The validity of the saddle/001 method for estimating the percentage of smectite layers in randomly interstratified illite/smectite (I/S) minerals as a routine laboratory technique has been examined with respect to the effects of the crystallite size distribution (N = number of layers) of I/S and the degree of preferred orientation of crystallites in the prepared specimen. X-ray powder diffraction experiments of I/S clays indicated that the crystallite size distribution was 3 < N < 12; these values were supported satisfactorily by the variation of the d(002) value of the samples. An analysis of the Lorentz factor concerning the degree of preferred orientation of crystallites indicated that a calibration curve calculated using the random powder Lorentz factor and the above crystallite size distribution fit the data better than an assumption of perfect orientation. Consequently, if a calibration curve of the saddle/001 ratio is used to estimate the percentage of smectite layers in I/S, an error of 10–15% should be expected from the variable crystallite size distribution of actual samples, in which I/S dominates over other phyllosilicate phases that give reflection between 10–14 Å. This method is useful, however, in estimating the relative percentage of smectite layers in randomly interstratified I/S for samples examined under identical experimental conditions.

Mössbauer spectra of 15 hematites with Al substitutions between 0 and 10 mole % were taken at room temperature. X-ray powder diffraction indicated dimensions of these hematites in the c-direction to range upwards from 27 nm to crystals large enough to show no line broadening. The Mössbauer spectra showed that magnetic hyperfine fields decreased both with increasing Al-for-Fe substitution and with decreasing crystal size. These relationships indicate that hyperfine field variations cannot, as has been done in the past, be unequivocally related to Al substitution alone. Hyperfine field reductions were paralleled by Mössbauer line broadening due to hyperfine field distributions. Only the hematites heated to 1000°C showed a significant variation of quadrupole splittings with Al substitution. No dependence of quadrupole splitting on crystal size was observed, indicating no detectable distortion of coordination polyhedra in the particle size range studied.

We used high-resolution X-ray texture goniometry to quantify changes in the mm-scale orientation of phyllosilicate minerals in a suite of Paleogene mudstones from the Podhale Basin in southern Poland. The sample set covers an estimated range of burial depths between 2.4 and 7.0 km, corresponding to a temperature range of 60–160°C. Although mechanical compaction has reduced porosities to ∼10% in the shallowest samples, the phyllosilicate fabric is only modestly aligned. Coarser-grained (>10 µm) detrital chlorite and mica appear to be more strongly aligned with (001) parallel to bedding, suggesting their deposition as single grains rather than as isotropic flocs or aggregates. From 2.4 to 4.6 km, R0 illite-smectite with 40–50% illite layers changes to R1 illite-smectite with 70–80%) illite layers. At the same time kaolinite is lost and diagenetic chlorite is formed. The mineralogical changes are accompanied by a strong increase in the alignment of illite-smectite, chlorite, and detrital illite, parallel to bedding and normal to the presumed principal effective stress. We propose that the development of a more aligned I-S fabric results from the dissolution of smectite and the growth of illite with (001) normal to the maximum effective stress. Water released by illitization may act as a lubricant for the rotation of all platy minerals into nanoporosity transiently formed by the illitization reaction. At greater depths and temperatures, further illitization is inhibited through the exhaustion of K-feldspar. After the cessation of illitization, a further 2.4 km of burial only results in a small increase in phyllosilicate alignment. At such small values for porosity and pore size, increasing stress does not substantially reorient phyllosilicates in the absence of mineralogical change.

