Halloysite clay minerals are ubiquitous in soils and weathered rocks where they occur in a variety of particle shapes and hydration states. Diversity also characterizes their chemical composition, cation exchange capacity and potassium selectivity. This review summarizes the extensive but scattered literature on halloysite, from its natural occurrence, through its crystal structure, chemical and morphological diversity, to its reactivity toward organic compounds, ions and salts, involving the various methods of differentiating halloysite from kaolinite. No unique test seems to be ideal to distinguish these 1:1 clay minerals, especially in soils. The occurrence of 2:1 phyllosilicate contaminants appears, so far, to provide the best explanation for the high charge and potassium selectivity of halloysite. Yet, hydration properties of the mineral probably play a major role in ion sorption. Clear trends seem to relate particle morphology and structural Fe. However, future work is required to understand the possible mechanisms linking chemical, morphological, hydration and charge properties of halloysite.

Mine-drainage waters and associated precipitates from several active and abandoned lignite mines and mine dumps in the Sokolov Basin, northwestern Czech Republic, were sampled and analysed. The data showed considerable variations of effluent composition and pH that can generally be related to differences in the local microenvironments. Temporal changes such as seasonal fluctuations of precipitation, leading to variations of water infiltration through and runoff over mines and mine dumps, were also observed to noticeably affect effluent composition. These variations have led to a wide range of precipitates, the principal constituents of which are generally one or several of the ferric minerals jarosite, schwertmannite, goethite, ferrihydrite and lepidocrocite.

The present paper consists of two parts: a short review of the genesis and properties of the named minerals, and a study of mine-drainage precipitates formed under different local conditions in various lignite mines and mine dumps of the Sokolov mining district. We show that variations in mine-drainage precipitate mineralogy, such as the presence or absence of specific ‘key’ minerals, can serve as indicators for factors such as the pH and sulphate concentration, and thus — with certain limitations — for the genetic environment during precipitate formation. Such variations in composition are reflected, among other properties, in the precipitate colour, which can therefore be used for a rapid identification and classification — both in the field and by remote sensing — of regions that are potentially prone to acid mine drainage.

The determination of the cation exchange capacity (CEC) of clays by exchange with the cationic copper complexes [Cu(en)2]2+ and [Cu(trien)]2+ is revisited. The procedures reported by Bergaya & Vayer (1997) and Meier & Kahr (1999) are modified slightly. The concentration of the copper complexes in the equilibrium solutions is measured by spectral photometry. Correct CEC values of the [Cu(en)2]2+ exchange are only obtained when a buffer (‘tris’, tris (hydroxymethyl) aminomethane, pH = 8) is added to the equilibrium solution after separation of the clay, because the molar extinction coefficient of this complex depends on the pH of the solution. A standard procedure is recommended as a reference method. In most cases, tris addition is not needed for the determination with [Cu(trien)]2+ cations. Nevertheless, tris addition is recommended in the standard procedure. Determination of CEC for 40 samples (kaolins, ‘common clays’, bentonites, montmorillonites and beidellite) shows a good agreement between measurements using [Cu(en)2]2+ and [Cu(trien)]2+ cations and with results by the ammonium acetate method.

From leaching experiments with metallic uranium-aluminium material test reactor fuel elements in repository-relevant MgCl2-rich salt brines, a Mg-Al-layered double hydroxide with chloride as interlayer anion was identified as a crystalline secondary phase component. In the present study, this Mg-Al-Cl-layered double hydroxide (Mg-Al-Cl-LDH) was synthesized and its ability to adsorb iodine was evaluated. The adsorption of I– was rapid, and equilibrium for an initial concentration of 4.25 x 10-5 mole/l was obtained within 48 h. Further results indicated that the adsorption isotherm for I– retention could be fitted to Freundlich and Dubinin-Radushkevich equations. The values obtained indicate that iodine is adsorbed on Mg-Al-Cl-LDH by an anion- exchange process. The adsorption of I– was also studied as a function of pH. In the pH range between 3.5 and 8.5, the adsorption was independent of the pH. The effects of competiting anions were investigated using an MgCl2-solution instead of water, and indicate that chloride anions have a greater affinity towards the Mg-Al-Cl-layered double hydroxide than iodine.

The Neogene volcanic region of Cabo de Gata, Almería, SE Spain, is dotted with many outcrops of bentonite, some of them of significant economic interest. The bentonites have their origin in the hydrothermal alteration of pyroclastic rocks (15-7 Ma). The deposits are usually associated with fractures. The major mineral is a dioctahedral Fe- and Mg-smectite (89-75%) and this is accompanied by minor amounts of feldspars, quartz, amphiboles, pyroxenes, biotite, zeolites, disordered tridymite, calcite, etc. This paper describes the geological background, the general characteristics of the bentonites and major aspects of their formation, e.g. type of low-temperature hydrothermal solutions, mass balance, chemical evolutions of the smectites and geochemistry of trace elements. Finally, the characteristics of three of the most important deposits are described.

