In recent years, experimental and theoretical methods have provided new insights into the size, shape, reactivity, and stability of clay minerals. Although diverse and complex, the surface chemistry of all clay minerals is defined spatially on a common scale of nanometres. This review is organized around the nanoscale architecture of clay minerals examined at several different length scales. The first, and perhaps most important, is the length scale associated with Hbonding in clay minerals. Hbonding interactions define the size and shape of 1:1 phyllosilicates and dominate the surface chemistry of many clay minerals. Structural and surface OHgroups contained within and on the surface of clay minerals provide a type of ‘molecular reporter group’ and are sensitive to subtle changes in their local environment. Examples of OH-reporter group studies in clay minerals, and the spatial scales at which they provide diagnostic information, are examined. The second length scale considered here is that associated with clay–water and clay–organic interactions. Inorganic and organic solutes can be used to explore the surface chemistry of clay minerals. Similar to the use of reporter groups, molecular probes have diagnostic properties that are sensitive to changes in their molecular environment. Clay–water interactions occur at a length scale that extends from the size of the H2O molecule (~0.3 nm) to the larger scales associated with clay-swelling (>10 nm). Similarly, clay–organic interactions are also defined, in part, on the basis of their molecular size, in addition to the type of chemical bonding interactions that take place between the organic solute and the clay surface. Examples illustrating the use of clay–water and clay–organic solute interactions as molecular probes are presented. The largest scale to be considered is that of the particles themselves, with scales that approach micrometres. Recent developments in the synthesis and characterization of ultrathin hybrid films of clay minerals provide complementary information about the nature and distribution of active sites on clay minerals, as well as providing new opportunities to exploit the surface chemistry of clay minerals in the design of functional materials.

The chlorite-group mineral chamosite occurs in nanocrystalline form (~200 nm grain size) as a naturally occurring clay in the Quaternary marine sedimentary deposits near Kudiamozhi, Tuticorin District, Tamil Nadu, India; samples were used in this study as a reusable catalyst. The clay has the usual alternating tetrahedral-octahedral-tetrahedral silicate/aluminate/silicate layer structural arrangement (sometimes called the 2:1 silicate or talc layer structure). The interlayer and the t-o-t layer are bound together by both electrostatic and hydrogen-bonding forces. This natural clay catalyst has been well characterized by various techniques such as Fourier transform infrared spectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS), transmission electron microscopy (TEM), X-ray diffraction (XRD), temperature programmed desorption (TPD), thermal analysis and BET surface area measurements (Sreedhar et al., 2009); it has been utilized for various organic transformations such as acylation of alcohols and amines, cyclization of arylaldehydes with O-phenylenediamines and C-O bond formation reactions.

Kaolin is found in deposits of economic concentration in the Jabal Al-Harad/ Batn El-Ghoul area in southern Jordan. Ten representative kaolin samples were collected from the area and investigated for their mineralogical and chemical composition. Mineral characterization was carried using X-ray powder diffraction (XRD), thermogravimetrical analysis (TGA), differential thermal analysis (DTA), Fourier transform infrared spectroscopy (FTIR) and scanning electron microscopy (SEM). X-ray fluorescence (XRF) studies were conducted to determine the chemical composition of the kaolin deposits. Kaolinite was the predominant mineral, followed by quartz, with traces of illite-muscovite, Fe-bearing minerals (hematite), anatase and feldspar. The average chemical composition of the kaolin samples was 58.02 wt.% SiO2, 28.00% Al2O3, 1.48% Fe2O3, 1.26% TiO2 and 0.41% K2O (ignited basis). Dehydroxylation and mullitization temperatures (from DTA) were close to the theoretical values. Hexagonal booklets and stacks of kaolinite, as well as individual platelets, were present in the Jabal Al-Harad kaolin. Based on granulometric and descriptive mineralogical analyses, the mineral assemblages and kaolinite morphology, the Jabal Al-Harad kaolin deposit is thought to have originated from greatly weathered surfaces related to the Precambrian basement rocks. The kaolin was found to be suitable for manufacturing of common bricks, medium-fired bricks and sanitary ware, although a beneficiation process would be required; it could also be used in the refractory, white cement, paper and advanced ceramic industries.

