Illite, the predominant component of the clay fraction of the Chinese loess, has been studied by X-ray diffraction (XRD) in 33 samples taken from the Holocene palaeosol (S0), the last glacial loess (L1) and the last interglacial palaeosol (S1) in the Louchuan loess section of the Loess Plateau in China. The XRD investigations indicate that it is mainly a 2M1 dioctahedral mineral with crystallinity values ranging from 0.23 to 0.36°Δ2θ (CIS Index). The 2M1 polytype illites with low IC values preclude a pedogenic formation of illites in the surface horizons of aridic and semi-aridic soils or deserts, and thus the clay mineral composition was largely determined by provenance. The illites are considered to be of detrital origin derived from pre-existing sediments and very low- to low-grade metamorphic rocks, eroded from the northern part of the Tibetan Plateau. The clay content of the loess and palaeosols was largely controlled by the strength of the winter monsoon, rather than by effects of in situ pedogenetic processes.

Well crystallized chlorite has been detected in a Typic Hapludult developed on quartz porphyry at Ulsan, Korea, after treatment of the <0.2 µm fraction with a high-gradient magnetic separation (HGMS) technique. The X-ray diffraction (XRD) characteristics of the chlorite suggest that it is an Fe-rich variety and this was confirmed by energy dispersive spectroscopy (EDS) analysis. The chlorite could not be detected with certainty in the <0.2 µm fraction but is clearly present throughout the soil profile in the <0.2 µm fraction. It is suggested that this chlorite is residual inherited chlorite, the rest of which has dissolved in the acidic environment (pH 4—5).

The IR spectra of -saturated smectites were examined in terms of their charge characteristics. The υ4 band near 1440 cm-1, observed in the DRIFTS spectra (obtained without use of a KBr matrix), was assigned to the vibrations of ions compensating the negative charge of the clays. When KBr was used as a diluting matrix, the υ4 band was located at 1400 and/or 1440 cm-1. The band at 1400 cm-1, related to NH4Br, originated from the replacement of in the clay by K+ from the KBr. For swelling clay minerals this band indicates that layers have permanent low charge density and/or variable charge. For non-swelling clay minerals, the 1400 cm-1 band characterizes the presence of variable charges only. The υ4 band at 1440 cm-1 suggests that in the clay was not replaced by K+ from KBr and remains in the interlayer space of the clay minerals. This absorption is due to compensating only permanent charge in the interlayers, or part of the interlayers with a high charge density. The presence of both bands at 1400 cm-1 and 1440 cm-1 in the IR spectrum suggests that the clays studied have a heterogeneous interlayer charge.

The high frequency OH-stretching band in micro-Raman spectra of kaolinites consistently exhibits two partly overlapping components with varying relative intensities. These bands, labelled A and Z, are located at ~3700–3690 and 3690–3680 cm-1, respectively. Band Z displays greater intensity in Raman spectra of highly crystallized kaolinites which have large coherent domains. Both components are also observed in the curve fitted infrared (IR) spectra of kaolinites; band Z is stronger in IR spectra of highly crystallized samples. Data for kaolinites with different particle size support the idea that bands A and Z are the longitudinal optic and transverse optic modes of the high frequency OH-stretching band.

The intercalation complex of kaolinite and potassium acetate (KAc) was studied by HTXRD, IR spectroscopy and DTA-TG. The HTXRD patterns indicate that the 14.06 Å basal spacing of the complex contracts to 11.77 Å and 9.35 Å after heating at 60°C. The DTA-TG data indicate that water is present in these new complexes, the decomposition of which occurs between 290°C and 400°C. Modifications observed in the high-frequency region of the spectra obtained after heating suggest that K ions occupy the ditrigonal holes in the OH surface of the kaolinite layers, whereas water is probably located between the KAc layer and the OH surface of the kaolinite. This structural arrangement would favour the H-bonding between inner-surface OH groups and water and justifies the presence of new bands at lower frequencies. Electrostatic interactions between the keyed K ions and O of the inner OH groups would justify the modifications of the 3619 cm-1 OH-stretching band.

Five Fe–bearing dioctahedral smectites (three nontronites and two Fe-rich smectites) were purified using a variety of physical and chemical procedures. The structural formulae indicate one nontronite and one Fe-rich smectite to be montmorillonitic, whereas the other three smectites are beidellitic. Mössbauer spectra showed Fe to be exclusively trivalent and were fitted with three doublets, two of which had quadrupole splittings characteristic of Fe3+ in octahedral coordination, whereas the third had a distinctly lower quadrupole splitting. Although the position of the Si–O stretching band in the infrared spectra could reflect tetrahedral Fe3+, the lack of distinctive features prevented a definitive attribution of this component to tetrahedral Fe3+. The 18O/16O data suggest that fractionation of nontronite-water at ambient temperatures (1000 lnα = 23 ± 2‰) is lower than that of Fe-rich smectite (1000 lnα = 27 ± 2‰). The estimated formation temperatures of the samples are below 70°C.