An investigation was carried out on the bioavailability of kaolinite and the role it plays in remediating oil-polluted seawater, since kaolinite is known to enable hydrocarbon-degrading bacteria to grow well. Experimental results revealed that significant amounts of Al and Si dissolved from kaolinite were not observed ((P > 0.05) in comparison with a control sample which contained no kaolinite) in the aqueous phase during the ~24 day experimental period. Transmission electron microscope observations and energy-dispersive spectroscope data revealed that some altered kaolinite particles appeared, connected to intact kaolinite and bacterial cells. Bacterial cells were associated and encrusted with intact and/or altered kaolinite clay particles, where mixed (C, O, Na and Si)-precipitates of kaolinite clays were formed on the surface of hydrocarbon-degrading bacterial cells. However, the uptake of Si (from kaolinite) by bacterial cells appeared to be more prevalent than Al, and there were no significant changes in basal spacings of kaolinite due to these altered kaolinite particles. Separate studies showed that hydrocarbon-degrading bacteria have a high resistance to Si. Thus, the present data suggest that Si from kaolinite may facilitate hydrocarbon-degrading bacterial growth as shown in our previous study (Chaerun et al., 2005), and the C-O-Na-Si complexes on the surfaces of bacterial cell walls may be the stimulator for hydrocarbon-degrading bacterial growth in seawater contaminated with oil spill.

Two different approaches used to express the tortuosity of water pathways in a solid clay body are known: (1) a static approach based on microscopic measurement in thin sections; and (2) a dynamic approach based on ‘imbibition’ speed. Each is expressed by a corresponding equation. The second method has the following advantages over the first: (a) it enables the testing of a much larger volume than that observed in a common thin section; (b) it measures the effects of all particles, including the finest fragments barely visible under optical microscope which also have influence on hydraulic conductivity or the imbibition speed of water; (c) in the case of expansible clays, the magnitude of k (tortuosity of pathways including the shape of particles) can be compared with k for non-expansible clays using a non-polar liquid, e.g. tetraline; (d) the measurement of the imbibition speed into the clay aggregate through a constant area is automated. Specific properties and processes important for sealing clays are discussed in the connection with the dynamic approach.

α-amylase was immobilized on acid-activated montmorillonite K-10 via adsorption and covalent linkage. The immobilized enzymes were characterized by X-ray diffraction (XRD), surface area measurements, 27Al nuclear magnetic resonance (NMR) and scanning electron microscopy (SEM). Surface area measurements indicate pore blockage due to linking of the enzyme in the vicinity of the pore mouth. The XRD demonstrates intercalation of enzyme upon immobilization. The NMR studies indicate that, during adsorption, tetrahedral Al sites are involved, while covalent binding occurs exclusively on the octahedral Al sites. The SEM images depict the changed morphology of the clay surface due to immobilization. The efficiency of immobilized enzymes for starch hydrolysis was tested in a batch and a fixed-bed reactor and the performances were compared. The immobilized α-amylase showed a broad pH profile and improved stability characteristics in both reactor types when compared to the free enzyme. The effectiveness factor increased in the fixed-bed reactor, implying that diffusional restrictions to mass transfer operate in the heterogeneous reaction and the use of a fixed-bed reactor leads to a reduction in these diffusional resistances. In the continuous run, 100% initial activity was maintained for 72 h, and after 96 h, >80% activity was retained.

SWy-1 and SAz-1 smectites and an Italian bentonite from Sardinia were mechanically deformed via high-energy ball milling for 20 h, in a controlled thermodynamic environment at constant temperature (25°C) under vacuum. The deformed smectites have a lower cation exchange capacity (CEC) and form thicker particles than the original ones, due to agglomeration of smectite crystallites. The 001 diffraction maximum shifted to lower d spacings, the intensity of the 060 reflection decreased and the background at 20-30°2θ increased, suggesting partial amorphization of the smectite. Moreover, the layer charge of the smectites decreased. The intensity of the complex stretching band at 3625 cm-1, and the AlAlOH, and AlFe3+OH bending bands at 916 cm-1 and 886 cm-1, respectively, decreased, while the band at AlMgOH bending at 849 cm-1, disappeared. Deformation mainly disrupted the octahedral sheet and preferentially destroyed those sites occupied by Mg cations, thus explaining the observed decrease in layer charge. Octahedral sites occupied by Fe were least affected. The disruption of the octahedral sheet is substantiated further by the almost total disappearance of the dehydroxylation peak, which is more pronounced in the Mg-rich SAz-1 smectite.

