Polytype diversity of hydrotalcite-like minerals is mainly a function of the nature of the interlayer anion. Among the varieties with CO32− anions, only two- and three-layer polylypes having the same structure as manasseite and hydrotalcite have been confirmed. Stichtite and reevesite, which have been previously identified as six-layer polytypes, are in fact three-layer polytypes.

Among SO42− varieties, one-layer and three-layer polytypes have been identified, but the one-layer types are only present in more hydrated minerals with larger interlayer spacings. The three-layer varieties are of three different polytypes, with both P- and O-types of interlayers. Both rhombohedral and hexagonal varieties exist. Interlayer type may change during hydration-dehydration or anion exchange. Thus, in contrast with the CO32−-bearing minerals, a complete description of the polytype of the SO42−-bearing minerals cannot be made by simply indicating the number of the brucite-like layers in the unit cell.

The two-layer unit cell seen in refined crystal structures of some minerals with SO42− interlayers is not due to a doubled periodicity of alternation of brucite-like layers but to periodicity of interlayer anions, or layer cations.

We report the first application of 39K solid-state NMR to the study of tecto- and phyllosilicates. Under high field (11.7 Tesla) and with the application of a spin-echo sequence, informative 39K spectra can be obtained for several compounds of interest to the geologist and the agronomist. Tectosilicates and phyllosilicates can be distinguished from the uncorrected frequency (δCG) of the observed NMR peak. A series of montmorillonites submitted to increasing numbers of wetting and drying cycles was studied in order to discriminate between mobile and “fixed” forms of K+: when the spectra are run on hydrated samples, two different signals are observed corresponding to K+ in different hydration states, and NMR data can be correlated with the amount of exchangeable K+ measured by ion exchange. Thus, it appears that NMR can provide useful information on K fixation complementary to classical chemical methods.

The effect of phosphate on the formation of Fe oxides from Fe(II) salts is important because phosphate is a ubiquitous anion in natural environments. For this reason, the products formed by oxidation of phosphate-containing Fe(II)SO4 solutions neutralized with bicarbonate were characterized. The rate of oxidation of Fe(II) increased with increasing P/Fe atomic ratio to 0.2 in the initial solution. Goethite (α-FeOOH) or lepidocrocite (γ-FeOOH) or both were produced and identified by powder X-ray diffraction (XRD). The ratio between lepidocrocite and goethite increased with increasing P/Fe. In the 5–8.5 pH range, the formation of goethite predominated at P/Fe < 0.005, but only lepidocrocite was detected by XRD for P/Fe > 0.02. Thus, phosphate favors lepidocrocite formation because lepidocrocite has (1) a layered structure (like its precursor green rust), and (2) a structure less dense than that of goethite, thereby requiring less complete removal of the green-rust interlayer phosphate to form. The lepidocrocite crystals were platy, with prominent {010} faces and the thickness of the plates decreased with increasing P/Fe from >25 nm for P/Fe < 0.005 to <5 nm for P/Fe > 0.1. The solubility of lepidocrocite in acid oxalate was nearly complete for P/Fe > 0.03. The lepidocrocite contained occluded phosphate, i.e., phosphate that could not be desorbed by alkali treatment. The decrease in the b unit-cell length with increasing P/Fe suggests that lepidocrocite may contain structural P.

Iron oxides in surface environments generally form at temperatures of 25 ± 10°C, but synthesis experiments are usually done at higher temperatures to increase the rate of crystallization. To more closely simulate natural environments, the transformation of 2-line ferrihydrite to hematite and goethite at 25°C in the presence of different Al concentrations and at pH values from 4 to 7 was studied in a long-term (16–20 y) experiment. Aluminum affects the hydrolysis and charging behavior of 2-line ferrihydrite and retards crystallization. Al also promotes the formation of hematite over goethite and leads to multidomainic discoidal and framboidal crystals instead of rhombohedral crystals. The strong hematite-promoting effect of Al appears to be the result of a lower solubility of the Al-containing ferrihydrite precursor relative to pure ferrihydrite. Hematite incorporates Al into its structure, as is shown by a decrease in the a and c-cell lengths and a decrease in magnetic hyperfine fields (Mössbauer spectroscopy). With hematite formed at low-temperature, these decreases were, however, smaller for the cell length and greater for the magnetic field than for hematite produced at higher temperatures. Both phenomena are removed by heating the hematite at 200°C. They are attributed to structural OH and/or structural defects. The relative content of Al in the structure is lower for hematite formed at 25°C than for hematites synthesized at higher temperatures (80 and 500°C). The maximum possible substitution of one sixth of the Fe positions was not achieved, similar to soil hematites. These results show that properties of widely distributed soil Al-containing hematites can reflect formation environment.

