Rhyolitic obsidian was reacted with natural waters to study the effect of water chemistry and biological activity on the composition and formation mechanisms of clay. Two sets of experiments (18 months, 6 years) used fresh, hypersaline water (Mg-Na-SO4-Cl- and NaCl-rich) and seawater. The 6-year experiments produced the transformation of obsidian into quartz, apparently by in situ re-crystallization (Cuadros et al., 2012). The most abundant neoformed clay was dioctahedral (typically montmorillonite), indicating chemical control by the glass (where Al > Mg). Altered glass morphology and chemistry in the 18-months experiments indicated in situ transformation to clay. Magnesium-rich (saponite) clay formed under water-chemistry control in the bulk and within biofilms with elevated Mg concentration (Cuadros et al., 2013). The contact between microbial structures and glass was very intimate. Glass transformation into quartz may be due to some characteristic of the obsidian and/or alteration conditions. Such combination needs not to be uncommon in nature and opens new possibilities of quartz origin.

Highly weathered soils of the tropics and subtropics commonly have kaolinitedominated clay fractions. Under acidic conditions prevailing in these soils kaolinite dissolution occurs, contributing to the high levels of soluble Al in these soils. This study evaluates the dissolution behaviour of kaolinites from subsurface horizons of highly weathered soils from Thailand, along with a soil kaolinite from Western Australia (WA kaolinite) and Georgia kaolinite (KGa-2). Kaolinite-dominated clay fractions were isolated from soils by sedimentation and chemically treated to remove iron oxides. The dissolution rate of kaolinites was measured in 0.01 M NaCl as a function of pH (1–4; HCl) at 25±1°C using non-stirred flow-through reactors. Kaolinite dissolution rates were calculated from the release of Al and Si at the steady state. In most of the experiments and at all pH values, the release of both Si and Al underwent a distinct transition from an initial period of rapid release to significantly slower release at the steady state. Aluminium and Si concentrations at the steady state were higher for soil kaolinites than the reference sample (KGa-2). At the steady state the dissolution of all kaolinites was stoichiometric except for the soil kaolinites from Thailand at pH 4, where the Al/Si ratio was well below the stoichiometric ratio. Log dissolution rate (RSi) of soil kaolinites ranged from –13.75 to –12.51, with the dissolution rate increasing significantly with decreasing solution pH. However, the dissolution rate was similar or pH independent between pH 2 and 3, which is the pH range of the point of zero net charge (PZNC) for both soil and reference kaolinites. The dissolution rate of soil kaolinite was significantly higher than the KGa-2 sample at pH < 3. The results obtained on kaolinite samples from highly weathered soils show that in highly acidic systems kaolinite is a source of soluble Al. Soil kaolinites with poorly ordered small crystals dissolve faster than better crystalline reference kaolinite (KGa-2) with larger crystals.

Basalts and other mafic rocks are frequently used for road construction. The aggregates have to provide intrinsic mechanical strength and skid resistance. However, smectite contents stemming from alteration processes may deteriorate these mechanical properties considerably because of the resulting contrast in hardness. Quantitative relationships between smectite content, kind of distribution, and aggregate stability are rare except for a recent study by Kaufhold et al. (2012). In this work thirteen basalt and andesite samples from German quarries were analysed by X-ray diffraction (XRD) for their mineral contents (including clay minerals), their cation exchange capacities (CEC) and their resistance against disaggregation in dimethyl sulfoxide. Smectite contents from XRD exceeded those calculated from the CEC regardless of the kind of occurrence of smectite observed in thin sections. The hypothesis that rocks with smectites occurring mostly in patches should differ in terms of CEC and mechanical properties from those rocks where smectites are more disseminated could not be confirmed. However, the amounts of smectite accessible to cation exchange correlated with the DMSO losses. Therefore, the fast CEC determination with Cu-Trien exchange can be substituted for the time-consuming DMSO test.

Ceramic tile production by the wet route requires clay suspensions with a high solid content and low viscosity. In this work the deflocculation of clays in aqueous suspensions was investigated by varying the type of clay and additive. Three kaolinitic and two illitic clays were characterized and dispersed with deflocculants based on lithium, sodium and potassium silicates and polyacrylates. The clays were characterized by chemical and mineralogical analyses, particle size distribution, zeta potential, organic carbon content, cation exchange capacity (CEC) and specific surface area (BET). Deflocculation curves were determined by measuring the viscosity for 50 wt.% clay slips. The results indicate that additive consumption is closely related to CEC and BET, which correspond respectively to the chemical and physical characteristics of the clay mineral's surface. Moreover, viscosity values at the deflocculation point are closely related to BET.

The incomplete miscibility of Al into the ferrihydrite structure has been a recurring issue in understanding the environmental geochemistry of this important oxyhydroxide. During co-precipitation from acidic aqueous solution, ferrihydrite has been observed to accept only up to ∼25 at.% Al without the formation of multi-phasic Al and Fe oxyhydroxides. Using basic chemistry and crystal-chemical relationships we propose here that the saturation limit of Al substitution in the structure of Fe oxyhydroxides is controlled by Al – Al avoidance in a manner that conforms to Pauling's distortion rule. Employing this hypothesis, we show that the predicted miscibility limit for Al incorporation is 25 at.% in ferrihydrite and 33 at.% in goethite, in agreement with previous observations. These results indicate that the classical f-phase model for ferrihydrite best represents observations. Incorrect assignment of Fe site occupancy and other shortcomings of the akdalaite/tohdite model for ferrihydrite are also discussed.

