Radiometric dating of fault gouges has become a useful tool for regional tectonics studies and for exploring and understanding fault and earthquake processes. Methods to define the absolute age of faults achieved a solid scientific foundation almost 25 years ago when the development and application of illite age analysis for investigating sedimentary burial and thermal histories found a new potential application – defining the age of fold-and-thrust development. Since then, the methods have benefitted from further development and incorporation of the 40Ar/39Ar micro-encapsulation method and quantitative clay mineral evaluation to distinguish polytypes (Wildfire). These refinements to the methods have improved their application in fold-and-thrust terrains and have opened up applications in normal and strike-slip fault environments. Another important development is the use of absolute dating methods in retrograde clay gouges in which clays in a fault develop from igneous or metamorphic wall rocks that contain no clays. In addition, the method has also been shown to be useful at dating folds in fold-and-thrust belts. We think the method is now an established part of the geological toolkit, look forward to future fault structural and tectonic studies that incorporate fault ages and hope that researchers continue to probe and discover ways that the method can assist fault process studies, including earthquake fault studies.

Bernard Kübler measured illite ‘crystallinity’, the half-height width or full width at half maximum (FWHM) of the X-ray diffraction line of illite/mica at 10 Å, directly on the diffraction traces; this procedure has since been followed by the vast majority of workers. However, some workers have recently measured the FWHM of the fitted Pearson VII function rather than on the diffraction traces. The FWHM of this function for low-angle phyllosilicate diffraction peaks (FWHM*PVII) is almost consistently ‘broader’ than those measured directly on the diffraction trace profiles (FWHMtrace) by up to 0.08°Δ2θ for the broader peaks. The Pearson VII function shows gentle curvature (‘smoothing’) at its tops and fast fading of the tails relative to virtually all 10 Å diffraction traces. The broad FWHM*PVII results from the consequent lowering/’under-fitting’ of the peak tops and the upper tails and compensatory broadening/’over-fitting’ of the intermediate peak flanks. FWHM*PVII ‘contraction’ with respect to FWHMtrace and enhancement of the peak maximum is found on traces of muscovite strips. The fitting reliabilities of the Cauchy function are almost invariably better than those of the Pearson VII function. Their FWHM*Cauchy values are narrower for both the illite/mica 10 Å and the chlorite 7 Å reflections; although they still differ somewhat from the FWHMtrace, they are much closer, usually within 0.02°Δ2θ. This markedly lesser broadening of FWHM* of the Cauchy of the Pearson VII function is the result of its stronger top curvature and notably faster tail fading (less ‘smoothening’). For higher-angle mica peaks, the FWHM* values of the Pearson VII and Cauchy functions converge, usually differing only by 0.01–0.03°Δ2θ for the 5 Å peak, and even less for the 3.3 Å peak. It is therefore strongly recommended that FWHM values of the illite/mica 10 Å reflections be measured on the diffraction traces rather than on fitted functions. Where peak fitting is unavoidable (e.g. in order to separate the contributions of adjoining, partly resolved or unresolved reflections on broadened 10 Å reflections), Cauchy rather than Pearson VII functions should be used.

A new set of clay mineral ‘Crystallinity’ Index Standards (CIS) is available for improved calibration of the half-peak-width values of the X-ray diffraction 001 illite reflection (the Kübler index) and the 002 chlorite reflection (the Árkai index), two widely used indices for determining the state of prograde diagenesis and low-temperature metamorphism. Calibration using mudrock standards removes the numerical differences between laboratories caused by variations in sample preparation, machine settings and measurement methods, thus avoiding erroneous grade determinations. The number of standards available has been increased to nine. These can be used to obtain Kübler index values for each CIS sample and Árkai index values for upper anchizonal and epizonal samples. The diagenetic and lower anchizonal mudrocks are not suitable for Árkai index measurements due to the absence of chlorite or overlap by the 001 kaolinite reflection. Applying the new Kübler-equivalent upper and lower boundary limits of the anchizone placed at 0.32°2θ and 0.52°2θ, respectively (Warr & Ferreiro Mählmann, 2015), the nine standards from the Palaeozoic mudrocks of southwest England now comprise two diagenetic, two lower anchizonal, three upper anchizonal and two epizonal grade samples. These range from weakly cleaved mudstones to strongly foliated slates.

