The atomic structure of dioctahedral 2:1 phyllosilicate edge surfaces was calculated using pseudopotential planewave density functional theory. Bulk structures of pyrophyllite and ferripyrophyllite were optimized using periodic boundary conditions, after which crystal chemical methods were used to obtain initial terminations for ideal (110)- and (010)-type edge surfaces. The edge surfaces were protonated using various schemes to neutralize the surface charge, and total minimized energies were compared to identify which schemes are the most energetically favorable. The calculations show that significant surface relaxation should occur on the (110)-type faces, as well as in response to different protonation schemes on both surface types. This result is consistent with atomic force microscopy observations of phyllosilicate dissolution behavior. Bond-valence methods incorporating bond lengths from calculated structures can be used to predict intrinsic acidity constants for surface functional groups on (110)- and (010)-type edge surfaces. However, the occurrence of surface relaxation poses problems for applying current bond-valence methods. An alternative method is proposed that considers bond relaxation, and accounts for the energetics of various protonation schemes on phyllosilicate edges.

We performed Monte Carlo and molecular dynamics simulations to investigate the interlayer structure of a uranyl-substituted smectite clay. Our clay model is a dioctahedral montmorillonite with negative charge sites in the octahedral sheet only. We simulated a wide range of interlayer water content (0 mg H2O/g clay — 260 mg H2O/g clay), but we were particularly interested in the two-layer hydrate that has been the focus of recent X-ray absorption experiments. Our simulation results for the two-layer hydrate of uranyl-montmorillonite yield a water content of 160 mg H2O/g clay and a layer spacing of 14.66 Å. Except at extremely low water content, uranyl cations are oriented nearly parallel to the surface normal in an outer-sphere complex. The first coordination shell consists of five water molecules with an average U-O distance of 2.45 Å, in good agreement with experimental data. At low water content, the cations can assume a perpendicular orientation to include surface oxygen atoms in the first coordination shell. Our molecular dynamics results show that ${\text{UO}}_2{\left({\text{H}}_2\text{O}\right)}_5^{2+}$ complexes translate within the clay pore through a jump diffusion process, and that first-shell water molecules are exchangeable and interchangeable.

Microstructural changes induced by the microbial reduction of Fe(III) in nontronite by Shewanella oneidensis were studied using environmental cell (EC)-transmission electron microscopy (TEM), conventional TEM, and X-ray powder diffraction (XRD). Direct observations of clays by EC-TEM in their hydrated state allowed for the first time an accurate and unambiguous TEM measurement of basal layer spacings and the contraction of layer spacing caused by microbial effects, most likely those of Fe(III) reduction. Non-reduced and Fe(III)-reduced nontronite, observed by EC-TEM, exhibited fringes with mean d001 spacings of 1.50 nm (standard deviation, σ = 0.08 nm) and 1.26 nm (σ = 0.10 nm), respectively. In comparison, the same samples embedded with Nanoplast resin, sectioned by microtome, and observed using conventional TEM, displayed layer spacings of 1.0–1.1 nm (non-reduced) and 1.0 nm (reduced). The results from Nanoplast-embedded samples are typical of conventional TEM studies, which have measured nearly identical layer spacings regardless of Fe oxidation state. Following Fe(III) reduction, both EC- and conventional TEM showed an increase in the order of nontronite selected area electron diffraction patterns while the images exhibited fewer wavy fringes and fewer layer terminations. An increase in stacking order in reduced nontronite was also suggested by XRD measurements. In particular, the ratio of the valley to peak intensity (v/p) of the 1.7 nm basal 001 peak of ethylene glycolated nontronite was measured at 0.65 (non-reduced) and 0.85 (microbially reduced).

Evidence for the formation of maghemite from goethite due to a bushfire on acid sulfate soil at East Trinity, Australia, is presented. Oxidation of pyrite-bearing acid sulfate soils led to precipitation of goethite-impregnated leaf litter. During a major bushfire, goethite with a crystal size calculated from broadening of the 110 reflection of ∼9 nm was converted to microcrystalline maghemite (size 12 nm, 220 reflection) and hematite (17 nm, 104 reflection) in a matrix of partly combusted plant litter. Replication of this natural formation of maghemite from goethite was achieved in the laboratory by burning goethite-impregnated leaf litter.

The structural and physical effects of partially substituting Cd for Fe in goethite have been investigated. The solubility of Cd2+ in goethite is ∼10 mol.%, i.e. Fe0.905Cd0.095OOH. The structures of the substituted goethites have been refined, using the Rietveld method, from synchrotron X-ray powder diffraction data. There is a progressive increase in the size of the unit-cell parameters and unit-cell volume, upon the incorporation of much larger Cd2+ ion (0.95 Å) compared with Fe3+ (0.645 Å) in the goethite structure, together with a reduction in crystallinity. Transmission electron microscopy measurements confirm the crystallite size decreases as the Cd2+ content increases in goethite structure.