A clayey soil from southern Tunisia has been studied in order to assess its possible use as an anti-pollution barrier. The soil has been characterized from both geotechnical and physicochemical points of view. The physicochemical characterization was carried out using X-ray diffraction, thermogravimetric and differential thermal analyses, specific surface area and cation exchange capacity measurements. Hydration tests were carried out using oedometers to determine the swelling behaviour of compacted specimens under different stress conditions. The large swelling pressure (420 kPa) is in agreement with the mainly smectitic character of the soil highlighted by the physicochemical tests. The changes in the fabric of the soil and in its porosity after the hydration tests were investigated using the same physicochemical methods, and related to the macroscopic hydro-mechanical properties. X-ray diffraction and mercury intrusion measurements show that there are changes in the interlayer spacing, particle size and inter-aggregate pore distribution. Comparing the properties of the soil with the Belgian guidelines for the construction of anti-pollution barriers showed that the material was suitable for that use.

The majority of investigations of soil electrical characteristics are concerned with tropical and subtropical regions and hence there is little information about soils from other regions. The present study attempts to discover the most important constituents affecting soil electrical charge characteristics in a temperate zone. The study area is located on southern coastal zones of the Caspian Sea in northern Iran (Guilan province). The soil moisture and temperature regimes of the region determined by means of Newhall software were udic and thermic, respectively. Six representative pedons developed on granite, andesite, basalt, phyllite and limestone as their parent rocks are described and their physicochemical, mineralogical and electrical properties studied. The results indicate that the point of zero charge (PZC) values are small in all samples and smaller at the surface than in subsurface horizons. There is a positive correlation between pH0 (PZC) values, organic carbon percentage and crystalline Fe (Fed) content. The points of zero net charge (ZPNC) values are <2.5 in all the pedons studied, which refers to large amounts of negative charge in these soils. The results also show that the differences in negative charge development in a pH range from 2.5 to 6 were largest for horizons rich in organic C and least for those with significant amounts of layer silicate minerals. Therefore, the most important variable charge component in all of the soils studied is organic matter which controls the negative charge at the soil surface. The permanent charge (бp) of the soils studied is also large and negative, which agrees with the amount of clay and the mineralogy of these soils.

Well developed mullite whiskers were obtained by heating compacts of kaolin and 2 wt.% sodium pyrophosphate powder at 1350ºC for 10 h, with or without a small addition of aluminium sulphate. Without aluminium sulphate, mullite whiskers were obtained with an aspect ratio of >8 and reasonably uniform diameter (~0.05–0.3 μm). When aluminium sulphate additions were employed a mass of relatively well developed and interlocked whisker-shaped crystals were produced, with an aspect ratio of >10 (~0.05–0.3 μm in diameter). Larger whiskers were also observed, with aspect ratio of >25 (~0.2–0.5 μm in diameter).

Mineralogical and chemical compositions of residual and sedimentary clays (bulk and <2 μm fraction) from the Nelas region (schist, aplite-pegmatites, granites and Tertiary sediments from both Mondego River margins), Portugal, were studied, aiming to establish indicators for raw materials in ancient ceramic provenance studies. The mineralogy of bulk material does not provide a clear distinction between samples. Among clay minerals, kaolinite dominates, except in the aplite-pegmatites where illite prevails. Smectite was only found in sediments of the left river bank.

A more successful result was the geochemical differentiation of clay types. The weathered schist presents greater enrichment in Cr, whereas the clay fraction of aplite-pegmatites shows enrichment in all the chemical elements studied. The sediments and weathered granites are not easy to differentiate; the best geochemical indicators are U (lower contents in clay-size fraction of sediments) and REE patterns in both bulk and clay-size fraction.