The [MnO|SiO2,Al2O3,FeO,MgO] balanced ratio (i.e. the isometric log-ratio of the MnO concentration relative to the concentration of SiO2, Al2O3, FeO and MgO) of chlorite and of whole-rock composition is an effective discriminant between Mesozoic stratigraphic formations in the Magallanes Basin (Chile). The MnO content in chlorite is only controlled by the host rock chemistry and is dependent on the geological environment. The MnO content in chlorite remains unchanged at low-grade metamorphic conditions. Single-grain chlorite analysis (n = 1042, electron microprobe) and whole-rock analysis (n = 40, X-ray fluorescence) were used to discriminate stratigraphic formations and to decipher differences in the depositional environment in the Magallanes Basin. The samples are from one Upper Jurassic and three Cretaceous sedimentary units that were affected either by low-grade regional metamorphism or by Miocene contact metamorphism. The highest [MnO|SiO2,Al2O3,FeO,MgO] values are recorded in the upper Zapata Formation. The Punta Barrosa, Cerro Toro and Tobífera Formations show slightly lower [MnO|SiO2,Al2O3,FeO,MgO] values. Elevated [MnO|SiO2,Al2O3,FeO,MgO] values at the transition between Zapata and Punta Barrosa Formations record an oxygenated shallow marine environment that can be linked to the closure of the Rocas Verdes Basin and the onset of fold-and-thrust belt formation. Decreasing [MnO|SiO2,Al2O3,FeO,MgO] values from the Punta Barrosa towards the Cerro Toro Formation indicate gradually increasing water depths during the Upper Cretaceous that correlate well with the global sea level.

The layered uranyl silicate clay-like mineral, uranophane-α, Ca(UO2)2(SiO3OH)2·5H2O, was studied by first-principles calculations based on the density functional theory method. The structure, observed in nature in a wide variety of compounds having the uranophane sheet anion topology, is confirmed here for the first time by means of rigorous theoretical solid-state calculations. The computed lattice parameters, bond lengths and bond angles were in very good agreement with the experimental ones, and the calculated X-ray powder trace accurately reproduced its experimental counterpart. The mechanical properties of uranophane-α, for which there are no experimental data for terms of comparison, were determined, and the satisfaction of the mechanical stability Born conditions of the structure was demonstrated by calculations of the elasticity tensor. The Raman spectrum was computed by the density functional perturbation theory and compared with the experimental spectrum. The vibrational properties of this mineral were well characterized, showing a good performance in all of the studied spectral range. Theoretical methods allowed assignment of the Raman bands to vibrations localized in different fragments within the crystal unit cell. Finally, the possibility of incorporation of strontium inside the uranophane structure was studied. The computed structure, X-ray powder trace and Raman spectrum of Sr-exchanged uranophane were very close to those of the ordinary Ca-uranophane.

Kaolinite can be used for many applications, including the underground storage of gases. Density functional theory was employed to investigate the adsorption of hydrogen molecules on the kaolinite (001) surface. The coverage dependence of the adsorption sites and energetics was studied systematically for a wide range of coverage, Θ (from 1/16 to 1 monolayer). The three-fold hollow site is the most stable, followed by the bridge, top-z and top sites. The adsorption energy of H2 decreased with increasing coverage, thus indicating the lower stability of surface adsorption due to the repulsion of neighbouring H2 molecules. The coverage has obvious effects on hydrogen adsorption. Other properties of the H2/kaolinite (001) system, including the lattice relaxation and changes of electronic density of states, were also studied and are discussed in detail.

The present study examined the microstructure and mechanical properties of ceramic composites based on a kaolin from Djebel Debbagh, northeast Algeria, composed mainly of kaolinite and halloysite with the addition of various amounts of BaCO3. The composites were prepared by high-energy ball milling and sintered at 1100°C and 1200°C for 3 h. The samples sintered at 1200°C without BaCO3 were composed mainly of mullite, which disappeared with increasing BaCO3 content. X-ray diffraction investigation showed the presence of hexacelsian (BaAl2SiO6 and BaAl2Si2O8), which disappeared at BaCO3 contents >50 wt.% in favour of barium aluminium and barium silicate phases. At 40 wt.% BaCO3 content, the porosity of the composites decreased from 0.7% to 0.1% and the microhardness increased from 7 to 8 GPa, respectively, at 1100°C and 1200°C due to the amorphous phase.