The Scherrer equation links the measured width of an X-ray diffraction peak (Scherrer width, SW) to the number of stacked cells (N) in the direction normal to the diffracting planes. The formula is only valid for one d-value occurring in the coherently diffracting domain. This equation can be modified for weakly swelling mixed-layer minerals. This assumes that the peak broadening caused by the mixed-layering is proportional to the amount of swelling component (S) and that the effects of size and mixed-layering are additive.

If two SW can be measured on XRD patterns from samples treated in two different ways (such as air dried or glycolated), N and S can be determined. This equation is applicable to illite-smectite mixed-layer minerals with high illitic content. The results are most accurate for N>30. The use of Scherrer's equation is discussed.

Sandstones and mudstones of the Skagerrak Formation from the Danish Sub-basin have been studied to assess the origin of the clay minerals, their diagenesis and their transformation/ conversion temperatures (compared to modelled maximum burial temperatures). Detrital smectite and vermiculite occur only at shallow levels. Conversion of smectite into random S-I took place at >47°C and of random S-I into ordered I-S at >74°C. The omnipresent illite increases in amount with burial. The amount of authigenic kaolinite (kaolin) decreases with increasing temperature (>75°C), possibly due to the growth of chlorite. Random S-C is converted into ordered S-C between 70°C and 105°C and chlorite is most abundant at maximum burial temperatures (150°C). This investigation suggests that diagenesis of clay minerals begins at relatively low temperatures, and emphasizes the importance of the availability of necessary ions (K+, Mg2+, Fe2+) for the reactions, besides time and temperature, as controlling parameters.

To gain a deeper understanding of the geochemical processes involved in the interactions between ionic solutions and clay minerals in natural systems, the dissolution rate of an Fe-rich saponite from Mt. Prinzera (Taro Valley, Italy) was measured as a function of pH and time at 25°C. Also, its ability to adsorb Cr was studied at varying metal concentrations (3.22, 5.50, 8.50 mEq l-1) and with different competing anions (CH3COO-, Cl-, NO3-). It was found that there is a correlation between the experimentally determined release rate constant (k), as defined by the relase of Si, and pH. In acidic solutions, the k values correlate negatively with pH, whereas the opposite occurs for basic solutions. The extent of Cr uptake was measured by analysis of the liquid portion separated by centrifugation after a controlled period of exposure. The Cr adsorbed for different concentrations and anionic environments shows the dependence of Cr uptake on the initial metal concentration and on accompanying anions in the order CH3COO-, Cl-, NO3-.

Due to its channels of molecular dimensions and a high specific surface area, sepiolite has many industrial applications which require high resistance to thermal effects in addition to a large surface area. On heating, sorbed water molecules are removed causing changes in the pore size distribution. In this study, the effects of thermal treatment on the pore structure of sepiolite and the acid-treated sepiolite samples were investigated. The solid density of sepiolite, measured by a He displacement technique, was 2.08 g cm-3 and total porosity was ~0.58. Both of these values showed an increase at 100°C, then decreased with further temperature increase due to crystal deformation and channel plugging which occurred at elevated temperatures. The BET surface area of the original sepiolite was 148 m2 g-1, and increased to 263 m2 g-1 at 100°C and then started to decrease. Approximately 16% of the total volume was in the micropores at 100°C. The acid pre-activation caused restrictions in possible crystal deformation during thermal treatment. The micropore volume increased to 20% and BET surface area reached values >500 m2 g-1 for the acid-treated samples.

Van Houten (1973), Walker (1976) and Turner (1980) have documented the authigenic origin of red coatings in continental red beds. Pigmentary hematite, ferrihydrite, goethite and clay-associated Fe-oxide crystallites have been suggested as the principal colouring agents of continental red beds (Walker, 1968, 1976; Walker & Honea, 1969; Larson & Walker, 1975; Turner, 1980; Fine, 1986, Besly et al., 1993). The red pigment in most Mesozoic red beds consists of hematite, whereas ferrihydrite and goethite are more common in younger sediments (van Houten, 1973). This has led several authors to the assumption that poorly crystalline Fe-oxides/hydroxides age into hematite (Walker, 1968, 1976; Walker & Honea, 1969; Larson & Walker, 1975; Turner, 1980).