Smectites are one of the most important groups of phyllosilicates found in soils and sediments, and certainly one of the most difficult to study. New information about the formation mechanisms, impact of structural features on surface properties, and long-term stability of smectites can best be gained from the systematic study of single-phase specimens. In most instances, these specimens can only be obtained through synthesis under controlled conditions. Syntheses of smectites have been attempted (1) at ambient pressure and low-temperature (<100°C), (2) under moderate hydrothermal conditions (100–1000°C, pressures to several kbars), (3) under extreme hydrothermal conditions (>1000°C or pressures >10 kbars), and (4) in the presence of fluoride. Of these approaches, syntheses performed under moderate hydrothermal conditions are the most numerous and the most successful in terms of smectite yield and phase-purity. Using hydrothermal techniques, high phase-purity can be obtained for beidellites and several transition-metal smectites. However, synthesis of montmorillonite in high purity remains difficult. Starting materials for hydrothermal syntheses include gels, glasses, and other aluminosilicate minerals. The presence of Mg2+ seems to be essential for the formation of smectites, even for phases such as montmorillonite which contain low amounts of Mg. Highly crystalline smectites can be obtained when extreme temperatures or pressures are used, but other crystalline impurities are always present. Although the correlation between synthesis stability fields and thermodynamic stability fields is good in many instances, metastable phases are often formed. Few studies, however, include the additional experiments (approach from under-and over-saturation, reversal experiments) needed to ascertain the conditions for formation of thermody-namically stable phases. Thorough characterization of synthetic products by modern instrumental and molecular-scale techniques is also needed to better understand the processes leading to smectite formation.

A procedure for structural investigations by X-ray diffraction of mixed-layer structures incorporating swelling layers has been developed. For each sample, specimens saturated with different cations (Na, Mg, and Ca), are analyzed both as air-dried and as glycolated. One structural model fitting all the observed patterns then provides the structure of the sample. Samples tested include: Mite-smectite (I-S) minerals from Kazachstan (a rectorite), Dolna Ves in Slovakia, Kinnekulle in Sweden, the North Sea, and Scania in Sweden. The fitting of the patterns of the Kazachstan rectorite demonstrated that the instrumental parameters applied in the modeling were correct. For the I-S minerals from Slovakia and Kinnekulle the observed patterns were fitted with one two-component I-S model. However, the Ca-saturated and air-dried specimen of the Kinnekulle bentonites had two types of swelling interlayers. For the Slovakian I-S with Reichweite = 2, an alternative two-phase I-S plus I–V (V = vermiculite) model fitted the experimental X-ray diffraction patterns equally well. The I-S mineral from Scania is in fact a three-component I-T-S (T = tobelite) and the North Sea sample is a four-component I-S-V-V, one type of the swelling layers having swelling characteristics intermediately between smectite and vermiculite. In addition to layer types and distribution, interlayer compositions, such as the amount of interlayer glycol and water and of fixed and exchangeable cations, were determined.

The ability of Na-activated bentonite to remove Ni2+ and Co2+ from aqueous solutions at room temperature (22 ± 1°C) was studied under various experimental conditions. The parameters studied were solid-to-liquid ratios and initial cation concentrations. Experiments involved the behavior of bentonite vs. Ni and Co separately and where Ni and Co were present in solution at different concentrations and ratios. Bentonite retained substantial amounts of both metals readily, but it showed a higher affinity for Ni. Over-exchange appears when initial metal concentration exceeds the concentration corresponding to the cation exchange capacity (CEC) of bentonite. The presence of both metals in solution may be either synergistic or antagonistic sorption, depending on the initial ion concentrations.