Adsorption by synthetic 2-line ferrihydrite and hematite of glyphosate and phosphate, separately and together, was compared with adsorption results for goethite, gibbsite and two kaolinites in order to determine adsorption differences and similarities, in particular competition and phosphate preference, of these variable-charge minerals. Hematite rapidly adsorbed both compounds, while adsorption by ferrihydrite was slow, in particular of glyphosate, probably because of very slow diffusion of the bulky glyphosate molecules into interior sites in ferrihydrite particles. Accordingly, the Langmuir adsorption capacity of glyphosate (GAC) was considerably smaller (1.85 µmol m−2)than GAC for hematite (2.61 µmol m−2). The phosphate adsorption capacities (PAC) for ferrihydrite and hematite were more alike, 2.91 µmol m−2 and 2.85 µmol m−2, respectively. Differences between surface coordination (mono- or bidentate) may also contribute to the observed differences but conflicting information about the nature of the surface complexes makes this a difficult contributary factor to assess. The minerals were found to exhibit great variation in extent of competition and phosphate preference. Little competition and phosphate preference characterized hematite adsorption, while phosphate almost completely outcompeted glyphosate on goethite; ferrihydrite adsorption fell between these extremes. These differences may be attributed to different numbers of common (competitive) and specific (selective) adsorption sites on the three Fe oxides with a decreasing number of common sites in the order: goethite>>ferrihydrite>hematite, i.e. almost all goethite sites are common but with strong phosphate preference, while most hematite sites are specific for either glyphosate or phosphate. Alternatively, the result may be explained by adsorption in more planes, e.g. glyphosate adsorption onto the inner-Helmholtz-plane-adsorbed phosphate. For all six minerals compared, desorption of glyphosate following phosphate addition was found to be significantly correlated with the difference between the amounts of phosphate and glyphosate adsorbed indicating that this difference may be used as a competition index for predicting the influence of phosphate on glyphosate adsorption.

Structural Fe3+ in kaolinites and dickites covering a broad range of disorder was investigated using electron paramagnetic resonance (EPR) spectroscopy at both the X and Q-band frequencies. A procedure based on a numerical diagonalization of the spin Hamiltonian was used to accurately determine the second and fourth-order fine-structure parameters. A least-squares fitting method was also developed to model the EPR spectra of Fe3+ ions in disordered local environments, including multimodal site-to-site distributions. Satisfactory fits between calculated and observed X and Q-band spectra were obtained regardless of the stacking order of the samples.

In well-ordered kaolinite, Fe3+ ions are equally substituted in sites of axial symmetry (Fe(II)sites, namely Fe(II)a and Fe(II)b) which were determined to be the two non-equivalent Al1 and Al2 sites of the kaolinite structure. In dickite, Fe3+ ions were also found to be equally substituted for Al3+ in the two non-equivalent Al sites of the dickite structure. In poorly ordered kaolinites, the distribution of the fine-structure parameters indicates that Fe3+ ions are distributed between Fe(II) sites and other sites with the symmetry of the dickite sites.

Hence, when stacking disorder prevails over local perturbations of the structure, the near isotropic resonance owing to Fe3+ ions in rhombically distorted sites (Fe(I) sites) is a diagnostic feature for the occurrence of C-layers in the kaolinite structure, where C refers to a specific distribution of vacant octahedral sites in successive layers.

Novel modified clay adsorbents were prepared by blocking cation-exchange sites of an expandable clay, such as montmorillonite, with polymeric or polyvalent inorganic ions and by using cationic surfactants as sources of surface organic carbon. Electrokinetic measurements demonstrated that the adsorbed polycations were essentially nonexchangeable. Adsorption and desorption experiments revealed that about 90% of the cationic surfactant was apparently irreversibly bound to the surface. Flocculation and peptization studies were performed to establish that the adsorbed surfactant moiety was oriented with its hydrocarbon tail towards the surface. Such a configuration of simultaneously adsorbed polycations and cationic surfactants was designated as an inorgano-organo-clay (IOC). As shown in an accompanying paper, these IOCs bind priority pollutants as strongly as granulated activated carbon.