The colloidal state (sol, flocs, gels) of Na-montmorillonite dispersions (from bentonite from Bavaria, Germany) was adjusted by the addition of poly(ethylene oxides) with terminal trimethylammonium groups. The colloidal state of the dispersions changed with the molar mass of these end-capped poly(ethylene oxides). The sol-gel diagrams (up to 4.5% (w/w) montmorillonite) in the presence of TMA-PEO 1500 and TMA-PEO 4000 (up to 40 g/l) were characterized by large fields of gel whereas TMA-PEO 20000 and TMA-PEO 35000 caused peptization by steric stabilization. Even dispersions with the highest montmorillonite content (4.5%) were peptized by >0.2 g/l TMA-PEO 35000. The critical coagulation concentration of NaCl was increased by the polymer addition, but the increase was modest (from 8-12 mmol/l to 50 mmol/l). The changes of the colloidal state, the flow behaviour and the salt coagulation reveal the sensitive interplay between stabilization mechanisms (electrostatic, steric) and destabilization effects (bridging, charge compensation, depletion flocculation).

Several new interlayer-grafted derivatives of kaolinite were synthesized by esterification of inner-surface hydroxyl groups with alkanols, diols and glycol mono-ethers starting with the dimethyl sulphoxide intercalate. The derivatives were characterized by X-ray powder diffractometry, thermal analysis, Fourier transform infrared spectroscopy and transmission electron microscopy. The grafted molecules are arranged in monolayers between the kaolinite layers, with typical basal spacings of ~11.3 Å. Rheological studies of aqueous dispersions of the modified kaolinites revealed an exponential increase of the yield value and apparent viscosity with increasing alkyl chain length of the grafted molecules.

Kaolinite was intercalated with n-hexylamine, n-octadecylamine and n-docosanamine, using methanol-kaolinite as the precursor. The intercalation compound with n-docosanamine presented the largest basal spacing for a kaolinite derivative thus far reported (64.2 Å). Five grafted derivatives of kaolinite were directly intercalated with n-hexyl- and n-octadecylamine. During intercalation, the grafted molecules rearrange from parallel to perpendicular orientation to the kaolinite surface, in order to maximize the interaction with the amine and minimize the interlayer expansion needed. The octadecylamine intercalation compounds were delaminated in toluene, accompanied by the deintercalation of the amine molecules. Thin kaolinite particles rolled into a halloysite-like morphology, but forming much smaller tubes, some of which possibly consist of single kaolinite layers. The delamination was more efficient with the grafted kaolinites than with raw kaolinite.

Diagenetic alteration in two tuff horizons (the lower and upper tuff units) in the GoÈrdes region of Turkey led to the formation of mainly heulandite-clinoptilolite-type zeolites and also clay and silica minerals and K-feldspar. Possible variations in the climate and geological environment followed by variations in the hydrological system and the composition of the Miocene lake water and groundwater at that time resulted in the mineralogical facies. The lower tuffs are unaltered or slightly altered in the northern part and are more altered, particularly to zeolite (clinoptilolite) in the southern part of the study area. Heulandite and K-feldspar are mainly authigenic minerals in the upper tuff unit. The ammonium-exchange capacities of the 16 samples were investigated to understand the effects of the mineralogical composition on their ion-exchange capacities, which vary in the range 0.19-2.00 mEq/g. It is observed that the ion-exchange capacities are strongly dependent on the zeolite contents of the rocks. The correlation coefficient of this linear relation is 0.86. The most significant increase in ammonium-exchange capacity (from 0.2 to ~1.0 mEq/g) was observed when the zeolite contents increased from 0 to 30 wt.%. The second increase in exchange capacity was observed for samples containing >80 wt.% zeolite. Although there is no significant effect from the other authigenic minerals, smectite has a positive effect, and K-feldspar and opal-CT have almost no effect on the ion exchange capacities.

Mineralogical and geochemical techniques have been used to determine the role of minerals in controlling the trace element composition of a set of clay-sized fractions from Albian siliciclastic rocks in NE Spain. These clay-sized fractions are composed of kaolinite and illite, minor quantities of quartz, and accessory heavy minerals. Kaolinite has a smaller crystal size than illite, accounting for its relative concentration in these fractions. The µm-sized heavy minerals are rutile, Fe-Ti- and Fe-oxides, zircon, pyrite, cassiterite, monazite and xenotime. Geochemical data indicate that most of the trace elements are relatively concentrated in clay-sized fractions, except for Zr, Hf, Y and HREE. Statistical treatment shows three different associations in the clay-sized fractions: (1) Rb, Cs and Ba with clay phyllosilicates, especially illite; (2) REE with Th, Y and P phases; and (3) Sc, Cr and V with Ti- and Nb oxides. Therefore, these data do not support the dominant REE control by clay mineralogy that other authors have reported.