The recent discovery of a Roman ceramics manufacturing workshop at Montelabate (Perugia, Italy), in use from the first century BC until the late-fourth to fifth centuries AD, offers a unique opportunity to study the technical processes for producing Roman amphorae. Ancient and modern clays were sampled and analysed; they do not differ significantly, supporting the hypothesis of the exploitation of the rich local clay source that allowed a continuity of production. Characterization of the clays was performed using geotechnical methods (Atterberg limits and size distribution) and by thermogravimetric and differential thermogravimetric analysis, Fourier-transform infrared spectroscopy, X-ray diffraction and X-ray fluorescence analyses. The material was suitable for pottery making with the addition of calcite and quartz sand temper. Production waste and discarded materials as well as good-quality products were also analysed with the same methodology. It is therefore possible to reconstruct the ancient technology by defining the recipe for the production of the amphorae and their firing temperature on the basis of the decomposition of clay materials and the presence of newly formed minerals.

Five clay deposits in Foumban, west Cameroon, were studied for their morphological, mineralogical and geochemical properties to determine their suitability for ceramics. The clays were examined with X-ray diffraction, X-ray fluorescence, thermal gravimetric analysis and Fourier-transform infrared spectroscopy. Field studies showed that a homogeneous clayey layer occurs at the upper part of the laterite cover of the interfluves, while the valleys are occupied by a clayey heterogeneous hydromorphic material. The clays are composed of kaolinite, illite, smectite and chlorite, associated with quartz, K-feldspars, plagioclase, goethite, traces of rutile and hematite. Geochemical analyses of these samples show a relatively large amount of SiO2 (45–71%), Al2O3 (14–31%) and relatively little Fe2O3 (up to 11%), suggesting weathering of mainly granitic and rhyolitic parent rocks. The majority of these clays may be used in the production of structural ceramics such as bricks (refractory or not) and tiles. The relatively high proportion of the alkalis (K2O + Na2O; 6–8%) in some samples from Marom and Njindare areas might be responsible for the low firing temperatures. The abundance of smectite limits the application of some Koutaba and Marom clays for structural ceramics, while the high Fe2O3 contents (>8%) in some Bangourain clays indicate that some pre-treatment might be necessary prior to use.

Three clayey materials named MY3, KK and KG originating from the Foumban region (west Cameroon) were analysed to determine their granulometry, plasticity, major-element chemistry and mineralogy. Dilatometric and ceramic behaviour were also investigated. Clays were shaped by uniaxial pressing in a steel mould. Shaped samples were heated at 1300, 1400 and 1500°C. The end products were characterized in terms of their density, porosity and compressive strength. Raw materials differ in terms of their mineralogical composition, grain-size distribution, Al2O3 content and the nature and abundance of impurities inducing specific thermal behaviour during dilatometric analysis and sintering tests. The final material properties may be related to the main features of the raw materials used.

Textural, mineralogical and chemical characterization of archaeological ceramics (zellige) from El Badi Palace (Marrakech, Morocco), the main Islamic monument from the Saadian period (sixteenth century), has been performed to enhance restoration and to determine the technology of manufacturing. A multi-analytical approach based on optical and scanning electron microscopy, cathodoluminescence, X-ray fluorescence and X-ray diffraction was used. Re-firing tests on ceramic supports were also performed to determine the firing temperatures used by the Saadian artisans. A calcareous clay raw material was used to manufacture these decorative ceramics. The sherds were fired at a maximum temperature of 800°C in oxidizing atmosphere. The low firing temperature for ‘zellige’ facilitates cutting of the pieces, but also causes fragility in these materials due to the absence of vitreous phases.

Compositional and microstructural analysis of mullites in porcelain whitewares obtained by the firing of two blends of identical triaxial composition using a kaolin B consisting of ‘higher-crystallinity’ kaolinite or a finer halloysitic kaolin M of lower crystal order was performed. No significant changes in the average Al2O3 contents (near the stoichiometric composition 3:2) of the mullites were observed. Fast and slow firing at the same temperature using B or M kaolin yielded different mullite contents. The Warren–Averbach method showed increase of the D110 mullite crystallite size and crystallite size distributions with small shifts to greater values with increasing firing temperature for the same type of firing (slow or fast) using the same kaolin, as well as significant differences between fast and slow firing of the same blend at different temperatures for each kaolin. The higher maximum frequency distribution of crystallite size observed at the same firing temperature using blends with M kaolin suggests a clearer crystallite growth of mullite in this blend. The agreement between thickening perpendicular to prism faces and mean crystallite sizes <D110> of mullite were not always observed because the direction perpendicular to 110 planes is not preferred for growth.