Products resulting from the reaction of toluene with Cu(II)-montmorillonite were analyzed using GC/MS, HPLC/MS, GPC, and FTIR methods. Numerous oligomers of toluene were observed, extending at least as high as the resolution limit (1500 g/mol) of the GPC column. The FTIR spectrum of the nonvolatile components of the extract was very similar to that of liquid toluene. GC/MS data on the volatile components revealed dimers, trimers, and a multitude of transmethylated products. Oligomerization proceeded via both ring-ring (i.e., polyphenyl) and ring-methyl linkages. The primary trans-methylated products were tert-butylbenzene and isopropylxylene, indicating a competition between ringand side-chain methylations. The side-chain substitutions cannot be explained in terms of the aromatic radical cation intermediate which typically forms in arene/clay reactions. A consideration of alkylbenzene reactions observed in various other media suggests that the present transmethylation reactions occur via a benzyl cation intermediate.

A theoretical model describing the interaction between crystalline swelling and cation exchange selectivity is proposed for expanding 2:1 phyllosilicates. The model is based on the assumption that changes in basal spacing of a clay are phase changes, and that each phase of a clay has a different selectivity constant for a particular cation exchange reaction. Energy barriers stabilize the various phases over a limited range of interlayer ionic composition. These energy barriers cause hysteresis in crystalline swelling, which in turn causes hysteresis in cation exchange. Results are presented for an experiment involving Ba-Mg exchange on a synthetic fiuoro-hectorite. The results demonstrate key aspects of the proposed model, including a correlation between measured selectivity coefficients and basal spacings (R2 = 0.85), an abrupt change in basal spacing that corresponds with an abrupt change in selectivity and corresponding hysteresis in crystalline swelling and cation exchange selectivity. The results also demonstrate increased selectivity for the preferred cation (Ba) at high solution mole fraction of the preferred cation. This trend is opposite of that observed for heterogeneous natural smectites but consistent with predictions of the model for a homogeneous smectite.

The association between clay silicates, and iron and aluminium oxides has a major influence on the chemical and physical properties of soils. In this work the interaction of a kaolin substrate with iron and aluminium oxides and/or hydroxides obtained by basification of solutions of the metal ions was compared to that of quartz. Both precipitates were obtained in the presence of the substrates.

The aluminium precipitates had higher crystallinity, and thus led to smaller increases in specific surface area than those of iron, and were more effective modifiers of the surface electrical properties of the kaolin-oxide mixtures. At concentrations as low as 0.43% Al (g/100 g of substrate) the point of zero charge (PZC) of the components with variable charge was measurable, while Fe required 2.23% and gave lower PZCs than those of corresponding concentrations of Al. In both cases the PZCs shifted to higher pH as metal concentration was increased, as did the flocculation interval of colloidal suspensions of kaolin, which were close to the PZCs (where these were evaluated).

The Al and Fe oxides precipitated on quartz had higher crystallinities. Both metals increased the specific surface area to a similar extent, with an almost linear relationship to metal concentration. Samples containing ca. 6.5% Fe or Al had similar or slightly higher PZCs than corresponding kaolin samples.

The results were interpreted by assuming, in the case of kaolin, the union of the metal precipitate with the basal faces of the substrate, so decreasing the negative charge at this surface; and in the case of quartz, the formation of a hydroxide coating that neutralizes the negative charge on the silica surface. The difference between the results obtained for each metal was attributed to the different morphologies of their oxide precipitates.

