Showing papers in "Clays and Clay Minerals in 1995"

Mechanism of Adsorption and Desorption of Water Vapor by Homoionic Montmorillonites: 2. The Li+ Na+, K+, Rb+ and Cs+-Exchanged Forms

Abstract: Methods previously used to distinguish between water adsorbed on external surfaces and in the interlamellar space of Na-montmorillonite during adsorption and desorption of water vapor have been extended to a set of homoionic Li-, Na-, K-, Rb- and Cs-montmorillonite. The textural and structural features have been investigated at different stages of hydration and dehydration using controlled-rate thermal analysis, nitrogen adsorption volumetry, water adsorption gravimetry, immersion microcalorimetry and X-ray powder diffraction under controlled humidity conditions. During hydration, the size of the quasi-crystals decreases from 33 layers to 8 layers for Na-montmorillonite and from 25 layers to 10 layers for K-montmorillonite, but remains stable around 8–11 layers for Cs-montmorillonite. Each homoionic species leads to a one-layer hydrate, which starts forming at specific values of water vapor relative pressure. Li-, Na- and K-montmorillonite can form a two-layer hydrate. By comparing experimental X-ray diffraction patterns with theoretically simulated ones, the evolution of structural characteristics of montmorillonites during hydration or desorption can be described. Using structural and textural data, it is shown that during adsorption: (1) the rate of filling of interlamellar space of the one layer hydrate increases with the relative pressure but decreases with the size of the cations; and (2) the different hydrated states are never homogeneous.

Definition of clay and clay mineral; joint report of the AIPEA nomenclature and CMS nomenclature committees

Abstract: The definition of"clay" was first formalized in 1546 by Agricola. It has been revised many times since, although the fundamentals involving plasticity, particle size, and hardening on firing were retained by most. For an exhaustive account of the history of the definit ion to 1963, the reader is referred to Mackenzie (1963). More recent developments may be found in Weaver (1989). The definition of clay raises the important issue of clay constituents and, implicitly, the definition of "clay mineral." Mackenzie (1963, p. 15) noted the inappropriateness of defining clay mineral as "any mineral which occurs in clay" since, among several reasons, it would include many accessory minerals that are not characteristic of clay. Previous definitions of "clay mineral" (e.g., Bailey 1980) simply equated clay minerals to phyllosilicates, thus endorsing the use of a term without apparent justification. The term "clay mineral," however, is useful when applied in context to clay constituents. The approach of equating "clay mineral" to phyllosilicate fails to consider the relationship of the properties of the bulk clay with the properties of the constituents of clay. This inconsistency is addressed here in part, without making major changes to current usage. Weaver (1989, p. 5) noted an additional complication: the conceptual problem of combining particle size

Monte Carlo simulation of interlayer molecular structure in swelling clay minerals; 1, Methodology

Abstract: Monte Carlo (MC) simulations of molecular structure in the interlayers of 2:1 Na-saturated clay minerals were performed to address several important simulation methodological issues. Investigation was focused on monolayer hydrates of the clay minerals because these systems provide a severe test of the quality and sensitivity of MC interlayer simulations. Comparisons were made between two leading models of the water-water interaction in condensed phases, and the sensitivity of the simulations to the size or shape of the periodically-repeated simulation cell was determined. The results indicated that model potential functions permitting significant deviations from the molecular environment in bulk liquid water are superior to those calibrated to mimic the bulk water structure closely. Increasing the simulation cell size or altering its shape from a rectangular 21.12 A × 18.28 A × 6.54 A cell (about eight clay mineral unit cells) had no significant effect on the calculated interlayer properties.

The hydrophilicity and hydrophobicity of clay minerals

Abstract: The terms “hydrophobic” and “hydrophilic” are typically used in a non-specific sense and, as such, they have a limited utility. Surface thermodynamic theory, as described here, allows a natural and potentially powerful definition of these terms. The boundary between hydrophobicity and hydrophilicity occurs when the difference between the apolar attraction and the polar repulsion between molecules or particles of material (1) immersed in water (w) is equal to the cohesive polar attraction between the water molecules. Under these conditions, the interfacial free energy of interaction between particles of 1, immersed in water (ignoring electrostatic interactions), ΔG 1 1 IF exactly zero. When ΔG 1 1 IF is positive, the interaction of the material with water dominates and the surface of the material is hydrophilic; when ΔG 1 1 IF is negative, the polar cohesive attraction between the water molecules dominates and the material is hydrophobic. Thus, the sign of defines the nature of the surface and the magnitude of may be used as the natural quantitative measure of the surface hydrophobicity or hydrophilicity.

