OH-A1 solutions were prepared by adding appropriate amounts of NaOH to A1C13 to obtain OH/Al mole ratios of 2.0, 2.5, 2.7, 3.0, and 3.3 in the final suspension. Solid Na2SO4 and Georgia kaolinite (KGa-2) were added individually and jointly to the OH-Al solutions. All samples were aged for 30, 70, and 180 d. X-ray diffraction, infrared spectroscopy, scanning electron microscopy, and energy dispersive X-ray spectrometry were used to characterize precipitates. Bayerite, gibbsite, and nordstrandite crystallized at mole ratios of 3.0 and 3.3, with bayerite being the most abundant. A morphology of clusters of triangular pyramids is described for bayerite. Despite the aging duration, only noncrystalline Al compounds were obtained in mole ratios of 2.0, 2.5, or 2.7. The addition of sulfate to OH-A1 solutions in mole ratios of 2.0 and 2.5 produced crystalline basic aluminum sulfates of variable morphology, but with similar chemical compositions. These phases lost crystallinity with aging. The product from a 2.7 OH-Al solution was X-ray amorphous hydroxysulfate. In contrast, products obtained at mole ratios of 3.0 and 3.3 contained no sulfate ion, which restricted the formation of gibbsite, bayerite, and nordstrandite. The addition of kaolinite to the solutions in OH/A1 mole ratios of 3.0 and 3.3 favored the formation of nordstrandite. The simultaneous addition of sulfate and kaolinite to the OH-A1 solutions in mole ratios of 2.0 and 2.5 produced prevalent sulfate over kaolinite, whereas the opposite occurred at mole ratios 3.0 and 3.3.

The persistence of many seemingly metastable mineral assemblages in sediments and soils is commonly attributed to their sluggish transformation to the stable-phase assemblage. Although undoubtedly kinetics plays a major role, this study shows that thermodynamic factors, particularly surface energy, significantly influence the free energy. Enthalpies of formation of boehmite samples with variable surface area were derived using high-temperature oxide-melt calorimetry. The average surface enthalpy for all faces terminating boehmite particles was calculated at +0.52 ± 0.12 J/m2. This value represents the surface enthalpy for surfaces exposed to vacuum assuming that H2O adsorbed on the surface of boehmite is loosely bound. These results show that the enthalpy of formation of boehmite may vary by ≤8 kJ/mol as a function of particle size. An overview of published values of surface energies of gibbsite, γ-Al2O3, corundum, and the results here indicates that the hydrated phases (boehmite, gibbsite) have lower surface energies than the anhydrous phases (corundum, γ-Al2O3). Lower surface energies allow the hydrated phases to maintain high surface area, i.e., small particle size. Similar surface energies of boehmite and gibbsite suggest kinetic control favoring the crystallization of boehmite or gibbsite from aqueous solution. The enthalpy of formation of bulk boehmite from the elements was calculated at −994.0 ±1.1 kJ/mol. Combining this result with the data in existing thermodynamic databases, we confirm that bulk boehmite is metastable with respect to bulk diaspore at ambient conditions.

The colloidal state (stable, coagulated, or gel-like) and the rheological properties of Na-rich montmorillonite (Wyoming) dispersions are strongly influenced by organic cations. This effect is shown for homologous organic cations: alkyl trimethylammonium ions, paraquat, diquat, alkyl bispyridinium ions, and the triphenylmethane dyes crystal violet, methyl green, and tris (tri-methylammonium phenyl) methane chloride. The critical coagulation concentrations, cK, are small (often < 1 mmol/L) because the cations are enriched in the Stern layer and influence the solvent structure near the surface. The strong adsorption of the counterions at the clay-mineral surface causes cK values to increase with the solid content. Charge reversal (recharging) of the particles was observed with the longer chain alkyl trimethyl-ammonium ions, dodecyl bispyridinium ions, and crystal violet. Other cations reduced the electrophoretic mobility to zero but positive particle charges were not observed.

The plastic viscosity increased sharply at the critical coagulation concentration and showed a minimum slightly below cK, which was caused by the electroviscous effect. Yield values were developed at concentrations above cK. In most cases, yield values reached a plateau where the amount of organic cations was ∼0.5 mmol/g, i.e., about half of the cation-exchange capacity. The cK values decreased with increasing hydrophobicity of homologous compounds, but the yield value showed maxima at intermediate chain lengths. The yield value of several 0.5% dispersions was high, e.g., dodecyl trimethylammonium ions, 71 Pa; paraquat, 100 Pa; diquat, 42 Pa; hexyl bispyridinium ions, 53 Pa (vs. Ca2+, 0.2 Pa; Al3+, 0.7 Pa). The storage modulus as a function of the number of organic cations changed in a similar way as the yield value, and high values were observed (e.g., dodecyl trimethylammonium ions, hexyl bispyridinium ions: 1000 Pa, paraquat: >4000 Pa). Thus, dispersions with high viscosity, yield value, and pronounced viscoelasticity are obtained by coagulating Na-rich montmorillonite dispersions with organic cations.

