down in detail, will be factors related to the nature of the mineral itself. Particularly important in this respect is the inherent susceptibility of the mineral to weathering, which is related to overall chemical composition and structure, as well as the distribution and density of defects, dislocations and exsolution features, which often control the progress of the weathering reaction.

Image analyses, carried out on thin sections made in consolidated and sheared kaolinite test pieces, allow the identification of three ‘microstructural domains’: (1) the initial isotropic matrix; (2) a partly anisotropic matrix resulting from simple particle arrangement; and (3) an anisotropic matrix resulting from rearrangement plus flattening and delamination of particles.

In order to explain the micromechanisms of the clay matrix behaviour, this paper proposes to link the ‘microstructural domains’ represented in the e vs. log p Cam-clay diagram and domains of hydraulic conductivity in the k vs. e diagram.

The hydraulic conductivities are calculated following the Kozeny-Carman relations, which take into account the micro-arrangement of particles via a tortuosity calculation. The generation of 2D images shows that the preservation of the isotropic arrangement of particles is limited by a minimum porosity value. A decrease of the porosity value below this limit can be explained only by a progressive anisotropic rearrangement of the particles.

The microtexture behaviour, induced by the superimposition of the compaction, orientation and particle flattening and delamination stages, causes an anisotropy of the hydraulic conductivity which affects (1) the interstitial water flow direction, (2) the rotation of particles itself, and (3) the damage mechanism of the clay.

The hydrothermal reaction of kaolinite in the systems Li2O–K2O–MgO–Al2O3–SiO2–H2O–HCl and Li2O–Na2O–MgO–Al2O3–SiO2–H2O–HCl, at pH between 9 and 11, has been investigated at 200–250°C from 1 to 180 days.The X-ray diffraction study of the K-bearing system indicates that the solid products formed were Li-A(BW) zeolite, crystalline silica, and randomly ordered chlorite-smectite mixed layers, dioctahedral on average, with a chlorite:smectite ratio of ~1:4.Study by electron microscopy reveals, however, the presence of mixtures of very thin particles of smectite, di,trioctahedral chlorite and mixed-layer phases.The X-ray study of the solid products formed in the Na-bearing system reveals that the neo-formed phases were Li-A(BW) zeolite, analcime and partially ordered chlorite-vermiculite mixed-layers with high chlorite:vermiculite ratio. The transmission microscopic study of these phases shows the presence of dioctahedral ‘vermiculitic’ curved particles and di,trioctahedral chlorite-like particles.In both cases, long reaction times cause formation of a 7 Å phase with composition intermediate between kaolinite and serpentine.The composition of the solutions suggests that the stable phases would be chlorite/smectite mixed-layers, with compositions near tosudite in the case of the K-bearing reaction and both a dioctahedral phase (tosudite?) and a trioctahedral one (clinochlore) in the case of the Na-bearing reaction.

Lateritic weathering profiles developed on serpentinized peridotites of Murrin Murrin (Western Australia) exhibit thick smectite zones (10–15 m). The smectites from plasma and fissures were characterized by XRD, chemical analyses (ICP-AES, SEM-EDX and TEM-EDX) and Mo¨ssbauer spectroscopy. These Fe-rich smectites, previously described as nontronites, are in fact more complex. Their layer charges originate from both the tetrahedral and octahedral sheets. Plasma and notably fissure smectites exhibit, from the bulk sample scale to the particle scale, large and continuous Al for (Fe+Cr) substitutions, covering a chemical gap previously described for dioctahedral smectites ranging between nontronite and beidellite end-members. Lastly, they exhibit an octahedral occupancy slightly above 2, due to a low (Mg+Ni) trioctahedral contribution. Thus, the smectites occurring in weathering profiles of ultrabasic rocks can have actual chemistries intermediate between four dioctahedral end-members (beidellite, nontronite, montmorillonite and previously rarely described ferric-montmorillonite) and a trioctahedral one ((Mg+Ni)-saponite).

