Natural zeolites have traditionally been used as supplements in animal nutrition with positive effects in terms of either faster growth or reduced incidence of diseases and death. The discovery that natural zeolites may act as drugs or coadjutants in pharmaceutical preparations has greatly enlarged the field of their applications in many sectors of biology and medicine, opening unexpected horizons for either basic or applied research. This review updates the state of the art of research in this sector, focusing on clinoptilolite-rich materials, with two main purposes: (1) trying to rationalize this subject, which is extremely variable and heterogeneous, and (2) raising doubts about unsuitable uses of natural zeolites in the light of their known properties and expected behaviour.

Fe substitution for Al in kaolinite (from Venezuelan laterites) is proved by infrared spectroscopy and chemical techniques. The location of Fe in the octahedral sheet is characterized by two absorption bands, at 865–875 and 3607 cm−1, assigned as δ Al-OH-Fe and v OH respectively. The detection of the 865 cm−1 band requires the use of CsCl for the preparation of the disks heated to 270°C so that the clay delaminates as a result of a kaolinite-CsCl-H2O complex formation. The 3607 cm−1 absorption is detected when KI disks are prepared. These two characteristic frequencies persist after either thermal decomposition or selective chemical dissolution of free iron and aluminium hydroxides.

Selective chemical dissolutions by consecutive treatments for; (1) the removal of free iron oxides, (2) gibbsite removal and (3) extractions of Fe and Al from kaolinite, give additional evidence about the occurrence of this solid solution.

Magnetite is a redox active Fe oxide common in most soil and sedimentary environments. In this study, we investigated magnetite transformations under extreme alkaline conditions (0.1–2 mol l–1 NaOH) similar to those found under high-level radioactive waste storage tanks at the Hanford site in the State of Washington, USA. X-ray diffraction (XRD) and transmission electron microscopy (TEM) results showed that magnetite was transformed into maghemite and goethite, and the XRD peak intensities for goethite increased with NaOH concentration. Goethite was presumably formed through reconstructive dissolution/crystallization reactions that can be written as: Fe3O4 + OH– + H2O = γ-Fe2O3 + Fe(OH)3–; 2Fe(OH)3– + 1/2O2 + 2H+ = 2α-FeOOH + 3H2O. Some of the newly formed maghemite may also redissolve to form goethite at greater NaOH concentrations.

Experiments carried out in the author's laboratory on the artificial weathering, in an open system, of potash felspar, albite, leucite, tremolite, olivine, sideromelane, muscovite, and clay minerals are reviewed. The results are assessed, and eventually compared on the basis of the amount of SiO2 removed after leaching with 4 1 solvent.

Middle Jurassic sandstones contain a variety of authigenic clay materials. The origin and distribution of these are related to the influence of several factors, principally depositional pore-fluid chemistry, sandbody geometry and the migration of aggressive fluids. Pore-lining illite, pore-lining chlorite and pore-filling vermiform kaolinite in the Ravenscar Group mutually exclude each other because of depositional pore-water chemistry; seawater in the case of the illite, anoxic freshwater in the case of chlorite and oxygenated freshwater in the case of the kaolinite. Blocky pore-filling dickite occurs ubiquitously within the large connected sandbodies. Its origin may be related to the migration of aggressive fluids and its distribution to depositional sandbody geometry. Mixed-layer chlorite-vermiculite also occurs, and is believed to have formed from chlorite during Recent weathering. Similar patterns occur in Ninian Field Brent Group sandstones, although the situation is more complicated (in these sandstones the kandite subgroup minerals are undifferentiated in this paper). Here too, pore-lining illite occurs in marine sandstones and pore-filling vermiform kandites in non-marine sandstones. However, vermiform kandites also occur in the marine sandstones, perhaps due to freshwater-table development following progradation. The more blocky kandites occur in large connected sandbodies. Finally, a second phase of illitization occurs, postdating blocky kandites, perhaps caused by alkaline formation waters. The occurrence of chlorite in the Broom Formation is anomalous and its possible origin is discussed.

During weathering of forsterite and tephroite under the humid, tropical conditions of the Ivory Coast, intermediate Mg,Fe3+− or Mn2+-smectites appear prior to widespread development of iron or manganese oxyhydroxides. In forsterite, the smectite varies from an Mg-trioctahedral variety near the centres of altered crystals to an Mg,Fe3+,Al-di/trioctahedral variety near the edges. The Al-content of this latter smectite is supplied by dissolution of adjacent minerals, notably enstatite. An Mn2+-trioctahedral smectite forms in cracks in tephroite crystals.