X-ray diffraction (XRD), Rb-Sr isotope analysis, transmission electron microscopy (TEM), energy-dispersive X-ray spectroscopy (EDS), and Fourier transform infrared (FTIR) methods were used to study diagenetic illite and illite-smectite (I-S) in Lower Cambrian unlithified clays of shallow depth of burial in the northern part of the intercratonic Baltic paleosedimentary basin of the East-European Platform. The studies focused on the <0.06-µm size fraction of the clay. This fraction consists of a highly illitic illite-smectite (I-S) and a poorly crystalline illite (PCI), with some traces of Fe-rich chlorite also present. Rb-Sr isotopic data for the <0.06-µm size fractions suggest that the illitic I-S and PCI have different formation ages. No precise isotopic ages were derived directly owing to the composite illite mineralogy and retention of radiogenic Sr. This retention occurred because of imperfect isotopic homog-enization at low water/rock ratios. The age of burial diagenesis is proposed to coincide with the time of maximum burial depth, which was achieved during the Middle to Late Devonian and continued until Permian-Triassic erosion. Because of the shallow depth of burial (<2 km), diagenesis was probably a low-temperature (<50°C) transformation process. The resident time of 100–150 million years at maximum burial had a major influence in the process.

The distribution and crystal-chemical characteristics of chlorite, corrensite, and mica in samples from a stratigraphic profile in the Cameros basin are controlled by changes in the sedimentary facies. The lacustrine marls and limestones from the base and the top of the profile contain quartz + calcite + illite ± dolomite ± chlorite ± albite ± paragonite ± Na, K-rich mica. Chlorite is rich in Mg, with Fe/(Fe + Mg) ratios ranging between 0.18–0.37. A formation mechanism involving reaction between Mg-rich carbonate and dioctahedral phyllosilicates is proposed for these Mg-rich chlorites, on the basis of the mutually exclusive relationship found between Mg-rich chlorite and dolomite, together with the relative increase in the proportion of calcite in samples containing chlorite.

The mudrocks from the middle part of the profile are composed of quartz + albite + illite + corrensite (with a mean coefficient of variability of 0.60%) ± chlorite. Corrensite and chlorite are richer in Fe2+ than those from the base or top of the profile, with mean Fe/(Fe + Mg) ratios of 0.51 and 0.56, respectively. Textural and compositional features suggest a formation mechanism for the corrensite, chlorite, and chlorite-mica crystals through replacement of detrital igneous biotite. Whether or not corrensite occurs with chlorite appears to be related to redox conditions. The presence of corrensite alone is apparently favored by oxidizing conditions, whereas the occurrence of corrensite + chlorite is related to more reducing conditions. Corrensite shows higher Si and Na + K + Ca contents, and slightly lower Fe/(Fe + Mg) ratios than chlorite. The presence of corrensite and the lack of random chlorite-smectite interlayering is discussed in terms of the fluid/rock ratio; the occurrence is related to the hydrothermal character of metamorphism in the Cameros basin.

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.

The composition of clinoptilolite formed from volcanic glass was investigated from cuttings collected from the offshore MITI-Somaoki borehole, 52 km east of Soma City, Japan. Unaltered volcanic glass and clinoptilolite-replaced glass were found coexisting in the same silicic tuff at 1000 m depth, but unaltered volcanic glass and clinoptilolite did not coexist in individual glass shards, and each material analyzed in this study was pure. The SiO2 and Al2O3 contents of the pure clinoptilolite and the pure, unaltered glass were 69.32 and 13.05 wt. %, and 70.03 and 11.54 wt. %, respectively. The SiO2 content was almost the same and the Al2O3 content was greater in the clinoptilolite-replaced glass compared with the unaltered volcanic glass. As a result, the SiO2/Al2O3 ratio in the clinoptilolite-replaced glass (SiO2/ A12O3 = 5.31) was slightly lower than that of the unaltered volcanic glass (SiO2/Al2O3 = 6.07). The extra-framework cation composition of the clinoptilolite-replaced glass was three times richer in K (K2O = 7.84 wt. %) than the unaltered volcanic glass (K2O = 2.59 wt. %). The cation selectivity of clinoptilolite, which shows a preference for K to Ca and Na, played an important role in the concentration of K in clinoptilolite during clinoptilolite formation from volcanic glass, although the initial pore waters and the volcanic glass were not especially relative K.