Three methods were developed that allow for the imaging of any clay mineral in aqueous solutions with atomic force microscopy (AFM). The methods involve fixing the particles onto special substrates that do not complicate the imaging process, but hold the particles sufficiently so that they do not move laterally or float away during imaging. Two techniques depend on electrostatic attraction under circumneutral pH conditions, between the negatively charged clay particles and the high point of zero charge substrate (either aluminum oxide or polyethyleneimine-coated mica) whereas the third technique depends on adhesion to a thermoplastic film. The first electrostatic technique involves a polished single crystal α-Al2O3 (sapphire) substrate. This was used successfully as a substrate for clay-sized minerals with high permanent layer charge localized on the basal planes (phlogopite and vermiculite) and when the AFM was operated in TappingMode to limit the lateral forces between the probe tip and the particles. However, electrostatic attraction between the sapphire surface and clay minerals such as smectite and kaolinite (low or no permanent layer charge) is not sufficiently strong to adequately fix the particles for imaging. The second electrostatic technique involves a polyethyleneimine-coated mica surface designed to immobilize a larger variety of clay minerals (phlogopite, vermiculite, montmorillonite, and kaolinite), and in this technique weak bonding between the clay and the organic film is also a factor. The third technique, which does not depend on electrostatic attraction, fixes clay particles into the surface of a thermoplastic adhesive called Tempfix. This has proven useful for fixing and imaging relatively large clay particles with well-defined morphology. The Tempfix mount also requires imaging in TappingMode because the Tempfix is relatively soft.

Transmission electron microscopy (TEM) and analytical electron microscopy (AEM) methods were used to study the crystal chemistry of phyllosilicates occurring in green grains of Miocene sediments from the Congo continental shelf. Using diagrams based on wt. % K and the (Fe + Mg)/Al ratio, minerals were distinguished from mixed-layer phases. The most abundant detrital mineral is Fe-kaolinite. The morphology and composition identify this mineral as a component of ferralitic soils. This Fe-rich kaolinite has undergone a complex process of partial dissolution and recrystallization and further enrichment in Fe and, to a lesser extent, in Mg in the marine environment. The detrital mica observed with TEM retains the original morphology and chemistry of muscovite. Alteration processes resulted in the crystallization of 1:1 trioctahedral Fe2+ and Mg-rich minerals and interstratified phases with 1:1 and 2:1 layers in varying proportions observed with the aid of high-resolution transmission electron microscopy (HRTEM) imaging. Included among the newly formed 7-Å phases are those apparently containing excess Si. The smectites are apparently neoform, and chemical analyses showed that these marine K-smectites belong to the beidellite-nontronite series and have tetrahedral substitutions similar to muscovite. Their compositions are closer to beidellite than to nontronite, although the latter was observed in association with goethite. The TEM observations and crystallochemical data show that mineral alteration ceased after forming mixed-layer minerals, and alteration did not reach the glauconitization stage. Apparently, the Miocene assemblages experienced rapidly changing environmental conditions and high sedimentation rates that continue today.

Li+, Na+, Ca2+, Sr2+, Cu2+, or Zn2+-saturated samples of a cis-vacant montmorillonite from Linden, Bavaria, were heated to temperatures between 200–700°C. Half of each heated sample was subsequently autoelaved under steam at 200°C (∼1.5 MPa) to promote rehydroxylation. The smectites were characterized by cation-exchange capacity (CEC), determination of exchangeable cations, infrared (IR) spectroscopy, and thermoanalytical investigations of evolved water in a thermobalance linked with a mass spectrometer.

Changes in the montmorillonite structure and dehydroxylation behavior are related to three respective mechanisms: type of the interlayer cation, interlayer cation radius, and the movement of the interlayer cation. The migration of the smaller Li+, Cu2+, and Zn2+ ions after heating produces a strong reduction of CEC due to the Hofmann-Klemen effect before the initiation of dehydroxylation. Thereafter, the CEC of these smectites remains constant over a large temperature interval during dehydroxylation. After rehydroxylation, Cu2+ and Zn2+-rich samples release 16–23 meq/100 g of Mg2+ from the structure. No Mg2+ release is observed for the Li+-rich montmorillonite. Also the dehydroxylation behavior after rehydroxylation differs between the Cu2+, Zn2+, and Li+-rich samples. The mass curves of the evolved water during thermoanalysis of the rehydroxylated Cu2+ and Zn2+-rich smectites show a peak doublet between 480–700°C. For the Li+, Na+, Ca2+, and Sr2+-rich montmorillonites, the second peak disappeared and a third peak at ∼760°C developed after rehydroxylation. The resulting structure after rehydroxylation of all samples is celadonite-like.

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.