Dioctahedral and trioctahedral smectites of very low Fe contents were exchanged with Ca (0.8 N CaCl2) and then with Cd (0.004–0.2 N CdCl2), resulting in the approximate Ca/Cd interlayer compositions 100/0, 75/25, 50/50 and 0/100. The samples were analyzed using 113Cd and 1H MAS NMR spectroscopy, at different hydration states, and by FTIR using films oriented at 90 and 45° to the infrared beam. When adsorbed from low-concentration CdCl2 solutions, as in this study, Cd is in the smectite interlayer as Cd2+, surrounded by water molecules. In smectite specimens with no tetrahedral charge, Cd occupies one single site in the center of the interlayer along the c axis but in smectite with tetrahedral charge, the basal oxygen atoms with excess negative charge interact with some of the Cd atoms more strongly, generating a second site in which Cd is displaced from the center of the interlayer and closer to those oxygen atoms. The 1H NMR study showed protons from hydration water and smectite hydroxyls and showed that both have more than one component. The FTIR and 1H NMR experiments indicate that Cd interacts with hydroxyl groups through one of the solvating water molecules in the first coordination sphere. Cadmium retains water molecules bridging Cd and OH groups even at low hydration states. Comparison with previous studies shows that, when adsorbed from high-concentration CdCl2 solutions, Cd species other than Cd2+ are present in sufficient quantities to enter the interlayer and create a different Cd-water complex, seemingly displacing Cd2+ from the center of the interlayer.

The size of quaternary ${\text{NH}}_4^{+}$ cations defines their packing configurations in the interlayers and the basal spacings of organoclays, and hence strongly influences the sorptive properties of organoclays. A series of organoclays (TAA-SACs) was prepared from a smectite (SAC) fully exchanged with symmetrical tetraalkylammonium (TAA) cations of progressively increasing sizes with the carbon number of single alkyl group from 1 to 6. X-ray diffraction analysis indicated the packing configurations of monolayer, monolayer-to-bilayer transition, bilayer, and bilayer-to-trilayer transition in the interlayers of SAC. Calculations of the interionic distances between TAA ions support such packing configurations. Sorption of benzene by TAA-SACs displayed a high-low-high uptake trend and progressively weaker sorptive interactions as the size of TAA ions increased. Both the siloxane surfaces and TAA ions contributed to the overall sorption, with their relative contributions dependent on the TAA interionic distances and the basal spacings of TAA-SACs. High benzene sorption by small TAA-SAC (tetramethylammonium(TMA)-SAC) was attributed to the strong interactions between the siloxane surfaces and benzene molecules. With large TAAs, high sorption was due to the effective solute partitioning. Compared to benzene sorption, TCE sorption by small TAA-SACs (TMA-SAC and tetraethylammonium(TEA)-SAC) was less effective and displayed an abnormal trend, due largely to the lack of the siloxane surface-TCE interactions and to the stronger hydration of TMA as compared to TEA ions. The results provide strong evidence to support the use of either small or large quaternary ${\text{NH}}_4^{+}$ cations in preparation of organoclays as effective sorbents for removing organic contaminants from water.

Nanostructural analysis of pillared clay samples using high-resolution transmission electron microscopy has been developed. Montmorillonite samples were pillared using partially hydrolyzed Al and Fe solutions. Two samples, M01 and M05, corresponding to Fe/(Fe+Al) ratios of 0.1 and 0.5, respectively, were analyzed. The different steps of image filtration, resulting from filtration by ring-shaped masks, are illustrated and discussed from lattice imaging of sample M01. This procedure is used to show the heterogeneous distribution of the basal spacings in the different ordered domains. Domains of mesoporosity and distribution of the different Fe species are studied specifically in the sample M05. The quantitative HRTEM results are discussed and compared with X-ray diffraction patterns obtained from the same sample.

The influence of cation size on the misfit between the tetrahedral and the octahedral sheets of serpentine layers, and thus on the curvature of serpentine minerals, has been studied using an experimental approach, based on scanning and high-resolution transmission electron microscopies (SEM and HRTEM, respectively) coupled with analytical electron microscopy (AEM), and a theoretical approach, based on the elastic theory of thin plates and the surface stress concept.

Various Ni3(Si,Ge)2O5(OH)4 serpentine syntheses were prepared, with progressive tetrahedral substitution for Si by the larger Ge ion, the ratio Ge/(Si + Ge) ranging from 0 to 100%. Other parameters (temperature, time duration, water pressure) were fixed. From SEM-HRTEM observations and AEM analyses of all samples, two types of serpentine minerals were characterized: (1) curved structures of tubular or ‘roman-tile’ shape (curvature radius of 10 nm for 0% Ge, increasing with the Ge content), when the Ge tetrahedral content is <25%; (2) perfectly plane structures, with hexagonal or triangular shape (view normal to the layers), for greater Ge contents. These results prove the direct influence of cation size on the crystal curvature.