Clinoptilolite-replaced glass in the calcite-cemented tuff at 1100 m depth was not enriched in Ca, suggesting that the exchangable-cation composition of clinoptilolite did not change by exposure to Ca-rich solutions at this depth. Apparently, the exchangable-cation composition of clinoptilolite, once formed, is relatively stable and difficult to change, even under conditions in which calcite is precipitating on clinoptilolite crystals.

High-resolution transmission electron microscopy (HRTEM), selected-area electron diffraction (SAED), annular dark-field scanning transmission electron microscope (STEM) images, and electron nano-diffraction were used to examine structures of synthetic 2- and 6-line ferrihydrite specimens. HRTEM images of 2-line ferrihydrite (2LFh) show scattered small (~1-3 nm) areas with lattice fringes surrounded by areas free of fringes. All SAED patterns show two bright rings corresponding to d-values of ~0.15 and 0.25 nm; each ring has a conspicuous shoulder on each side. Faint rings corresponding to d-values of 0.08, 0.095, 0.100, 0.106-0.114 (very broad ring), and 0.122 nm are visible in strongly exposed SAED patterns. Nanodiffraction patterns show conspicuous streaks and a lack of superlattice formation.

HRTEM images of 6-line ferrihydrite (6LFh) display larger crystallites (typically ~5-6 nm) with lattice fringes visible in many thin areas. SAED patterns show rings corresponding to d-values of 0.148, 0.156, 0.176, 0.202, 0.227, and 0.25-0.26 nm and a shoulder extending between d-values of ~0.25 and 0.32 nm. Faint rings corresponding to d-values of 0.086, 0.093, 0.107, 0.112, 0.119, 0.125, and 0.135 nm are visible in strongly exposed SAED patterns. Small quantities of hematite, magnetite or maghemite, and an acicular material tentatively identified as goethite were observed in the 6-line ferrihydrite, but these quantities do not contribute significantly to the overall diffracted intensity from the sample.

The reduction of hexavalent chromium species in aqueous solutions by interaction with Fe(II)-bearing solid surfaces was studied using a 0.96 × l0−3 M Cr(VI) solution and iron-rich clays with different Fe(II)/Fe(III) ratios, layer charge, and exchange properties, i.e., chlorite, corrensite, and montmorillonite. Experimental studies demonstrated that Fe(II)-bearing phyllosilicates reduce aqueous Cr(VI) ions at acidic pH. Chlorite and corrensite, owing to the high Fe(II)/Fe(III) ratio, are electrochemically reactive, as rapid Cr(VI) reduction indicated. In contrast, montmorillonite showed minimum to nil reactivity towards Cr(VI). Furthermore, corrensite, which is high in both Fe(II)/Fe(III) ratio and exchange capacity, adsorbs the greatest amount of chromium.

X-ray absorption spectroscopy at Al, Mg, Fe, and Cr K-edges was used to investigate the adsorbed chromium species. The montmorillonite sample, unaffected by treatment with Cr(VI) solution, displays no change at any investigated edge. Edge shape and energy also do not change for the Mg and Al spectra in corrensite, and changes are minor in chlorite. By contrast, the Fe K-edge changes both in chlorite and corrensite, and indicates an increase of Fe(III) in treated samples at the expense of pre-existing Fe(II). Cr K-edge spectra show that chlorite and corrensite sorb Cr(III), which implies its reduction from Cr(VI) in the interacting solution.

An environmental infrared microbalance (EIRM) cell was used to study H2O sorption on two montmorillonite samples as a function of water content and type of exchangeable cation. The vibrational spectra showed that H2O sorbed to the clay at low-water content was strongly influenced by the exchangeable cation and by the close proximity to the clay surface. At water contents <6 H20 molecules per exchangeable cation, the H-O-H bending mode of H2O (v2 mode) shifts to a lower frequency and is characterized by an increase in molar absorptivity. In contrast, the positions of the asymmetric and symmetric OH-stretching modes of sorbed water (v1 and v3 modes) shift to higher energies. These observations indicate that H2O molecules sorbed to the clay surface at low-water content are less hydrogen bonded than in bulk H2O. In addition, the vibrational-stretching and bending bands of the structural OH groups of the 2:1 layer are also strongly influenced by H2O content and type of exchangeable cation. By using the EIRM cell, the molar absorptivities of the structural OH-bending vibrations were measured as a function of H2O content. The position and molar absorptivity of the structural OH-bending bands at 920, 883, and 840 cm-1 are strongly influenced by H2O content and type of exchangeable cation. The molar absorptivity of the 920-cm-1 band, which is assigned to the AlAlOH group, decreased strongly at low-H2O content. This reduction in intensity is assigned to a dehydration-induced change in orientation of the structural OH groups resulting from the penetration of H2O molecules into siloxane ditrigonal cavities that are not associated with a net negative charge from isomorphous substitutions.