The sorption of Cd, Cu, Pb, and Zn ions by Na-rich bentonite, Al and Zr-pillared Na-rich bentonite (Al-MX80, Zr-MX80), the uncalcined hydroxy-intercalated precursors (HAl, HZr-MX80), and commercial Al-pillared bentonite EXM 534 was investigated. Experiments were conducted in ultrapure water and artificial leachate with varying pH. The experiments were performed over periods to 30 wk. Sorption characteristics were described with one and two-site Langmuir isotherms. The non-exchangeable quantities of heavy metals were determined by fusion of the sorbents after ion exchange with ammonium acetate.

The sorption of Cd, Cu, Pb, and Zn by bentonite was dominated by cation exchange. In artificial leachate, the sorption was reduced due to competition with alkali and alkaline-earth cations. The sorption of Cu, Zn, and Pb at pH 4.9 and Cd at pH 6.9 by Al and Zr-hydroxy-intercalated and pillared MX80 was governed also by cation exchange. In contrast, the sorbed quantities of Zn at pH 6.9 exceeded the cation exchange capacity (CEC) of HAl, HZr, Al, Zr-MX80, and EXM 534 and were partially non-exchangeable. The increase of the sorption of Zn with pH and its independence of the ionic strength of the solution at neutral pH suggest a complexation of Zn ions to surface hydroxyl groups of the intercalated Al and Zr-polyhydroxo cations and pillars. This complexation is the dominating sorption mechanism. Removal of dissolved Zn from solution with time is attributed to surface precipitation. Al-hydroxy and pillared bentonites are considered potential sorbents of Zn ions from neutral pH aqueous solutions, such as waste waters and leachates.

The objective of this study was to investigate the influence of layer charge on the hydration of Mg-saturated expandable 2:1 phyllosilicates. Water retained by 12 Mg-saturated clays at 54% relative humidity was quantified gravimetrically. X-ray diffraction and total chemical analysis were used to determine the hydratable surface area (447–759 m2 g−1) and layer charge [0.327–0.754 electrons per formula unit (e f.u.−1)] of each sample. Water retained by the clays increased with both hydratable surface area and layer charge of the clays. However, the increase in H2O content with layer charge occurred only on external surfaces of the clays. This result suggests that the H2O on external surfaces is localized around the cation/charge sites rather than forming multi-layers as was suggested previously. A model is proposed for the hydration of expandable 2:1 phyllosilicates. The model assumes that interlayer volume controls interlayer hydration and that the number of cation/charge sites on external surfaces controls hydration of external surfaces.

Clay mineralogical studies by X-ray diffraction performed on extracted <2-µm fractions do not always represent all clay mineral constituents present in the soil. In this work, transmission X-ray diffraction (TXRD) was applied to undisturbed microsamples of optically homogeneous mineral soil fabrics and features. These microsamples were isolated by microdrilling their periphery in soil thin sections, then removing them, and transferring them to glass capillaries for TXRD analysis. The usefulness of this technique for supplying in situ mineralogical information on identification, structure, and natural orientation of soil constituents was tested on mineral microfabrics and features of primary and secondary phyllosilicates. The study demonstrated that TXRD allowed detailed, representative interpretations of undisturbed mineral features and fabrics. In particular, this technique allowed us (1) to compare mineralogical compositions at selected microlocalities, (2) to study natural preferred orientations, and (3) to detect small amounts of minor mineral interstratification phases. In addition, complementary information on crystallography and crystal chemistry may be obtained by performing analytical transmission electron microscopy on the same microsample.

Amorphous derivates prepared by aqueous reaction of various aluminosilicate clay minerals with concentrated KF solution at 80–110°C were studied for their gas adsorption properties. The four clay minerals studied are halloysite, a well-crystallized kaolinite, a poorly crystallized kaolinite, and a montmorillonite. The gases tested are N2, O2, CH4, CO, CO2, and C2H2. The kaolin-group mineral derivatives are characterized by substantial reduction in particle size, high specific surface, and significant selectivity towards CO2 and C2H2 relative to the other gases. The montmorillonite derivative shows no increase in adsorption over the starting material, however, for all the materials high adsorption of CO2 and C2H2 was observed. Levels of gas adsorption and gas adsorption ratios are comparable to pillared clays.