Previous work on the structural and thermal properties of various types of kaolinite have led to different conclusions, rendering comparison of analytical results difficult. The objectives of the present study were to investigate the thermal behaviour of kaolinite and to carry out a kinetic analysis of the decomposition of kaolinite at high temperatures. X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR) and thermogravimetry-differential scanning calorimetry (TG-DSC) were used to study the mechanism of the thermal decomposition. The modified Coats–Redfern, Friedman, Flynn–Wall–Ozawa and Kissinger decomposition models were used to determine the decomposition mechanism of the kaolinite sample. The dehydroxylation of kaolinite occurred at ∼600°C with the formation of metakaolin, which then transformed into either γ-alumina or aluminium-silicon spinel together with amorphous silica. The results of the XRD and FTIR analyses indicated that the γ-alumina, or aluminium-silicon spinel and amorphous silica phases, transformed into mullite and α-cristobalite, respectively, after decomposition at 900°C. Good linearity was observed with the modified Coats–Redfern, Flynn–Wall–Ozawa and Kissinger models from room temperature to 1400°C and the range of the activation energy determined was 120–180 kJ/mol.

Although the kinetics of the dehydroxylation of kaolinite have been widely studied, there is no definitive explanation of its mechanism due to its dependence on a variety of parameters. In this study, the dehydroxylation of kaolinite has been studied using controlled-transformation rate thermal analysis (CRTA), allowing precise control of the reaction rate, and thus of both the temperature and pressure above the sample. Modelling of the experimental results obtained by CRTA as well as those from TEM and MAS-NMR show that two elementary processes (diffusion and firstorder) can occur in competition. At the start of the decomposition the diffusion mechanism is predominant, but as the reaction progresses, the first-order mechanism prevails. It would seem that the importance of each process depends, in particular, on the presence of defects as well as on the local vapour pressure.

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.

Lower Palaeozoic shales and slates in the External Domain of the southern Canadian Appalachians are composed predominantly of illite and chlorite with minor occurrences of I-S mixed-layer minerals (restricted to samples with illite crystallinity, IC > 0·62°Δ2Θ) and paragonite (restricted to samples with IC < 0·42°Δ2Θ). Inverted diagenesis has occurred in the NW part of the Chaudière Nappe, indicating pre-orogenic deep burial diagenesis at the original depositional site, whereas to the SE, the diagenetic pattern was affected by synorogenic heating. Within the east-dipping thrust-fold belt and the St Lawrence Lowlands, increasing grade towards the S suggests a gradual southward increase in post-tectonic burial depth. Narrow (3–5 km) thermal haloes around the Cretaceous Monteregian intrusions show limited effects on the country rocks.

The percentage of 2M1 mica polytypes and bo increase with decreasing IC. Chlorite crystallinity (CC) increases with increasing IC. Good correlations between IC, %2M1, CC and bo of micas indicate that these parameters are reliable monitors of high-grade diagenesis and low-grade metamorphism in clay-rich sedimentary rocks. IC and CC improve with increasing grain size, illustrating the effect of grain size on IC and CC. Organic material affects IC more strongly in strata with lower permeability than in those with higher permeability. In the diagenetic zone, glycolation does not uniformly produce a narrowing of the 10 Å illite peak, but may also broaden it by up to 15%, probably due to the presence of Kalkberg-type mixed-layers.

Cationic dye, methylene blue (MB), was intercalated into films of expandable clay minerals with low, moderate and high layer charges (saponite, montmorillonite and synthetic expandable mica-fluorohectorite, respectively) . Tilted orientation of the dye cations on the silicate surface was observed by polarized ultraviolet-visible (UV-Vis) transmission spectroscopy. The tilting angle of the dye cations increased with the formation of high-order H-aggregates that absorb light at 525 nm. These dye species were formed only on the surface of fluorohectorite, which has the highest layer charge, and exhibited dichroism. Models for the orientation of MB cations on a clay surface were proposed based on MB orientation models previously reported, and compared with the basal spacings determined by X-ray diffraction.