Mixed layering of illite-smectite was studied both experimentally, by using high-resolution transmission electron microscopy (HRTEM) and analytical electron microscopy (AEM), and theoretically, by using lattice-energy calculations.

Samples from a hydrothermal origin show the transformation of smectite to illite with different ordering types in the illite-smectite layer sequences. Ordering ranges from complete disordered (Reichweite, R = 0 type) in the less transformed samples to increased local order, with IS and IIS sequences (R = 1 and R = 2, respectively; I = illite, S = smectite) in more illitized samples.

Lattice-energy calculations are used to determine the structure of the illite-smectite sequence, which corresponds to the minimum energy. The unit layers are: O0.5TI′TO0.5 (O, T, and I′, respectively, denote the octahedral and tetrahedral sheets, and the interlayer. The 0.5 signifies half of the octahedral cations.) For example, the arrangements of the perfectly ordered … ISIS … and … IISIIS … sequences are respectively … OM(TI′T)IOM(TI′T)S … and … OM(TI′T)IOI(TI′T)1OM(TI′T)S … (the subscripts I, S, and M, respectively, refer to compositions of illite, smectite, and midway between at 0.5). Such arrangements produce a polar model for TOT layers, which display a TIOMTS structure in the case of IS adjacent layers. Furthermore, the lattice energies of … ISIS … and … IISIIS … are found to be nearly equal to the corresponding sums of the lattice energies of illite and smectite. This result indicates that interstratified illite-smectite and the two-phase assemblage of illite + smectite have similar stabilities.

On the basis of the above model, the solid-state transformation of one smectite layer to one illite layer, which produces mixed-layer sequences, involves the transformation of an O0.5TI′TO0.5 unit of smectite into the same corresponding unit of illite.

Reduction of structural Fe in Na-exchanged dioctahedral smectites decreases swellability in water, but because clay interlayers also collapse in the process the concomitant effect on surface hydration energy is uncertain. This study examined the hydration behavior of oxidized and reduced dioctahedral smectite clays exchanged with polar (Na) and weakly-polar (organic) cations to determine the nature of the surface before and after Fe reduction, and to determine if clay surfaces are hydrophilic or hydrophobic. The H2O content in various dioctahedral smectites decreased if Na was replaced by tetramethylammonium (TMA), trimethylphenylammonium (TMPA), or hexadecyltrimethylammonium (HDTMA). Among the organo-clays, H2O adsorption decreased with increasing complexity of the cation. For oxidized smectites, those exchanged with TMPA retained less H2O than those exchanged with Na at all pressures. The extent of this difference depended on the clay and decreased with increasing applied pressure. Reduction of Fe(III) to Fe(II) in the octahedral sheets decreased the swelling of Na-saturated smectites, apparently causing some previously swelling interlayers to collapse. If the Na interlayer cation was exchanged to alkylammonium after reduction, but prior to swelling-pressure measurements, the swelling increased or remained near constant, suggesting that the organo-cation disrupted the collapse process of the interlayers associated with the reduced smectite layers. Reduced TMPA-saturated smectite surfaces are more strongly hydrated if the octahedral sheet is reduced than if oxidized. Thus, reduction of structural Fe increases the hydration energy of smectite basal surfaces, but swellability could decrease or increase depending on the extent of interlayer collapse occurring with different exchangeable cations.

The oxidation state of structural Fe greatly alters surface chemistry, which may have a large influence on clay-organic interactions. The effect of structural-iron oxidation state on chlorinated hydrocarbons at the clay-water interface was examined. Pentachloroethane (5CA) was reacted with oxidized, reduced, and reoxidized forms of three different smectites: montmorillonite, ferruginous smectite, and nontronite in aqueous suspension under controlled-atmosphere conditions. Pentachloroethane was found to adsorb at different rates for the three smectites. A series of 5CA-adsorption rate constants in the presence of these clays showed a strong correlation with the Fe(II) content of the clay (r2 = 0.98). The clay surface behaves as a Brønsted base and promotes 5CA dehydrochlorination. The adsorption kinetics at the clay-water interface were described by the formation of a precursor complex prior to 5CA dehydrochlorination.