The most abundant silicates formed under the Earth's 'normal' surface conditions, i.e. clay minerals, are always of small grain size. Under the same conditions, other mineral species such as carbonates, sulphates and oxides may form much bigger crystals. The reason why phyllosilicates formed in soils or in weathered rocks are always of small grain size is not related to the low-temperature-pressure conditions but rather to particular aspects of their crystal structure. Many recently published works describe the order-disorder cation distribution in the tetrahedral and octahedral sheets and the crystal defects in the layer stacks. Related to the Periodic Bond Chains (PBCs) theory, these data suggest that the size and the shape of clay crystallites could depend on the amount of crystal defects in the three axes of symmetry [100], [10] and [0]. The accumulation of crystal defects poisons the crystal growth along one, two or three PBCs. Then, nucleation becomes less energy-consuming than crystal growth and favours the formation of numerous smaller crystals rather than fewer bigger ones.

Intercalation of surfactant and polymer chains between the clay platelets gives rise to molecular ordering that changes both the chain conformation and mobility with respect to the bulk phase. As a local probe, nuclear magnetic resonance is particularly suited for such investigations, and this review reports the main results obtained in the solid state. The properties of the modified clays are studied as a function of the surfactant loading, the nature of the head group, and the length of the hydrocarbon chain(s). The structure and charge of the mineral also influence the behaviour of molecules in the gallery space. Among papers on polymer/clay nanocomposites, those dealing with poly(ethylene oxide) have been studied in particular, using a multinuclear approach. Natural clays often contain paramagnetic species such as Fe(III) that perturb the nuclear magnetic resonance (NMR) relaxation processes and can prevent observation of appropriate NMR data. Accordingly, most organic/clay hybrids are studied with hectorite or synthetic smectites. However, the paramagnetic effect has also been found useful in characterizing clay dispersion within the polymer matrix of the nanocomposites.

Inceptisols are the major forest soils in northern Taiwan. Some chemical, physical and morphological properties have been documented for these soils, yet there is little information on the mineralogy and the charge characteristics of their constituent 2:1 clay minerals. In this study we conducted a detailed characterization of the clay mineralogy of two Inceptisols. Two pedons were sampled at diagnostic horizons and the clay mineralogy was examined by X-ray diffraction. The magnitude of the layer charge of the 2:1 phyllosilicates was estimated using the alkylammonium exchange method (nC = 12). The clay mineralogy of both soils was dominated by vermiculite and mica with small amounts of kaolinite. The surface horizon contained more mica and kaolinite than the lower horizons. The mean layer charge of vermiculite ranged between 0.60 and 0.86 cmolc/(O10(OH)2). The distribution of clay layer charge decreased with increasing soil depth in two pedons. Differences in layer charge between samples are due to differences in weathering processes. The difference in the extent of clay mineral weathering in the A and Bsm horizons could be partly because the mineral surfaces in the Bsm horizon were coated with organo-Fe complexes which protected them from weathering.

Widespread chemical weathering of Tertiary alkaline volcanic rocks in the vicinity of Trabzon (NE Turkey) has led to the formation of well developed reddish-brown saprolite. These saprolites are dominated by kaolin minerals (predominantly kaolinite), with minor quantities of halloysite, pyrophyllite, chlorite, smectite, hematite and illite. Other common minerals are opal-CT, quartz and occasional feldspar, clinopyroxene, hematite and Fe-Mn-Ti oxides. The significant degree of fracturing and fragmentation of primary igneous minerals within the pyroclastic units, and subsequent penetration by reactive fluids, resulted in partial chloritization of clinopyroxene, albitization of plagioclase and precipitation of Fe-Mn-Ti-oxides within fractures and dissolution voids. Kaolin minerals occur as spongy fibrous meshes surrounding partly dissolved volcanic glass and devitrified sub-rounded grains that may be relict allophane. Field observations and mineralogical-chemical characteristics indicate that the alteration of the Tertiary volcanic units of the Black Sea region was controlled by chemical weathering during circulation of oxic meteoric or mixed meteoric and marine waters. Intense fracturing and fragmentation of the pyroclastic rocks resulted in the formation of clay minerals in the upper saprolite level under acidic and largely oxidizing conditions. Incongruent dissolution was associated with the leaching and downward transport of Si, Na, K and Ca and the concentration of Al. In such an environment, early precipitation of halloysite from allophane was probable, and it gradually transformed into kaolinite during aging and progressive weathering.

Triassic sequences from 'intermediate units' between the Alpujárride and the Malâguide complexes (Betic Cordilleras, Spain) of the westernmost part of the Cordilleras (Casares area) occur as four superimposed tectonic units; the uppermost unit shows lithological characteristics similar to those of the Malâguide complex, changing progressively at increasing depth, towards lithologies typical of the Alpujárride complex. The units studied, with a maximum thickness of ~400 m, record important variations in metamorphic pressures, according to the b parameter of white micas: from low-pressure metamorphism (in the unper unit) to high-pressure faciès series (in the deepest one). The mean b values range from 8.988 Å in the uppermost unit (Crestellina) to 9.042 Å in the lowermost one (Jubrique). The lowest metamorphic grade is represented by mineral assemblages consisting of phengite + intermediate Na-K white mica ± Fe-chlorite ± sudoite ± pyrophyllite, which record temperatures of ~300°C and pressures of 1.5—3 kbar. At increasing tectonic depth, intermediate Na-K mica and pyrophyllite disappear and the metamorphic assemblages consist of phengite ± paragonite ± margarite + Mg-chlorite ± sudoite, which record minimum pressures of ~7 kbar and temperatures in the order of 400—450°C. These mineral assemblages provide evidence of the passage from collisional to extensional geotectonic settings. The units showing different metamorphic patterns were juxtaposed tectonically, after the development of metamorphic mineral assemblages.

