Stable isotope and chemical compositions of vesicle- and fissure-filling smectites (i.e. montmorillonite and saponite) in Penghu subaerial basalts were analysed to establish their possible formation conditions. The results show that montmorillonites have higher Fe contents (as FeOtotal), as well as higher O-isotope and H-isotope compositions than saponites. These differences may largely reflect dissimilar formation conditions and/or fluid compositions. It is suggested that saponites probably precipitated from fluids dominated by meteoric groundwaters at 40–50°C, whereas montmorillonites were probably formed at 20–40°C from mixtures of groundwaters and seawater. Considering all the possible uncertainties, the above estimated temperature conditions are suggested as reasonable lower limits.

It is established that the associated calcite is not in O-isotope equilibrium with the montmorillonite. The calcite might have been formed in environments with ambient fluid dominated by meteoric groundwaters at temperatures lower than those for montmorillonite.

The composition, texture and genetic evolution of kerolite and related Mg clays belonging to the Intermediate Unit of the Miocene from the Madrid Basin have been studied. About 400 samples from the Esquivias deposit were analysed by several mineralogical and/or chemical techniques. Two genetic pathways for the development of Mg clays during early diagenesis have been observed: (1) mudflat environment: Al-smectite (beidellite) → Mg-smectite (saponite); and (2) palustrine environment: Si-Mg gel → kerolite → kerolite-stevensite → stevensite.

In the mudflat deposit the transformation processes predominate, whilst in the palustrine environment, kerolite is neoformed, probably from a gel-like medium. Stevensite seems to have originated from the transformation of mixed-layer kerolite-Mg-smectite, but also through neoformation at a later stage.

The textural features, isotopic data and sedimentary evolution within each lithofacies are indicative of shallowing-upward sequences with development of palaeosols. A post-sedimentary origin for sepiolite, calcite, authigenic quartz, zeolites and baryte is inferred.

Detrital smectite in a sandy claystone and a phosphorite, and authigenic palygorskite in a dolomitic marl and a porcellanite from Cretaceous-Tertiary phosphorite deposits of the Ganntour Basin (Morocco) were purified using cation exchange resin, leached with dilute acid, and analysed for the contents and distribution patterns of their REE before and after acid treatment. The normalized patterns confirm a detrital origin for the smectite in the sandy claystone, whereas the origin of the smectite from the phosphorite is obscured by the addition of REE from the phosphogenic environment. The normalized REE patterns of the palygorskite suggest formation in non-oxidizing restricted environments. The Al2O3/ΣREE ratio of the two clay types suggests formation of diagenetic palygorskite (and mixed-layer illite-smectite) from Al-bearing detrital smectite by a dissolution-crystallization process.

Infrared reflectance spectra of Li-, Na-, K-, Rb- and Cs-saturated samples of montmorillonite Zenith-N (Milos, Greece) have been measured in the 400–1300 cm-1 region in an attempt to elucidate the behaviour and migration properties of alkali cations after heating the montmorillonites at 300°C for 24 h. Deconvolution of the complex Si–O stretching band reveals that the component band that arises from the asymmetric stretching vibrations of the silicon-apical oxygen units exhibits the biggest change upon heating the montmorillonites. The normalized absorption area of this band has been correlated with the decrease in the layer charge due to the cation migration. Analysis of the IR data for each alkali-saturated montmorillonite has shown that only Li+ migrates near, or to, vacant octahedral sites.

In order to assess the influence of tetrahedral sheet charge on the hydroxyl-stretching vibration in clay minerals, a series of synthetic Na-saponites, with increasing layer charge, was studied by infrared (IR) spectroscopy. When recorded after KBr dilution either in transmission or diffuse reflectance, the IR spectra exhibit a component at ~3710 cm-1, the intensity of which increases linearly with increasing layer charge. When experiments are carried out without any dilution of the samples, this component is not observed, suggesting a Na+/K+ exchange upon mixing of the sample with KBr. The spectra obtained for K-saponite confirm this exchange. This shows that the dilution procedure can lead to serious misinterpretations of IR spectra in the OH-stretching region.