Polynuclear Al13 tridecamer species are the major hydrolyzed species of aluminum, but their occurrence in terrestrial environments has not been established. X-ray diffraction (XRD), 27Al nuclear magnetic resonance (NMR), and scanning electron microscope (SEM) analyses show that the presence of tartaric acid (concentration range of 10−5–10−3 M), one of the commonly occurring low-molecular-weight organic acids, inhibits the formation of the Al13 tridecamer species.

In the absence of tartaric acid, the basic aluminum sulfate crystals were of tetrahedral morphology and conformed to isometric symmetry with a = 17.748 Å and space group of P4232. Increasing amounts of tartaric acid [tartaric acid/Al molar ratio (R) ranging from 0.01 to 0.05] modified the crystal morphology from the tetrahedral particles of isometric symmetry (R = 0) to rod-shaped particles of monoclinic symmetry (R = 0.01) to irregularly shaped X-ray noncrystalline microparticles (R = 0.05). Failure to detect the presence of Al13 tridecamer, the dominant hydrolyzed species of aluminum, in terrestrial environments may be partially attributed to the presence of low-molecular-weight organic acids, which inhibit the formation of Al13 tridecamer species.

This study investigates the complexes formed between amino acids, which are the natural degradation products of organic matter, and smectites. Thus, the adsorption and desorption behavior of cysteine and Na-, Ca-, Cu-homoionic smectites with different layer-charge location, a montmorillonite, and a beidellite, were studied. The clay samples were treated with Na, Ca, and Cu 1 N solutions and then with a 0.2 M cysteine solution. To test smectite-cysteine stability at acidic pH, the solids obtained were repeatedly treated with distilled water acidified to pH = 5. All treated samples were characterized by thermal, X-ray diffraction, chemical, and infrared analyses. The results showed that: 1) Na- and Ca-rich smectites adsorbed and retained small amounts of cysteine, and did not show interlayer cation-cysteine complexes, whereas the amino acid was strongly retained in the interlayer by Cu-rich smectites; 2) d(001)-values for Na- and Ca-rich smectites showed little or no expansion, whereas for the Cu-rich smectites the intercalation of the organic molecule produced a swelling of the structure; 3) the interaction mechanism of homoionic smectites with cysteine in an aqueous medium occurs by weak interactions, (e.g., van der Waals interactions, hydrogen bonding, dipole-dipole interactions, and other electrostatic forces such as entropy-driven hydrophobic bonding), and/or by complexes involving interlayer cations and organic ligands. The formation of a stable chelate complex with the saturating ion permits cysteine to be adsorbed by Cu(II)-rich smectites and to be resistant to migration in soils and groundwaters.

Montmorillonites saturated with Li+, Na+, K+, NH4+, Mg2+, Ca2+, Ba2+, or Ni2+ ions can form complexes with propylene carbonate (PC) by intercalation; the d(001) of montmorillonite expands to 19 Å. In the infrared absorption spectra of these complexes, the C=0 stretching band of the intercalated PC molecules shifts to lower frequencies, and the amount of the shift increased with the increase of the polarizing power of the interlayer cations. Water molecules were strongly bound to the cations. The PC molecules interacted with the cations by way of H2O molecules. With the removal of H2O, the PC molecules directly coordinate to the cations and the PC molecules show a much larger red-shift in the C=O frequency. The PC-montmorillonite complexes exhibited osmotic swelling, even in aqueous electrolyte solutions. This finding is interpreted in terms of the formation of thick electric double-layers consisting of PC and H2O between the 2:1 layers.

A new osmotic-swelling system for montmorillonite with formamide (FA) is described. Water and a variety of polar organic liquids were used to swell FA-montmorillonite complexes. The post-swelling behavior of the complex is interpreted in terms of the bi-functionality of FA as an electron donor/acceptor and the magnitude of the donor number (DN) of the polar liquid relative to that of FA. The FA-montmorillonite complex shows limited swelling of d(001) <2.3 nm for polar liquids with DNs larger than that of FA. In contrast, the FA-montmorillonite complex shows osmotic swelling when the DN of the polar liquid is smaller than that of FA.