Mineralogical and chemical variations were studied in the upper half of a 1 m thick discontinuous bentonite bed interlaminated in the Lower Cenomanian sedimentary formations of the northern Aquitaine basin (France). X-ray diffraction patterns obtained from the <2 mm fraction in the Ca and K-saturated states were decomposed and compared to those calculated from decomposition parameters. They revealed the presence of two highly expandable illite-expandable (I-Exp) mixedlayer minerals (MLMs). The relative proportions of the two MLMs evolve steadily with depth leading to the decrease of the cation exchange capacity and of the (Na + Ca) content towards the centre of the bentonite bed. However, the system is essentially isochemical and Mg, Al, Si, K and Fe are roughly constant in the bulk samples. It is thought that the mineralogical zonation results from the initial stages of the smectite formation in an ash layer.

In the Ca-saturated state, the expandable component of these MLMs was for the most part homogeneous with the presence of 2 sheets of ethylene glycol molecules in the interlayer. However, the heterogeneous hydration behaviour of these expandable layers was enhanced by the K-saturation test. From this test, the presence of three layer types with contrasting layer charge was evidenced from their contrasting swelling abilities. The C12-alkylammonium saturation test applied to samples in which the octahedral charge had previously been neutralized (Hofmann-Klemen treatment) showed that the tetrahedral charge is located on specific layers. These layers are responsible for the heterogeneous hydration behaviour. Low-charge smectite layers are mostly octahedrally substituted, whereas for intermediate- and high-charge layers this montmorillonitic charge is complemented by additional tetrahedral substitutions (0.30 and 0.35–0.40 charge per O10(OH)2, respectively).

This study uses transmission electron microscopy (TEM) and analytical electron microscopy (AEM) supported by X-ray diffraction (XRD) and scanning electron microscopy (SEM) to investigate the mineralogical and textural changes produced in carbonated clays by firing. Sample bars were prepared using raw clays composed of quartz, illite and carbonates with minor amounts of smectite, chlorite, feldspars and Fe oxides. The raw samples were then fired at temperatures of between 800 and 1050°C. The XRD data show that increases in firing temperature result in dehydroxylation of clay minerals, carbonate decomposition and the formation of Ca-bearing silicates (e.g. gehlenite, wollastonite, pyroxenes and anorthite). The sizes of the Ca-silicate crystals make the use of the SEM inappropriate since they lie below the resolution threshold. However, TEM/AEM do provide the required textural and compositional characterization, revealing that there is a broad range of pyroxene compositions, some of which resemble fassaite, and that Ca/Mg ratios increase with temperature. The TEM also shows significant dehydroxylation and vitrification of the clay-rich matrix at T of ~800°C. Observed mineralogical and textural changes probably occurred in a system with a local disequilibrium much like small-scale, high-temperature metamorphic reactions (i.e. pyrometamorphism). The importance of these results is that they enable the selection of more appropriate raw clay composition and firing dynamics (temperature, firing duration and cooling rate) for both the brickmaking industry and brick conservation in the field of cultural heritage.

Fe3+-doped K10 montmorillonite combined with 2,6-di-tert-butyl-4-methyl-phenol is a potent catalytic system for trans-2-methylene-1,3-dithiolane 1,3-dioxide as a dienophile. Cycloaddition occurs with cyclopentadiene and furan as a diene at room temperature in short time and with good yields (64% overall).

Kaolinites were modified by controlled hydrothermal deuteration. Both infrared and near infrared vibration spectra of deuterated kaolinite were investigated by Diffuse Reflectance Infrared Fourier Transform spectroscopy. The stretching vibrations (v) of deuteroxyl groups of the kaolinite were observed at 2726, 2709, 2697 and 2667 cm-1, respectively. The overtone bands (2n) of the deuteroxyl groups were visible in NIR regions at 5454, 5419, 5388 and 5342 cm-1, respectively. The lattice vibrations of the kaolinite taking part in deuteration included 937, 915, 792, 751 and 690 cm-1 which are assigned to hydroxyl group-related deformation or translation-vibration mode. The hydroxyl band at 3435 cm-1 with its deuteroxyl band at 2542 cm-1 resulted from unevenly distributed structural defects, either due to the isomorphous substitution of foreign cations or the imperfections of the kaolinite lattice structure.