Montmorillonite was modified by replacing the exchangeable cations with betaines

(CH3)3N+ - (CH2)n - COO-M+ (n = 3, 5, 7, 10)

The betaine derivatives delaminated in water and formed a colloidal dispersion. Air-drying of this material yielded hard pieces which were difficult to redisperse. The dried material became redispersible in water when the Na ions (counterions to the carboxyl groups) were replaced by Li ions. Colloidal dispersions of this material were more stable against salts than Li+- or Na+ - montmorillonite. Extremely high LiCl concentrations (>1 mol/l) were needed to coagulate the betaine derivatives (n>5) in the presence of diphosphate. The increased salt stability resulted from lyospheres around the silicate layers or thin packets of them which reduced the van der Waals attraction. Addition of organic solvents destabilized the dispersion by compressing the diffuse ionic layer (DLVO theory). The delaminated particles then aggregated to small flocs which settled very slowly. Neither band-type structures nor cardhouses were formed at conditions comparable to network formation and stiffening of Li- and Na-montmorillonite dispersions. Rheological measurements revealed the liquefying action of the betaines. Dispersions of butyrobetaine montmorillonite (15 g solid/l) revealed a relative viscosity (related to the dispersion medium water) ηrel ≈ 2. The longer chain derivatives showed a value slightly >1 whereas Li+-montmorillonite had ηrel = 8. Yield values were not formed at pH ≈ 7. Only at acidic conditions did the butyrobetaine montmorillonite dispersion showed a small yield value (<200 mPa).

The adsorption of the metachromic dye acridine orange (AO) by Na-beidellite was investigated by visible spectroscopy. Different types of clay-AO association were identified from the appearance and location of absorption bands α or β. The colloidal properties were determined from curves of the absorbance vs. the degree of saturation. Three regions were identified in the absorbance curve. In the first region beidellite is peptized with small amounts of AO and the dye penetrates into the interlayer space where it undergoes metachromasy due to π interactions between the aromatic entity and the oxygen plane of the clay. With larger amounts of AO (second region), the clay flocculates due to the aggregation of the dye cations in the interparticle space of the flocs. In excess AO (third region), beidellite is gradually peptized, forming small tactoids with monomeric AO in the interlayer space and at the same time adsorbing dimeric and polymeric AO cationic species at the solid-liquid interface. Compared with the other smectites, AO shows the greatest tendency to undergo metachromasy in the presence of beidellite.

The adsorption by different silicate minerals of some heavy metals, present in industrial waste water, has been studied. These adsorbents (mainly clay minerals) are readily available, inexpensive materials and offer a cost-effective alternative to conventional treatment of wastes from the metal finishing industry. The results show that some mineral species are suitable for the purification of such residual waters down to the limits prescribed by current legislation concerning industrial wastes. The Langmuir model was found to describe such adsorption processes best. Sepiolite (Orera, Spain) has an adsorption capacity of 8.26 mg g-1 for Cd2+, the capacities depending on the metal adsorbed in the order: Cd2+ > Cu2+ > Zn2+ > Ni2+. This mineral shows the highest sorption capacity relative to the other minerals studied. Factors in the reaction medium such as pH and ionic strength influenced the adsorption process.

Raman spectra of a selection of kaolins with widely varying TiO2 contents revealed the presence of anatase in 17 of 18 samples. A significant positive correlation was observed between the intensity of the main anatase Raman band at ~145 cm-1 and the total TiO2 contents of the kaolins as determined by chemical analysis. No TiO2 modification other than anatase was detected in any of the samples.