Chromium(VI) in the environment is of particular concern because it is toxic to both plants and animals, even at low concentrations. As a redox-sensitive element, the fate and toxicity of chromium is controlled by soil reduction-oxidation (redox) reactions. In-situ remediation of chromium combines reduction of Cr(VI) to Cr(III) and immobilization of chromium on mineral surfaces. In this study, Fe-rich smectite, montmorillonite, illite, vermiculite, and kaolinite were examined to determine reactivity in sorption-reduction of Cr(VI). The clays were compared to forms that were reduced by sodium dithionite. Clays containing Fe(II) efficiently removed soluble Cr(VI) from solution. Chromium K-edge X-ray absorption near edge structure (XANES) suggested that clays containing Fe(II) reduced Cr(VI) to Cr(III), immobilizing Cr at the clay/water interface. Adsorption of Cr(VI) by the Fe(II)-containing clay was a prerequisite for the coupled sorption-reduction reaction. Sodium dithionite added directly to aqueous suspensions of non-reduced clays reduced Cr(VI) to Cr(III), but did not immobilize Cr on clay surfaces. The capacity of clays to reduce Cr(VI) is correlated with the ferrous iron content of the clays. For dithionite-reduced smectite, the exchangeable cation influenced the sorption reaction, and thus it also influenced the coupled sorption-reduction reaction of Cr(VI). The pH of the aqueous system affected both the amount of Cr(VI) reduced to Cr(III) and the partition of Cr(III) between aqueous and adsorbed species. A plot of pH vs. amount (adsorption envelope) adsorbed for the coupled sorption-reduction reaction of Cr by reduced smectite exhibited a similar pattern to that of typical anion-sorption.

The clay fraction of a Spodosol and its parent rock in the Apennine mountains of central Italy were studied by powder X-raydiffraction (XRD) and infrared (IR) spectroscopy, to evaluate the possibility of transformation of chlorite into low-charge expandable minerals. Results indicated that the main phyllosilicate in the rock was a slightly weathered trioctahedral chlorite, rich in both Mg and Fe, together with dioctahedral mica and minor amounts of kaolinite. In the BC horizon, chlorite has undergone partial transformation into 2 vermiculitic components, in 1 of which the interlayer could be removed by hot Na-citrate treatment; the presence of a regular interstratified mineral (high-charge corrensite) was also observed. Further changes in the structure of chlorite were detected in the Bsl horizon, becoming more evident towards the soil surface. The first stage of weathering of chlorite involved Fe oxidation and partial expulsion of Mg from the hydroxide sheet, followed by deposition of Al in the interlayer space. Iron is also removed from the interlayer sheet, possibly remaining, in the oxidized state, in the 2:1 octahedral sheet, and so contributing to the lowering of layer charge and transformation to a dioctahedral structure. When approaching the surface, Al removal from the interlayers is enhanced by complexing agents, and further charge reduction leads to the formation of 2:1 minerals with a smectite nature. Illite, because of its low content in the soil clay fraction, contributes marginally to this weathering sequence, forming the high charged expandable component observed in the Bhs horizon. At the soil surface, arandomly interstratified vermiculite/illite was detected, which probably originated from K fixation by the higher-charged expandable minerals. This study of weathering in a natural soil strongly supports the hypothesis, previously ascertained by laboratory experiments, that chlorite can transform into a low-charge expandable mineral.

Small-angle X-ray powder diffraction analyses and high-resolution electron microscopy of allophane samples (SiO2/Al2O3 ratio, 1.12 to 1.68) showed that allophanes consist of nearly identical spherical particles with diameters of about 40 Å and retain their characteristic “hollow” spherical morphology at different ambient moisture and even after dehydroxylation by heating at 500° to 600°C. Unheated allophane samples gave another X-ray powder diffraction band whose maximum position varied from 12.3 to 14.5 A depending on their SiO2/Al2O3 ratio. The appearance of this band may denote some long-range ordering in the structure of allophane. Unlike the spherical particles of allophane, the tube unit of imogolite collapsed on dehydroxylation. This observation suggests that imogolite and allophane are different in their framework structures and that a Si- or Si(Al)-tetrahedral sheet rather than an Al-octahedral sheet constitutes the framework structure of allophane, irrespective of its SiO2/Al2O3 ratio.