The weathering of a trachytic pumice within a pyroclastic flow underlying an andic-brown soil on the volcano Vico has been studied. The main mineral formed is a spherical 10 Å halloysite which has been shown by SEM and in situ microprobe analysis to have formed directly from the glass. The major mineralogical characteristics as determined by XRD, IR, DTA, TEM and microdiffraction are typical of 10 Å halloysite. However, some minor mineralogical properties and the high Fe and K contents, suggest that it is an interstratification of 74% halloysite and 26% illite-smectite. The calculated formula of the hypothetical 2:1 minerals reveals an Fe- and K-rich clay, with high tetrahedral substitution, like an Fe-rich vermiculite, but the detailed structure of this mineral remains uncertain.

Methylene blue (MB) has been used as a probe molecule to examine how the uptake dynamics and the equilibria between this dye and the surface of Na+- and H+- montmorillonite were affected by the presence of a second dye. To prevent spectral interference, the yellow dyes thioflavin T, TFT, proflavine, PFH, and acridine yellow, ACY, were chosen to compete with MB for the exchange sites. The MB was initially adsorbed as trimer (MB+)3 and then redistributed via collisions between clay particles until equilibrium was reached. At equilibrium in the clay/MB systems, the protonated species (MBH2+) predominated at low loadings (1–5% CEC), whereas at higher loadings the trimer (MB+)3 was the major species. The presence of the second, competing dye slowed the approach to equilibrium, significantly reduced the amount of MBH2+ formed and provided evidence for the monomeric MB+, dimeric (MB+)2, and trimeric (MB+)3 forms of MB. Moreover, the presence of PFH and ACY, which are structurally similar to MB, resulted in more dimeric character in the aggregated form of MB compared to the aggregate formed in the presence of the structurally dissimilar TFT.

Although numerous models for the formation of glauconite have been presented, the precise process and micro-environment of glauconitization are still poorly constrained. We characterize the special micromilieu of glauconitization developed during early diagenesis and present a model for glauconite formation in fecal pellets.

Glauconitization at Oker (Central Germany) occurred predominantly in fecal pellets deposited in a shallow marine-lagoonal environment during the Kimmeridgian. Within the fecal pellets, rapid oxidation of organic matter provides the post-depositional, physicochemical conditions favourable for glauconitization. Replacements of matrix calcite, dissolution of detrital quartz, K-feldspar, and clay minerals, and Fe redox reactions were observed within the early micro-environment, followed by the precipitation of euhedral pyrite, matrix-replacive dolomite, and megaquartz accompanied by I-S formation as thin section analyses and SEM observations show. Carbonate geochemical compositions based on ICP-OES and stable oxygen and carbon isotope signatures demonstrate that glauconite formation started in a suboxic environment at a pH of 7–8 and a temperature of 22±3°C to 37±2°C at maximum.

TEM-EDX-SAED and XRD analyses on separated glauconite fecal pellets and on the <2 μm clay mineral fraction reveal the predominance of authigenic 1Md-glauconite, 1Md-glauconite-smectite, and 1Mdcis-vacant I-S, besides accessory detrital 2M1-illite and montmorillonite. Kinetic modelling of the glauconite (93–94% Fe-illite layers and 6–7% Fe-smectite layers, R3) and of I-S (66–68% Al-illite layers and 32–34% Al-smectite layers, R1) leads us to conclude that the I-S formed solely by slow burial diagenesis, whereas the glauconite formed close to the seafloor, suggesting significantly faster kinetics of the glauconitization reaction compared with smectite-illitization related to burial diagenesis. Thermodynamically, the substitution of octahedral Al3+ for Fe3+ and Mg2+ during the Fe-Mg-smectite to glauconite reaction via the formation of glauconite-smectite mixed-layered clay minerals may have resulted in a higher reaction rate for this low-temperature glauconitization process.

Micropore distribution and effective size of the channels of natural sepiolite from Turkey were measured by the BET method. Before the BET measurement, the samples were treated under a water vapour atmosphere at various pressures to fill progressively the sepiolite micropores with water. The surface areas measured by means of N2 adsorption decreased with increased vapour pressures of water. The outer surface area was estimated by comparison of the surface area of the vacuum-dried sepiolite with that filled with adsorbed water. The total surface area was ∼290 m2/g, and the outer surface area was 170 m2/g, the difference being attributed to the structural micropores of the sepiolite. The ratio of the surface areas possessed by the channels and that of the outer surface suggest that the mean thickness of the sepiolite fibre was ∼12 nm. The effective size of the channels was estimated from the number of various-sized molecules sorbed by the sepiolite, the results showing that molecules larger than benzene could not migrate into the channels.