The adsorption and desorption of cesium onto layered minerals, zeolite and geochemical reference samples were studied. 0.5 g of bentonite and mica were able to adsorb 71.2 and 51.5 mg of cesium, respectively, from 50 mL of deionized water containing 200 mg/L of cesium under neutral pH condition. These amounts of cesium adsorption were greater than those reported for vermiculites (8.9 and 5.6 mg, respectively). Additionally, the cesium adsorption on mica and vermiculite remained essentially unchanged under seawater conditions, but it decreased drastically on zeolite. The cesium desorption from the layered minerals was promoted by the addition of ammonium ions, namely trioctylmethylammonium chloride and zephiramine. These ammonium ions desorb cesium from the interlayers of the minerals without destroying the mineral structure. The cesium desorption procedure using quaternary ammonium ions would be extremely useful for decontamination of soil containing the layered minerals with adsorbed radioactive cesium.

This work presents the synthesis of minerals with a layered structure in supercritical water/ethanol mixtures to decrease the critical coordinates of the solvent regarding water. Depending on the water/ethanol ratio of the solvent, we obtained three different minerals adopting an octahedral brucitic sheet: (1) without a Si-tetrahedral sheet (O); (2) associated with one Si-tetrahedral sheet (T-O); or (3) intercalated between two Si-tetrahedral sheets (T-O-T). We have thus shown that ethanol in a supercritical water/ethanol mixture changes the solubility of silicon with a direct consequence on the formation of the tetrahedral silicon sheets and thus makes it possible to control the structure of the synthesized layered material.

The present study investigates the use of magnesium sulfate (MgSO4) instead of (NH4)2SO4 as a lixiviant in the recovery of rare earth elements (REEs) from clays. Experiments were carried out to investigate the influence of leaching conditions such as leaching time, lixiviant concentration and liquid:solid ratio on the leaching efficiency. The optimum leaching conditions, leading to 75.48% of total REE leaching efficiency, required a stirring speed of 500 rpm, a leaching time of 30 min, a lixiviant concentration of 3 wt.% and a liquid:solid ratio of 3:1. After extension of the leaching process by a second stage, the leaching efficiency may reach up to 96.19%, which is slightly higher than that obtained by (NH4)2SO4. Leaching varies from element to element, with Ce presenting the lowest leaching efficiency, and the partition in leaching solution is in agreement with that in raw ore other than for Ce. Based on these findings, MgSO4 lixiviant is an excellent alternative leaching agent for a sustainable REE industry because it reduces or eliminates NH4+–N pollution.

The Gümüşköy kaolin clay deposit, located ~40 km northwest of Kütahya, Turkey, has promising reserves for ceramic production. In order to evaluate the potential use of this region, five representative clay samples were collected and characterized with X-ray diffraction, polarized light optical microscopy, X-ray fluorescence spectrometry, differential thermal analysis and scanning electron microscopy coupled with energy-dispersive X-ray spectroscopy. According to the Chemical Index of Alteration and Plagioclase Index of Alteration, the kaolin samples were formed by strong chemical alteration of the Tavşanlı volcanites. The clays contain kaolinite, illite, pyrophyllite, montmorillonite and minor halloysite, as well as quartz, plagioclase (andesine), biotite, sanidine, rutile and goethite. The clay samples were used for thermal and physical tests. The plasticity, particle-size distributions firing properties (water absorption, apparent porosity, linear shrinkage, dry bending strength, fired bending strength) and colour parameters of the fired samples were determined. The kaolinitic clay samples from Gümüşköy are suitable for making wall tiles, floor tiles and kiln wares after shaping by dry pressing and extrusion.

The adsorption capacities of nano-sized organoclays composed of a stevensite-rich clay (R), phosphorus dendrimers (GC1 and GC2) and Na+-saturated clay were evaluated for their capacity to adsorb chromate and methylene blue (MB) in the range of 298–318 K. The adsorption kinetics and the isotherms were analysed based on kinetic equations and isotherm models and by adopting a non-linear regression procedure. In addition, the organoclays and the Na+-saturated clays were characterized principally by solid-state nuclear magnetic resonance spectroscopy. The pseudo-second-order rate equation described kinetics data well, and the adsorption rates were not limited by the intraparticle diffusion or by the liquid film diffusion. Both chemical species were adsorbed spontaneously (–31 < ΔG°T< –10 kJ/mol), but the adsorbents had a high affinity for MB species. The adsorption isotherms of chromate were fitted better by the Freundlich model, while those of MB followed the Langmuir model. Chromate adsorption took place at the edges and the free surfaces of stevensite, particularly at the protonated aluminols. MB was adsorbed as MBH2+ and MB+. The MB protonation occurred at the clay surfaces, and MB+ ions were located at the planar surfaces of stevensite as well as at the external surfaces of aggregates. Moreover, the tetrahedral sheet of stevensite involved in the formation of GC1-based organoclays was the subject of a partial chemical modification.