An O2 oxidation and freeze-dry procedure has been used to synthesize birnessite through the oxidation of Mn2+ in alkali media. The effects of O2 flow rate, the fluxion velocity of reaction suspension, the reaction temperature, pretreatment the reaction solutions with N2, and the hydration conditions on the purity of birnessite, the crystallinity, the ion-exchange properties, and the phase transformation of intermediate phases were examined. Buserite with a 1 nm basal spacing, produced after the oxidation, is transformed to 0.7 nm Na birnessite by complete freeze drying. Increasing the fluxion velocity of the reaction suspension and the O2 flow rate facilitated oxidation of Mn(OH)2. Prephase I (a phase related to hausmannite, γ-Mn3O4 (Luo and Suib, 1997; Luo et al., 1998), and feitknechtite (β-MnOOH) were formed as intermediates during the synthesis. Mechanical stirring was used to change the fluxion velocity of the reactive suspension. When the speed of stirring and the O2 flow rate were raised to 250 rpm and 3.0 L/min, respectively, or 450 rpm and 2.0 L/min, respectively, birnessite was the only phase in the final product. Irrespective of temperature in other reactions, pure birnessite was synthesized as long as the temperature during the initial mixing of the reaction solutions was maintained below 10°C. Increasing the reaction temperature led to a larger crystal size, better crystallinity and lower surface area. The pretreatment of solutions with N2 or O2 had little effect on the synthesis; synthesized birnessites had the same purity (100%) as, and similar crystallinity to, that of the no-pretreatment control. Dehydration of the buserite by freeze drying and heating at 60°C did not affect the production of birnessite; however, the latter caused partial loss of ion-exchange capacity with Mg2+. The pathways of the birnessite formation in this study might be:

1. (1) Mn(OH)2 (amorphous) → feitknechtite → buserite → birnessite, and

2. (2) Mn(OH)2 (amorphous) → prephase I → feitknechtite → buserite → birnessite

Mn(OH)2 existed in an X-ray amorphous state, not in the form of ‘pyrochroite’, during the synthesis.

The adopted conditions for synthesis of pure birnessite were NaOH to Mn molar ratio of 13.7, O2 flow rate of 2 L/min and oxidation for 5 h during vigorous stirring at 450 rpm at room temperature. The birnessite synthesized had a hexagonal platy morphology with good crystallinity, an average composition of Na0.25MnO2.07.0.66H2O, and a surface area of 38 m2/g.

A laboratory study of the hydraulic conductivity (HC) of a marine clay with monovalent, divalent and trivalent cations revealed large differences in HC. The exchangeable cations employed in this study are Na, K, NH4, Mg, Ca and Al in order of increasing valency. An interpretation of the results derived from consolidation tests suggests that HC is significantly affected by the valency and size of the adsorbed cations. An increase in the valency of the adsorbed cations leads to quicker rates of consolidation and higher HC, while, for a constant valency an increase in the hydrated radius of the adsorbed cations results in a lower rate of consolidation and HC. The reduction in HC was related to the dispersion and deflocculation of clay. Lower valency and higher hydrated radii of the exchangeable cations enable the double layer repulsive forces to predominate, thereby increase dispersion and deflocculation.

Bromide-containing impurities were found to decrease the thermal stability of quaternary alkyl ammonium-modified layered silicates. Improved purification procedures completely removed bromide and led to a 20°C to >100°C increase in organic modified layered silicate thermal stability. Using mass spectrometry and thermal and electrochemical analysis, N,N-dimethyl-N,N-dioctadecyl quaternary ammonium-modified montmorillonite and fluorinated synthetic mica were found to degrade primarily through elimination and nucleophilic attack by these anions. The nature of residual bromides was identified and quantified, and the efficiency of removing these anions was found to be solvent dependent; sequential extraction, first ethanol then tetrahydrofuran, gave the best results. This exhaustive extraction method represents a viable alternative to the use of expensive, more thermally stable oniumion treatments for layered silicates.

The correlation between illite/smectite (I/S) diagenesis and mean vitrinite reflectance (Ro) data is examined in mudrocks from a hydrocarbon exploration well (geothermal gradient 35°C km−1) from the Great Hungarian Plain of the Pannonian Basin System. The expandability of I/S decreases with depth and there is a change from random to ordered mixed-layering at about 2500 m depth. At this depth Ro is about 0.6%. Comparison of the correlation of expandability and Ro from this study to published data for the Vienna Basin and the Transcarpathian Basin, sub-basins of the Pannonian Basin System, shows that the correlation is systematically different for each sub-basin, according to their thermal histories. In the Vienna Basin (geothermal gradient 25°C km−1), for any given value of Ro, the expandability of I/S is less than in the Transcarpathian Basin (geothermal gradient 55°C km−1) and the sediments are older and more deeply buried. Data from the present study are intermediate. This variation is believed to be due to the effect of time on the smectite-to-illite reaction. Results of an optimization procedure to calculate the kinetics of the smectite-to-illite reaction, using as input the expandability depth profiles, and thermal histories constrained by comparison of observed and calculated Ro data, showed that I/S diagenesis in the Pannonian Basin System can be modelled by a single first order rate equation:

$-dS/dt=e^{log(A)-E/RT}\cdot S$

Transmission electron microscopy (TEM) has been used to demonstrate the biological formation of a hollow spherical halloysite-like mineral in freshwater systems. The interaction between clays and microbes was investigated in microbial films from laboratory cultures derived from natural sediments. Optical and electron microscopic observations of cultured microbes revealed that thin clay films covered areas of the bacterial cell wall. X-ray diffraction of the thin films after 2 y of ageing showed a 7.13 Å d spacing, consistent with a 7 Å halloysite-like phase [Al2Si2O5(OH)4 H2O]. Fourier transform infrared analysis of the thin film exhibited the characteristic adsorption bands for O−H (3651 cm−1), C−H and C−N (2925, 1454 and 1420 cm−1, respectively), suggesting that the phase was closely associated with adhesive organics. Observation by TEM of the thin films revealed that spherical, hollow, halloysite-like material formed on both coccus- and bacillus-type bacterial cells. Electron diffraction analysis of this material showed 2.9, 2.5, 2.2 and 1.5 Å d spacings. The present investigation strongly suggests that the thin film wall of the spherical halloysite-like material was associated with bacteria as a bio-organic product. This material, referred to hereafter as bio-halloysite, is further evidence for the microbially-mediated formation of clay minerals. The identity of the bacteria responsible for bio-halloysite formation is unknown, but is tentatively assigned to sulfate-reducing bacteria on the basis of morphology and the presence of reducing conditions in the microcosm at the end of the experiments.

A new appraisal of radiation-induced defects (RID) in natural kaolinite, i.e., positive trapped holes on oxygen atoms, has been undertaken using Q-band EPR spectra, recorded at 93 K, of irradiated annealed and oriented kaolinite samples originating from various environments. Three different centers were identified. Two of the centers, A- and A’-centers, are trapped holes on oxygen from Si-O bonds. They have a distinct signature and orthogonal orientation, i.e., perpendicular and parallel to the (ab) plane, respectively. The third center, the B-center, is a hole trapped on the oxygen bonding Al in adjacent octahedral positions (AlVI-O−-AlVI bridge). This confirmed some previous assignments from the literature, some others are no longer considered as valid.

A least squares fitting procedure is proposed to assess the RID concentration in any kaolinite. It allows a quantitative approach of the thermal stability of RID. Isochronal annealing shows that the thermal stability of the centers decreases in the order A, A′, B over the temperature range 0–450°C: (1) B-center is completely annealed above 300°C; (2) A′-center can be annealed by heating at 400°C for more than two hours; (3) A-center is stable up to 450°C. The activation energy and the magnitude of the mean half-life for A-center is evaluated through isothermal annealing at 350, 375 and 400°C, with Ea = 2.0 eV ± 0.2, and t½ > 1012 years at 300 K. The stability of A-center seems to decrease with increasing crystalline disorder. Nevertheless, it is high enough for radiation dosimetry using kaolinites from any environment on the Earth's surface.

The results of a combined chemico-osmotic/diffusion experiment conducted on a geosynthetic clay liner (GCL) containing Na-bentonite illustrate the destructive role of diffusion on the ability of the GCL to act as a semipermeable membrane. The experiment is conducted by maintaining a concentration difference of 5 mM CaCl2 across the GCL specimen while preventing the flow of solution through the specimen. A time-dependent membrane efficiency is derived from measured pressure differences induced across the specimen in response to the applied concentration difference. The diffusive mass fluxes of the solutes (Cl− and Ca2+) through the specimen are also measured simultaneously. An initial increase in induced pressure difference across the specimen to a peak value of 19.3 kPa is observed, followed by a gradual decrease to zero. The decrease in induced pressure difference is consistent with compression of diffuse double layers between clay particles and particle clusters due to diffusion of Ca2+, resulting in a concomitant increase in pore sizes and decrease in the observed membrane behavior. The time required for effective destruction of the initially observed semipermeable membrane behavior correlates well with the time required to achieve steady-state Ca2+ diffusion. The results have important implications for the ability of clays to sustain membrane behavior.