A survey of the clay fraction at the surface of 86 loam and silt loam agricultural soils from the Central United States was undertaken to observe the possible influences of climate and parent material (dominantly phyllosilicates in this study) on the clay mineralogy of the upper portions of the soils, for the most part in Mollisol–Alfisol order soils.

Decomposed X-ray diffraction spectra show that the most abundant clay minerals are two disordered illite-smectite (I-S) minerals, one with ∼50% smectite layers and another, less abundant, disordered I-S phase with 20% smectite. These minerals usually co-exist in the same samples. The charge site in both of these I-S minerals is both beidellitic and montmorillonitic in the expanding layers. A relatively large proportion of these smectite layers (up to 20%) are of high charge.

There seems to be a convergence in mineralogy towards mixed-layer phases formed under a range of mid-Continent climates from a range of phyllosilicate parent materials.

Mixed-layer clay minerals of illite-smectite, having smectite contents ∼25% and IS ordering, kaolinite-smectite, and a three-component variety with kaolinite-smectite-chlorite-like material are described from a Carboniferous paleosol in South Wales. The clays, which are restricted to particular horizons, are unusual occurrences in a pedogenic sediment and pose particular problems as to their genesis. Positive geological evidence, principally in the form of mineralogical and organic thermal maturation data, demonstrates that the rocks of the region cannot have reached diagenetic temperatures > 100°C. This would appear to suggest a pedogenic rather than a diagenetic origin for these clays. In these sediments the only plausible mechanism of illitization to produce the illite-smectite would be by repeated wetting and drying cycles causing irreversible K fixation. Such cycles would be entirely realistic for a vertisol-like paleosol formed on overbank deposits in a floodplain environment subject to strong seasonal moisture contrasts. If this interpretation is correct, this illite-smectite represents the most illitic yet recorded from a non-burial or non-hydrothermal environment.

Bentonite samples from Choushan treated at various acid concentrations were studied using chemical analysis, XRD, 29Si and 27Al MAS NMR to investigate the microstructure of the activated montmorillonites. Two types of structural units were formed during the activation: (1) (SiO)3SiOH, in which up to 15% of Si is bound; (2) Q4(0Al), a major building unit when the acid concentration is >10%. A significant 27Al signal at 55.0 ppm was recorded for both untreated montmorillonite and (to a much greater extent) in acid-treated montmorillonites. This was interpreted as arising from four-fold coordinated Al located in the ‘bulk’ octahedral sheet of montmorillonite. In the course of activation, the removal of one of a pair of octahedral Al ions from montmorillonite removes two hydroxyl groups and leaves the other Al of the pair in four-fold coordination.

The thermal decomposition of hydrotalcite (HT), with chemical composition Mg1-xAlx(OH)2(CO3)x/2.(1-3x/2)H2O, (0.20 < x ≤ 0.33), is a complex sequence of dehydration, dehydroxylation and decarbonization and leads to the formation of a series of metaphases: HT-D (dehydrated HT), HT-B (partially dehydroxylated HT) and MO (mixed oxides with periclase-like structure). The evolution of water and CO2 in natural and synthetic hydrotalcites (a Mg/Al ratio between 2:1and 3.7:1), heated to 800°C, was investigated by differential thermal analysis, thermogravimetry and evolved gas analysis. At least six endothermic and two exothermic effects were established by computer-aided resolving of the curves. The formation of each HT metaphase was related to the release of a discrete number of water molecules depending on the Al content in the samples and each appeared as a corresponding endothermic peak in the DTA curves. The exothermic processes associated with the crystallization of HT-B and MO metaphases were specified by decomposition of DTA curves. The evolution of CO2 during the thermal decomposition of the carbonate groups was found to be different for the samples studied. The preservation of CO3 even at high temperatures was established for synthetic samples with a high Al content. The release of volatile H2O and CO2 (which comprise ~40% of the sample mass) provokes fine cracking both along and across the layers.

The amounts of potassium retained, after exchange with normal ammonium acetate solution at pH 7, were measured with six montmorillonite-beidellite minerals, whose interlayer charge ranged 0·72-1·00 M+/Si8O22 and where tetrahedral substitution of Al for Si ranged 0·14-0·93 M+/Si8O22. The amount of potassium retained increased with increasing interlayer charge, but there was no correlation between the amount of potassium retained and the tetrahedral charge.