The composition and origin of vanadium-bearing clay minerals in the Jurassic (Morrison and Entrada Formations) sandstones of the Colorado Plateau are reassessed using microanalyses (microprobe and scanning electron microscope). The main V-clays are authigenic illite and chlorite of various petrologic habits: clay casts and matrix, pore lining, replacement of detrital grains. The chemical composition of the V-clays is similar in three different localities in the Morrison Formation separated by about 50 km, suggesting that the V-clays are the result of a large regional event. In both illite and chlorite, Al and V are inversely correlated, showing that V replaces Al in the octahedral position. The chlorite contains a complex mixture of divalent and trivalent cations that cannot fit within a sudoite structure. A classification of V-micas is proposed that employs V3+/sum of the octahedral cations vs. the sum of the interlayer charges. V-illite and roscoelite from the Colorado Plateau are characteristic of diagenetic/hydrothermal environments. For a given locality the composition of the V-clays does not vary with habit, showing that these minerals formed at thermodynamic equilibrium.

A new model is proposed to explain, within the framework of the theory of spiral growth of Frank, the formation on inhomogeneous mica polytypes. This model relates the interaction and cooperative growth of two components (spirals and/or crystals) to produce a new stacking sequence. Depending on the relative orientation between the two components, a mismatch of the interlayer positions occurs, which is compensated through either a growth defect or a crystallographic slip at the octahedral (O) sheet. Both these adjustments transform the Ml layer into the M2 layer. These two types of layers have the same chemical composition but differ in cation distribution in the O sheet. The coalescence and cooperative growth of crystals occurs in fluid-rich environments and is most frequent in druses and volcanic fumaroles. These environments favor the inhomogeneous polytypes, especially those with complex stacking sequenc¬es. In addition, the Ml → M2 transformation is most probable in micas with an oxybiotitic composition, where the removal of the OH dipole strengthens the interlayer bonding and the presence of high-charge cations destabilizes the O sheet. Three examples of inhomogeneous polytypes of titaniferous oxybiotite from Ruiz Peak (a volcanic environment where many inhomogeneous polytypes have been reported) are presented.

Experimental alteration of obsidian was performed in 0.001 to 0.5 N NaOH and KOH solutions at 150 and 200 °C for 1 to 30 d. The products were examined by X-ray powder diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), and energy dispersive X-ray analysis (EDX). Changes in chemical composition and pH value of solutions during the reactions were also measured. As the pH of reacting solutions was increased, smectite, phillipsite and rhodesite crystallized progressively in NaOH solutions, while smectite, merlinoite and sanidine grew successively in KOH solutions. In addition, a small amount of less-soluble, poorly ordered boehmite was present as products of all the experiments. Smectite mainly appeared at slightly high pH, Si/Al and Na/K conditions, whereas rhodesite should be produced in extremely high pH, Na/K and Si/Al conditions. Sanidine was also formed in conditions of very high pH and Si/Al and very low Na/K. In intermediate conditions of pH and Si/Al, crystallization of phillipsite was stimulated in solutions of Na/K > 10, while formation of merlinoite was favored in conditions of Na/K < 1.

Palygorskite and sepiolite show a high sorption capacity for organic molecules. Adsorption of 2 organic cations, methylene blue (MB) and crystal violet (CV), by palygorskite and sepiolite were examined. The maximum sorption of MB and CV far exceeded the cation exchange capacity (CEC) of these minerals. This shows that, besides the contribution of free negative sorption sites (P-), the sites satisfied with sorption of single cations (PXi0) and neutral sorption sites (N) on clay surfaces may contribute to the sorption of organic cations. The number of neutral sites was determined by examining the sorption of 2 neutral organic molecules, triton-X 100 (TX100) and 15 crown ether 5 (15C5), and by application of the Langmuir isotherm.

To determine the contribution of different sites, an adsorption model that applies the Gouy-Chapman equation and takes into account the formation of different clay-organic complexes in a closed system was employed. Application of this model to sorption data provided the calculation of binding coefficients for neutral sites, as well as the surface potential of the minerals at different sorbate concentrations.

At sorption maxima, for both palygorskite and sepiolite, the contribution of neutral sites for sorption of organic cations was the highest, followed by the PXi0 sites in case of CV sorption, while in sorption of MB the contribution of P- sites was the second highest. The Fourier transform infrared (FTIR) patterns of clay-organic cation complexes compared with pure clays confirm that the sorption of organic cations is by silanol groups located at the edge of fibrous crystals, which account for neutral sorption sites.