From a theoretical point of view, a single serpentine layer with a misfit between its tetrahedral and octahedral sheets can be considered as an elastic thin plate subjected to two different surface stresses, σ+ and σ−, on its two faces. The difference σ+ − σ− between these two surface stresses was calculated from the above geometrical misfit, and the curvature of the serpentine layer was related to σ+ − σ−, according to the elastic theory of thin plates. The calculated curvature radii, and the Ge content of transition from curved to plane structures, are in agreement with the above observed values. Curved serpentine crystals may then be considered as a stacking of such elastically curved single serpentine layers.

We report infrared spectroscopic observations of the incorporation of CO2 into pyrophyllite that has been heated between 200°C and 1250°C for periods of 15 min, 1 h and 5 days. The presence of CO2 is characterized by the ν3 band of CO2 near 2347 cm−1, detectable in samples in which dehydroxylation has commenced after heating above 450°C. With increasing temperature, the CO2 signal becomes more intense. The signal reaches its maximum intensity near 800°C with an annealing time of 15 min. Further heating leads to a decrease in the CO2 signal and the occurrence of an extra signal near 2156 cm−1 that implies the presence of CO. The process is characterized by significant time-dependence, indicating its kinetic nature. The peak positions of CO2 signals show systematic variations with temperature. Our results suggest that the CO2 molecule is associated with the local structure rather than being present as free gaseous CO2, and that the local structure of pyrophyllite is gradually modified during high-temperature treatments. However, no signals related to carbonate molecules ($({\text{CO}}_3^{2-})$) were detected. The results suggest that CO2 or other carbon-based molecules may diffuse into some clay minerals during dehydroxylation and may become altered due to structural modifications at high temperatures. This may have significance for possible CO2 sequestration in shales and clay formations.

In this study, the feasibility of using surfactant-modified palygorskite (PFl-1) and sepiolite (SepSp-1) for removal of anionic contaminants from water was evaluated from batch experiments. The results showed that both minerals had strong affinity for hexadecyltrimethylammonium (HDTMA), a cationic surfactant used for surface modification. The HDTMA sorption capacities were 520 and 260 mmol/kg for PFl-1 and SepSp-1, respectively. Accompanying HDTMA sorption, the sorption of counterion bromide reached 380, and 210 mmol/kg, for PFl-1 and SepSp-1, respectively, indicating that the sorbed surfactant molecules form admicelles on the minerals’ surfaces. After modification by HDTMA to sorption maxima, these clays showed strong affinity for anionic contaminants such as chromate and nitrate. The chromate sorption capacities were 42 and 34 mmol/kg for HDTMA-modified PFl-1 and SepSp-1, respectively. Desorption of counterion bromide due to sorption of chromate followed a straight-line relationship, suggesting that the sorption of chromate on surfactant-modified palygorskite and sepiolite was also due to anion exchange as with other surfactant-modified clay minerals and zeolites.

Vermiculites and phlogopites can be exfoliated by chemical and thermal treatment methods to obtain chemically inert, adsorbent, fire-resistant, low-density materials with excellent thermal and acoustic insulation properties. The water content of the clay generally determines the extent of exfoliation and the presence of interstratification is claimed to increase the rate of exfoliation. Considering the strong interaction between water and microwaves, the effect of microwave power on exfoliation characteristics of vermiculites and phlogopites after treatment with water and hydrogen peroxide solution were studied at 600, 950 and 1300 W for microwave exposure times of 10, 20, 30, 60, 120 and 300 s. It was observed that the water molecules in the interlayers of the individual flakes were driven off quickly by microwave treatment causing layer separation in the samples. The vermiculite sample showed 2.8 and 5.6 times, respectively, the exfoliation ratio of the phlogopite samples, in accord with their water contents.

The chemical and structural properties of Mg smectites in the Vicálvaro sepiolite deposit have been studied in detail. The characterization was performed on different size-fractions of selected smectitic samples (5−2 µm; 2−1 µm; 1−0.5 µm; <0.5 µm and <0.1 µm). The chemical compositions of individual particles (5−1 µm) and of bulk undifferentiated fine fractions (1−<0.1 µm) were determined by energy dispersive spectroscopy-scanning electron microscopy and interpreted with the aid of X-ray diffraction (XRD) and infrared spectroscopy (IR) methods. The XRD and IR data demonstrate that all of the Mg smectite materials studied are mainly composed of a complex mixture of stevensite, saponite and mica-type minerals. Although the presence or absence of saponite cannot be confirmed absolutely, stevensite is a significant component of these Mg smectites. This is proven by the calculated layer charge reduction after the Hofmann-Klemen effect. The results are in close agreement with the suggested mechanism of topotactic overgrowth of stevensite on pre-existing phyllosilicate templates. This characterizes clay diagenesis in saline-lake systems.