The determination of the anatase contents in kaolins by XRD becomes increasingly difficult as the concentration of this mineral decreases below ~0.5%. In contrast, anatase and brookite concentrations an order of magnitude lower can still be readily identified by Raman spectroscopy. The specificity and sensitivity of Raman spectroscopy for these TiO2 polymorphs, coupled with the simplicity and rapidity of measurement, therefore make this a viable technique for the routine study of these accessory minerals in kaolins.

The ability to produce humic-like polymeric compounds, with D-glucose and L-tyrosine as starting materials, was evaluated in different mineral systems: (1) Ca-, Al- and Cu(II)-saturated montmorillonite; (2) Ca-, Al- and Cu(II)-saturated kaolinite; (3) quartz in the presence of two different amounts of the same cations (according to the cation exchange capacity of the clays); and (4) untreated quartz (as control). All systems proved to be effective in the formation of humic-like compounds, particularly quartz, in the presence of cations. The effectiveness in promoting humification reactions was strictly related to the amounts of added cations. In the reaction conditions considered, the humification appears to be due more to the cations than to the type of clay minerals. The clayey systems synthesized more complex (aromatic) substances than the quartz ones.

The redox potential and pH of aerated suspensions of iron(II) hydroxide in sulphate–containing aqueous solutions are measured during the oxidation process. Plateaux corresponding to the equilibrium conditions between Fe(OH)2(s) and Fe(II)-Fe(III) hydroxysulphate GR2(SO2-4)(s) on the one hand, and between GR2(SO2-4)(s) and FeOOH(s) on the other hand, are displayed. Potentiometry, voltammetry, pH-metry and Mössbauer spectroscopy are applied to follow all reactions. The thermodynamic meaning of the measured potential of the first plateau which corresponds to the GR2(SO2-4)(s)/Fe(OH)2(s) equilibrium is demonstrated. The chemical composition of GR2(SO2-4)(s) is found to be FeII4 FeIII2(OH)12SO4·nH2O all along the oxidation process, implying that this compound must be considered as a pure phase with a well-defined composition. The Gibbs standard free energy of formation or chemical potential μ°[GR2(SO2-4)(s)] in ‘anhydrous form’ (n = 0) is determined at −3790±10 kJ mol-1. A consistent value of μ°[Fe(OH)2(s)] at −490±1 kJ mol-1 is obtained.

Both magnetic and electric fields have been used to induce optical birefringence in colloidal suspen- sions. These have formed the bases of fast methods for measuring particle sizes and their distributions (Mekshenkov, 1965; Tsvetkov et al., 1975; O'Konski, 1976; Maret & Dransfeld, 1977; Stoylov, 1991; Jennings & Stoylov, 1992; Trusov & Vojtylov, 1993). Whereas the majority of studies have related to the electric-field procedure, we recently reported a magneto-optic procedure suitable for sizing magnetic particles (Wilson et al., 1996). By increasing the viscosity of the suspensions, we are now able to extend the size range down into the sub-micron region with relative ease. This note reports magnetic birefringence data for a montmorillonite sample and justifies the technique via supplementary data on vermiculite of greater size range.

Ferric oxides or hydroxides are common minor components of kaolinites. If free, these minerals can be removed by Fe reduction and leaching (Mehra & Jackson, 1960), but Fe3+ ions can substitute for Al3+ ions in the octahedral sites of kaolinite (Weaver et al., 1967; Malden & Meads, 1967). Moreover, Fe2+ ions can also substitute trioctahedrally in place of dioctahedral Al, leading to a local composition of Fe-antigorite (Cuttler, 1980).

The Font-Bouillant quarry (Charentes, France) is the source of disordered kaolinite known as ‘FU7, U-7-10 or FBT’ (Cases et al., 1982, 1986). The samples used in this study were considered in an earlier genetic analysis of the deposit (Delineau, 1994; Delineau et al., 1994) and were analysed previously by X-ray diffraction (XRD), infrared (IR) spectroscopy, electron paramagnetic resonance (EPR) and UV-visible spectroscopy by Delineau et al. (1994) (Table 1).