Clay minerals from the MacAdams Sandstone, Kettleman North Dome, California, have been studied by electron microscopy. The clay minerals fill pore space associated with fractured and brecciated clasts of K-feldspar. Curved packets of muscovite and kaolinite are caused by deformation of detrital muscovite that resulted in opening of fissures subsequently filled with dominant kaolinite and minor intergrown mixed-layer illite/smectite (I/S). Regions of authigenic R1 I/S (rectorite) with characteristic ~20 Å periodicity are intergrown with kaolinite in microfissures within K-feldspar or detrital muscovite. Clusters of small grains of muscovite with nearly ideal composition occur as stacks and intergrown with kaolinite and are tentatively inferred to be authigenic. Contrary to previous reports, no illite was found in these samples.

Electron microprobe analyses previously obtained on Kettleman Dome “illite” and subsequently used as a prime example of analyses of illite rich in excess interlayer water (H2O) and hydronium ion (H3O+) are shown to have been obtained on mixtures, and are not representative of the actual clay mineral compositions. Previous conclusions regarding significant H3O+ and H2O contents of illite are invalid because of inaccuracies inherent in bulk and EMPA analyses of illite, and do not affect arguments regarding the metastability of illite. Hydronium substitution should be favored via the reaction H2O + H+ = H3O+ only in highly acidic fluids. Ordinary illite forming in sedimentary environments with carbonates and iron oxides is unlikely to have significant H3O+ substituted for K+.

Imogolite is a tubular aluminosilicate which is common in Andosols and Spodosols. The high pH at point-of-zero charge at the outer parts of the tube and the anomalously high chloride adsorption of imogolite suggested that there may be structural charge associated with this mineral. The structural charge may arise because of changes in bond valence imposed by the incorporation of orthosilicate anions in a gibbsite-type sheet. By using a Basic Stem Model approach, it is shown that the surface charge properties of imogolite are explained if the mean Al-O bond valence of the outer -Al2OH groups is higher than the inner -Al2OHSiO3 groups. Hence, a weak positive charge is developed on the outer tube walls whereas a negative charge develops in the tubular pores. The best model fits were obtained where either one or two units of structural charge per unit cell of tube were assumed. The model may also explain why imogolite tubes are normally aggregated in large bundles in close hexagonal packing, because bound counterions may hold the tubes together. However, to arrive at good model descriptions, the deprotonation of -Al2OH groups must occur at a higher pH than that expected when assuming that all surface oxygens form two hydrogen bridges with H2O. A more precise structure of imogolite is required to test fully this hypothesis.

Competitive adsorption between glyphosate and phosphate on goethite was evaluated. The influence of background electrolyte on the adsorption of glyphosate and phosphate was also investigated by using 0.01 M KCl, 0.1 M KCl and 0.01 M CaCl2 as background electrolytes. Experiments showed that phosphate displaced adsorbed glyphosate from goethite, whereas glyphosate did not displace phosphate. Results also showed that the background electrolyte had a strong effect on phosphate adsorption, but little effect on glyphosate adsorption. Thus, there are differences between the adsorption of glyphosate and phosphate. The study also showed that 0.01 M KCl caused dispersion of goethite, resulting in inefficient filtering, and that phosphate precipitated as calcium phosphates in 0.01 M CaCl2 background electrolyte solutions. The results suggest that 0.1 M KCl is a more suitable background electrolyte to determine competitive adsorption processes involving glyphosate and phosphate.

A new technique utilizing Raman microscopy and Fourier transform infrared (FTIR) microsacopy is described. This technique uses thin films of oriented clay aggregates on glass slides suitable also for X-ray diffraction (XRD). Raman microscopy proved the most useful technique providing both better resolution of the OH-stretching bands and greater spectral resolution. Kaolinites from Washington County, Georgia, with varying defect structures involving layer stacking were intercalated with formamide and additional Raman bands were observed at 3610 and 3627 cm−1. A concomitant decrease in the inner-surface OH band intensities at 3695 and 3685 cm−1 occurred. These bands are attributed to the inner-surface OH hydrogen bonded to the formamide molecule through the C=O group. The 3627 cm−1 band is sharp with a half width of 7.5 cm−1 and comprises 11% of the total normalized band area. When two additional OH bands are observed at 3610 and 3627 cm−1 two C=O bands at 1674 and 1658 cm−1 are observed also. The two additional Raman inner-surface OH bands were not observed in the IR spectra. However, a band of low intensity was observed at 3590 cm−1. Models for the intercalation of formamide in kaolinites are proposed.