Clay and glauconite fractions separated from sedimentary rocks in Israel were analysed for their mineralogical composition by X-ray diffraction and dated by the K-Ar method. The main goal was to evaluate early diagenetic processes which might have affected the K-Ar system. One series of samples was taken from two contemporaneous mid-Albian to mid-Turonian (~105–91 Ma) shallow marine successions in which early diagenetic K-feldspar was found to be widespread. Another series was of samples of large spatial distribution associated with a mid- Turonian emergence event. None of the outcrops had been subjected to high temperatures due to deep burial or other thermal events. The abundant clay minerals in the first series are randomly ordered illite-smectite (I-S) phases of various discrete and mixed compositions. Illite, kaolinite, ordered I-S and palygorskite are sometimes present. The variability in clay assemblages and I-S composition within short vertical distances and the proximity to the provenance implies mostly a detrital origin for the clay minerals. The K-Ar ages of clay fractions in the first series are no older than 30% of the stratigraphie ages, much younger than the known detritus provenance ages. A few have known stratigraphie ages and one is younger by 10%. The K-Ar ages of associated glauconite fractions are younger by 5–13% than the stratigraphie ages. The ages of the clay fractions of the second series, which are on average more illitic, are mostly younger and no older than 22% more than the stratigraphie age. The age distribution and the clay mineralogy in the two series suggests that illitization of detrital I-S phases may start right after deposition under specific conditions, namely, contact of the sediments with K-rich brines, produced by evaporation in restricted marine environments. The process may continue from a few to many millions of years after burial, as long as K-enriched solutions are available. However, in most cases the near-surface temperatures did not enable a completion of the K-Ar reset process. It seems that low-temperature illitization is better explained as being dependent on solution composition and not on wetting-drying cycles.

MX-80 is a bentonite clay from the Clay Spur bed, Wyoming, USA, consisting of ~80 wt.% Na-rich montmorillonite and is a leading candidate material for use in engineered barriers within underground nuclear waste disposal sites in Switzerland and Sweden. The diffusive invasion of groundwater into the engineered barriers is likely to cause the dry high-density bentonite to swell and evolve into a less-dense, water-rich gel during re-submergence of the shafts and galleries. Data on H20 diffusivity within water-rich bentonite gels are therefore essential to the safe design of the engineered barriers. To address this need, the self-diffusion coefficient, D, of 1H2O molecules in water-rich MX-80 bentonite gels was measured using the pulsed-gradient spin-echo method of 1H nuclear magnetic resonance spectroscopy at 0.47 T; D was measured over a wide range of temperatures (20.6–70.1°C) and of bentonite volume fractions (0 to 17.2 vol.%). The results showed that D increased markedly to values of bulk water self-diffusivity as the bentonite volume fraction decreased towards 0 vol.%. The activation energy of the diffusion process for the gels was approximately equal to that for bulk water. As a result, the normalized H20 self-diffusivity, D/D0, was approximately independent of temperature, where D0 is D in bulk water. The D/D0 data for ≤6.39 vol.% bentonite fraction were successfully fitted to a porous clay gel model that involves unbound water molecules that diffuse in the pore space by avoiding randomly placed discoidal obstacles (clay particles) of a high aspect ratio. The D/D0 data for MX-80 were similar to data for SWy-2 (Wyoming bentonite from the Newcastle formation) and Kunigel-Vl (bentonite from Yamagata, Japan), suggesting that all three bentonites are equally suitable candidate materials, in terms of water diffusivity performance, for use within engineered barriers.

K-Ar dating of mixed-layer illite-smectite from clay fractions extracted from pyroclastic horizons was used to address the controversy about the age and mechanism of the thermal alteration of Carboniferous rocks from the Upper Silesia Coal Basin (USCB). The clay fractions were also investigated by X-ray diffraction in order to select for dating samples possibly rich in illitesmectite, and to evaluate the K-Ar dates for possible contamination by K-bearing pre-diagenetic minerals, of pyroclastic or epiclastic origin.

The K-Ar dates document intense Variscian tectonic burial illitization produced by thrusting (~290 Ma) in the SW of the basin, and the lack of intense burial illitization in the NE, which is consistent with sedimentological models of the basin. The burial illitization in its final phase (<30%S in illite-smectite) involved incorporation of measurable amounts of ammonium cation in the illite structure (substitution for K).

Advanced illitization in the NE of the basin is much younger than its tectonic inversion (uplift and erosion started in Permian), and the corresponding K-Ar dates have to be interpreted as the result of a Mesozoic thermal event, which produced widespread pervasive illitization at shallow depth. This event was dated at 175 Ma, but it may have started earlier and could have lasted longer. This conclusion is consistent with widespread evidence of a major Mesozoic thermal event all over Central Europe, produced by rifting and lithospheric thinning during the opening of the Tethys and Atlantic oceans.

This study demonstrates that smectite illitization histories may be very complex, and that the nature of the illitization mechanism results in mixed K-Ar dates encompassing pro-longed or multiple illitization histories. Dating of several grain-size fractions may help to unravel such histories.

When a calibration using data from Neogene sedimentary basins is applied, vitrinite reflectance and %S in I-S indicate similar palaeotemperatures of tectonic burial diagenesis in the USCB, but they produce very different estimates of the temperature of the thermal event.