The Linkuo terrace, situated to the west of the Taipei Basin in NW Taiwan, has thick red soils that have been little studied. This paper aims to interpret the development of these soils through chemical and micromorphological investigations, and relate the soils to their palaeo-environments. The soil samples were air dried, crushed and passed through a 2 mm sieve, and then subjected to conventional soil chemical and physical analyses, together with clay mineralogical and morphological characterization. Pedons I and II were clay with low base saturation (BS), cation-exchange capacity (CEC), and exchangeable cations. The pH of pedons I and II ranged from 3.80 to 5.26. The low magnetic susceptibility of these soils indicates that no magnetite (Fe3O4) or maghemite (γ-Fe2O3) are present. X-ray diffraction patterns of the magnetic clay fraction showed lepidocrocite, goethite and hematite, the amounts of which may relate to water fluctuation in the soil environments. Illite, kaolinite and quartz are major clay minerals in the red soil clay fractions. The micromorphology of all horizons showed a great accumulation of Fe-oxides. The upper horizon showed darkened isotropic Fe-oxide materials, and lower horizon showed a black to reddish dense plasma with soil matrix. The groundmass of the oxic horizon is generally characterized by a homogeneous distribution of the different coarse and fine constituents. The thick (4 m depth) and homogeneous red soils of the Linkuo terrace were developed from fine sediments after the gradual subsidence of the Taipei Basin. Alarge amount of gravel was flushed from the Xindian River before the Taipei Basin subsided. The Linkuo red soils can be classified as mesic, Typic Kandiudox. From the chemical compositions of clay fractions and the red soil features, these red soils can be considered as lateritic red earths or red earths that do not reach the criteria for laterite.

A set of Silurian black shales from Sierra de Albarracín (NE Spain) corresponding to two different sections was studied to determine the relative influence of diagenesis, igneous activity, and regional tectonics on the clay-mineral genesis. The coexistence of pyrophyllite, I-S interstratifications (R1), ammonium-rich illite, potassium illite, kaolin, and chlorite is not the result of prograde evolution during diagenesis – very low-grade metamorphism. Three different stages may be inferred: (1) sedimentation of black shales (Aeronian, Lower Silurian, to basal Ludfordian, Upper Silurian) and the subsequent diagenetic process producing the coexistence of quartz, illite, kaolinite, organic matter, etc.; (2) intrusion of andesitic sills producing hydrothermal alteration and crystallization of pyrophyllite, ammonium-rich illites, smectite, I-S R1 phases and jarosite; and (3) and folding of shales and sills and development of penetrative schistosity during the late Variscan leading to illite and paragonite recrystallization reaching the anchizone grade.

Ammonium illite and ammonium illite-smectite mixed layers, together with potassium illite, smectite and minute amounts of kaolinite were identified in hydrothermally altered andesite rocks from the Harghita Bãi area of the Eastern Carpathians, Romania. K-Ar dating and oxygen isotope tracing, as well as rare-earth elemental analyses were made to provide new information on the timing and crystal-chemical processes characterizing the crystallization and further evolution of these illite-type mineral phases.

The combined results suggest the occurrence of hydrothermal activity in two distinct episodes with nucleation of two generations of illite-type particles of different chemistry and morphology. About 9.5 Ma ago, potassium illite crystallized in alteration halos of the porphyry Cu system, probably at a temperature of ~270ºC from fluids having a δ18O of ~2.9% (V-SMOW). Associated smectite seems to have precipitated slightly later in external alteration halos at a similar temperature, but from fluids depleted in alkalis and with a different δ18O. Alternately, ammonium-rich illite-smectite mixed layers formed very recently, less than ~1 million years ago at a temperature of ~90ºC from fluids of probable meteoric origin that altered the previously crystallized potassium illite, resulting in the crystallization of a new generation of ammonium illite-smectite mixed layers. Evidence of this dissolution-precipitation process is provided by a significant increase in the δ18O of the mixed-layer structures and by a significant change in their REE contents and distribution patterns. Occurrence of potassium in the ammonium-rich mixed layers probably relates to the progressive alteration of the first-generation potassium illite and a discrete concomitant take up of released K by the new NH4-rich interlayers of the ammonium mixed layered sequence.