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.

Rheological measurements were used to evaluate the particle-particle associations of Na-rich montmorillonite in suspensions, under various electrolyte concentrations. A 2% free electrolyte clay suspension showed pseudoplastic flow behavior and had a high apparent viscosity, attributed at low shear rates to the high volume fraction of the suspended clay platelets, the flexibility of the platelets, and the presence of edge-to-edge association. The breaking of edge-to-edge associations and the progressive orientation of the individual platelets in the direction of flow contribute to the reduction in viscosity with increasing shear rate.

The compression of the diffuse double layer at a NaCl concentration of 10 mEq L-1 contributes to the free movement of the individual platelets, even at low shear rates. The flow behavior changed from pseudoplastic to plastic at an NaCl concentration of 100 mEq L-1. At this electrolyte concentration, face-to-face associations of specific junction points at certain areas of the planar surface are probably occurring.

The apparent viscosity of the clay suspension for the two particle-size ranges (<2 and <0.02 μm) at all shear rates converged to a minimum value of 4.5 mPa s at NaCl concentrations of 10–20 mEq L-1. On both sides of the minimum, the lower the shear rate, the greater the slope. The apparent viscosity of a 2% suspension of Na-rich montmorillonite <0.02 μm particles, however, was significantly greater than that observed for a suspension of <2 u,m particles. This high apparent viscosity is attributed to the increase in edge surface area and the number of clay particles in a unit volume.

We suggest that edge-to-edge association between Na-rich montmorillonite platelets prevails when the NaCl concentration is below the electrolyte critical concentration, for which the apparent viscosity of the suspension is at its minimum value, whereas face-to-face association prevails at NaCl concentrations above this critical value.

During electrochemical remediation of radionuclide, 235U, 238U, and 99Tc-contaminated aqueous solutions, pyroaurite-like phases, ideally [M(II)M(III)(OH)16CO3·4H2O] where M = Fe, were synthesized following coprecipitation with iron from metal iron electrodes. The effect of radionuclides on the transformation of amorphous precipitates to crystalline pyroaurite-like phases was investigated using X-ray diffraction (XRD), scanning electron microscopy with energy dispersive X-ray analysis, Fourier-transform infrared (FTIR) spectroscopy, and fluorescence spectroscopy. The synthetic iron carbonate hydroxide phases showed primary XRD peaks at 0.7 and 0.35 nm and FTIR spectra that indicated the presence of a brucite-like sheet structure with carbonate anions occupying the interlayer. Divalent and trivalent iron, eroded from the electrode, occupies the octahedral sites of the brucite-like sheets. The carbonate anions in the interlayer balance the excess positive charge from isomorphous substitution of the Fe2+ or Fe3+ by reduced uranium (U4+) and technetium (Tc4+). Because of the lower solubility associated with crystalline phases than amorphous phases, incorporation of radioactive contaminants into pyroaurite-like phases by electrochemical syntheses represents a more effective approach for removing U and Tc from contaminated aqueous solutions than traditional technologies.

This investigation was carried out to study the effect of different concentrations of citric acid and glycine, which are common in freshwaters, on the kinetics of the adsorption of Hg by kaolinite under various pH conditions. The data indicate that Hg adsorption by kaolinite at different concentrations of citric acid and glycine obeyed multiple first order kinetics. In the absence of the organic acids, the rate constants of the initial fast process were 46 to 75 times faster than those of the slow adsorption process in the pH range of 4.00 to 8.00. Citric acid had a significant retarding effect on both the fast and slow adsorption process at pHs of 6.0 and 8.0. It had a significant promoting effect on the fast and slow adsorption process at pH 4.00. Glycine had a pronounced enhancing effect on the rate of Hg adsorption by kaolinite during the fast process. The rise in pH of the system further increased the effect of glycine on Hg adsorption. The magnitude of the retarding/promoting effect upon the rate of Hg adsorption was evidently dependent upon the pH, structure and functionality of organic acids, and molar ratio of the organic acid/Hg. The data obtained suggest that low-molecular-weight organic acids merit close attention in studying the kinetics and mechanisms of the binding of Hg by sediment particulates and the subsequent food chain contamination.