The aqueous chemistry of water films confined between clay mineral surfaces remains an important unknown in predictions of radioelement migration from radioactive waste repositories. This issue is particularly important in the case of long-lived anionic radioisotopes (129I-, 99TcO4-, 36Cl-) which interact with clay minerals primarily by anion exclusion. For example, models of ion migration in clayey media do not agree as to whether anions are completely or partially excluded from clay interlayer nanopores. In the present study, this key issue was addressed for Cl- using MD simulations for a range of nanopore widths (6 to 15 Å) overlapping the range of average pore widths that exists in engineered clay barriers. The MD simulation results were compared with the predictions of a thermodynamic model (Donnan Equilibrium model) and two pore-scale models based on the Poisson-Boltzmann equation under the assumption that interlayer water behaves as bulk liquid water. The simulations confirmed that anion exclusion from clay interlayers is greater than predicted by the pore-scale models, particularly at the smallest pore size examined. This greater anion exclusion stems from Cl- being more weakly solvated in nano-confined water than it is in bulk liquid water. Anion exclusion predictions based on the Poisson-Boltzmann equation were consistent with the MD simulation results, however, if the predictions included an ion closest approach distance to the clay mineral surface on the order of 2.0 ± 0.8 Å. These findings suggest that clay interlayers approach a state of complete anion exclusion (hence, ideal semi-permeable membrane properties) at a pore width of 4.2 ± 1.5 Å.

An experimental program was conducted to investigate the water-vapor sorption characteristics of smectite and kaolinite mixtures. End-member smectite and kaolinite were slurry-mixed together at mass-controlled ratios corresponding to 0%, 20%, 50%, 70%, 80%, 90% and 100% smectite. Vapor desorption isotherms for the mixtures were measured at 24°C for relative humidity (RH) ranging from ∼95% to 0%.

Results show that the amount of water adsorbed by the clay mixtures at a given RH increases systematically with increasing smectite content. Derivative analysis of the sorption isotherms shows evidence of transitions between the two-, one- and zero-layer hydrate-states for the smectite-rich mixtures. The transitions become less apparent as the smectite content decreases. Monolayer coverage, specific surface area, and heat of adsorption were estimated from the isotherms using BET theory. It is shown that monolayer coverage and specific surface for the clay mixtures can be reasonably approximated by weighted averaging of the end-member clay properties. General methodologies are presented for predicting the sorption behavior (i.e. soil-water characteristics) and effective specific surface area from measurements of the end-member sorption isotherms.

Clays have played an important role in medicine since the dawn of mankind and are still applied widely as active ingredients and/or excipients in pharmaceutical formulations. Due to their outstanding properties of large retention capacity, swelling and rheological properties, and relative low cost, they have been used widely as advanced carriers for the efficient delivery of drugs by modifying their release (rate and/or time), increasing the stability of the drug, improving the dissolution profile of a drug, or enhancing their intestinal permeability. In addition, recent studies have shed new light on the potential of clay minerals in the nanomedicine field due to their biocompatibility, beneficial effects of clay nanoparticles on cellular adhesion, proliferation, and differentiation. Use as active ingredients and excipients are exerted via the oral and topical administration pathways. Skin drug delivery represents an attractive alternative to the oral route, providing local and/or systemic drug delivery. Due to their complex structures, however, most drugs penetrate the human skin only with difficulty. Enormous efforts have been invested, therefore, in developing advanced drug delivery systems able to overcome the skin barrier. Most strategies require the use of singular materials with new properties. In particular, and on the basis of their inherent properties, clay minerals are ideal candidates for the development of intelligent skin drug delivery systems. In this article, the properties of clay materials and their use in the skin-addressed pharmaceutical field are reviewed. A brief introduction of skin physiology and biopharmaceutical features of penetration by a drug through the skin layers is also included and is designed to shed light on the optimum properties of ideal nanosystems for advanced skin drug delivery. Special attention is devoted to the pharmacological functions of clays and their biomedical applications in pelotherapy, wound healing, regenerative medicine, antimicrobial, and dermocosmetics.