This study investigated a method for characterizing the behaviour of clays in an aqueous medium in the presence of an electric field. The experiments were performed with different clays (kaolinite, smectite, kaolinite-smectite) suspended in deionized water. An apparatus was developed for measuring a continuous electric current under the singular effect of adding sodium silicate to the aqueous suspensions. The results showed that the method allows the differentiation of the investigated clays through their electrophoretic behaviour in an aqueous solution.

The physicochemical and mineralogical characterization of the <250 μm particle-size fraction from six clay-rich samples from the Sabga deposit (north-west, Cameroon) were carried out to evaluate their potential applications. The major clay mineral is dioctahedral smectite and minor kaolinite is present in three of the clay samples. Cristobalite, feldspars, ilmenite and heulandite are accessory minerals. Application of the Greene-Kelly test revealed that the smectite present is montmorillonite. The chemical composition (wt.%) of the bulk clays consists of (66–70%) SiO2, (13–16%) Al2O3 and (2–7%) Fe2O3. Physico-chemical characterization of the clays showed that the cation exchange capacity (CEC) and the specific surface area range from 38 to 46 meq/100 g and from 33 to 90 m2/g respectively. The physical and chemical properties are fully compatible with potential uses in environmental applications. After purification and chemical modification, these materials could also be used in refining edible oil as adsorbent, waste water treatment and wine technology.

Alkali- and Ca-rich smectites occur in association with Neogene lacustrine sedimentary rocks and high-K calc-alkaline volcanic rocks with compositions ranging from andesite to rhyolite in western Anatolia (Turkey). Major clay occurrences are associated predominantly with the Foça and Akçaköy ignimbrites and with the ignimbrites within the Rahmanlar pyroclastics. Experimental studies indicate that the main clay minerals present are Na- and Ca-smectite and subordinate illite, associated with silica polymorphs, trace clinoptilolite and chlorite. The authigenic minerals formed by weathering and metasomatic reactions between hot volcanic material and lake water. Smectite was produced as a result of argillic alteration of the volcanic glass and feldspars of the ignimbrites, Rahmanlar pyroclastics and reworked volcaniclastic sediments through chemical weathering and dissolution-precipitation processes and formed as authigenic phases both in terrestrial and nearby shallow lacustrine environments under prevalent humid or semi-humid climate during the Neogene.

This paper provides evidence of appreciable amounts of Zn (up to 0.52 a.p.f.u.) residing in the crystal structure of trioctahedral vermiculite; the results were obtained by a combination of experimental methods (X-ray diffraction, energy dispersive X-ray spectrometry, electron microprobe analysis and selective chemical extractions) and theoretical studies based on density functional theory (DFT). Vermiculite occurs in size fractions ranging from 10 to 40 μm of near-neutral soils naturally enriched in Zn (up to 8110 mg kg–1) due to the weathering of carbonate-hosted Zn-Pb deposits in SW Spain. Zinc was partitioned among the various pedogenic phases during soil formation, but the larger part (>80%) was concentrated in the residual silicate fraction. The soil exchangeable Zn pool was found to be virtually negligible. The DFT calculations support the hypothesis that the role of vermiculite is an effective sink for geogenic Zn; this is consistent, in terms of energetic stability, with the assumption that most Zn is structurally bound in octahedral sheets rather than in the interlayer exchange sites. The findings conclusively indicate that the potential mobility of Zn should remain low under the present soil conditions.

Virtually all 2:1 clay minerals contain some Fe in their crystal structure, which may undergo redox reaction with surrounding redox-active species causing potentially significant changes in the chemical and physical properties of the clay mineral and its surrounding matrix. This phenomenon was originally of interest mostly as a laboratory experiment using strong inorganic reduction agents, but the discovery that the structural Fe could be reduced by microorganisms in natural soils and sediments opened the way for this to become a practical method for altering the chemical and physical properties of soils and sediments in situ. The purpose of this report was to review the body of literature that has been published since the inception of this field of inquiry and to complement, update, and complete three other reviews that have been published during the intervening years. Studies of microbial reduction of structural Fe in smectites have revealed the extent of reduction, effects on chemical and physical properties, reversibility (or lack thereof) of microbial reduction, stoichiometry, possible reaction mechanism, and types of organisms involved. Some organisms are also capable of oxidizing structural Fe, such as in biotite or reduced smectite, while one appears to be able to do both. Illitic layers resist reduction by microorganisms, but this can be partially overcome by the presence of an electron shuttle compound such as anthraquinone-2,6-disulfonate, which also enhances the extent of reduction in smectites. Microorganisms may be employed as an in situ reducing agent to drive redox cycles for structural Fe in constituent clay minerals of soils and sediments, which in turn can serve as an abiotic source for redox-mediated remediation of environmental contaminants.