Monte Carlo simulation of interlayer molecular structure in swelling clay minerals; 2, Monolayer hydrates

Abstract: Monte Carlo (MC) simulations of interlayer molecular structure in monolayer hydrates of Na- saturated Wyoming-type montmorillonites and vermiculite were performed. Detailed comparison of the stimulation results with experimental diffraction and thermodynamic data for these clay-water systems indicated good semiquantitative to quantitative agreement. The MC simulations revealed that, in the monolayer hydrate, interlayer water molecules tend to increase their occupation of the midplane as layer charge increases. As the percentage of tetrahedral layer charge increases, water molecules are induced to interact with the siloxane surface O atoms through hydrogen bonding and Na § counter-ions are induced to form inner-sphere surface complexes. These results suggest the need for careful diffraction experiments on a series of monolayer hydrates of montmorillonite whose layer charge and tetrahedral isomorphic substitution charge vary systematically.

The Influence of Uranyl Hydrolysis and Multiple Site-Binding Reactions on Adsorption of U(VI) to Montmorillonite

Abstract: Adsorption of uranyl to SWy- 1 montmorillonite was evaluated experimentally and results were modeled to identify likely surface complexation reactions responsible for removal ofuranyl from solution. Uranyl was contacted with SWy- 1 montmorillonite in a NaC104 electrolyte solution at three ionic strengths (I = 0.001, 0.01, 0.1), at pH 4 to 8.5, in a N2~) atmosphere. At low ionic strength, adsorption decreased from 95% at pH 4 to 75% at pH 6.8. At higher ionic strength, adsorption increased with pH from initial values less than 75%; adsorption edges for all ionic strengths coalesced above a pH of 7. A site-binding model was applied that treated SWy-1 as an aggregate of fixed-charge sites and edge sites analogous to gibbsite and silica. The concentration of fixed-charge sites was estimated as the cation exchange capacity, and non-preference exchange was assumed in calculating the contribution of fixed-charge sites to total uranyl adsorption. The concentration of edge sites was estimated by image analysis of transmission electron photomicrographs. Adsorption constants for uranyl binding to gibbsite and silica were determined by fitting to experimental data, and these adsorption constants were then used to simulate SWy-1 adsorption results. The best simulations were obtained with an ionization model in which A1OH2 + was the dominant aluminol surface species throughout the experimental range in pH. The pH-dependent aqueous speciation of uranyl was an important factor determining the magnitude of uranyl adsorption. At low ionic strength and low pH, adsorption by fixed-charge sites was predominant. The decrease in adsorption with increasing pH was caused by the formation of monovalent aqueous uranyl species, which were weakly bound to fixed-charge sites. At higher ionic strengths, competition with Na + decreased the adsorption of UOz 2+ to fixed-charge sites. At higher pH, the most significant adsorption reactions were the binding of UP22+ to A1OH and of(UO2)3(OH)5 + tO SiOH edge sites. Near-saturation of A1OH sites by UOz 1+ allowed significant contributions of SiOH sites to uranyl adsorption.

Clay mineral thermometry : a critical perspective

Abstract: Diagenetic clay minerals usually occur as heterogeneous assemblages of submicroscopic layers consisting of different structure types such as illite, smectite and chlorite, with variable composition within a given structure type, and with highly variable concentrations of imperfections. The dimensions of mixed-layering, the semi-coherent to coherent nature of the structures across the layering, and compo- sitional heterogeneity occur at a scale well below that of an individual thermodynamic phase. These relations imply that most clays are not distinct minerals or phases, and that assemblages of clays in shales and mudstones are incompatible with the phase rule. Such relations are better evaluated in terms of the formation of metastable materials with each small unit having unique chemical properties, rather than as a small number of stable homogeneous phases. Consequently, treatment of most clay minerals in terms of equilibrium stability with either a thermodynamic or experimental approach is subject to error. Chemical reactions involving most clay minerals are best understood with kinetic models. These involve a great variety of parameters such as time, fluid/rock ratio, deformation history, nature of starting materials and transformation mechanisms, as well as the variables, such as temperature, pressure and composition, that are commonly used to define equilibrium. Solubility experiments on the stabilities of clay minerals are unlikely to attain equilibrium at low temperatures. Moreover, the activity of soluble species may be controlled by surface equilibria, or by absorbed or exchangeable cations. Interpretations of available experiments on the solubility of illite vs. other mineral assemblages are in violation of Schreinemakers' rules and indicate lack of equilibrium. Predictable sequences of clay minerals as a function of temperature are best understood through the Ostwald step rule, in which clay mineral assemblages undergo reactions in response to kinetic factors that represent reaction progress rather than an approach to equilibrium. Currently used clay mineral ther- mometers (illite crystallinity, smectite/illite reaction, chlorite composition) are not based on equilibrium reactions. Such systems are not accurate thermometers and therefore have questionable utility.

Hydraulic conductivity of thirteen compacted clays

Abstract: Hydraulic conductivity tests were conducted on thirteen compacted clayey soils being used for compacted clay liners at landfills throughout the United States The soils were prepared to various molding water contents and then compacted and permeated in the laboratory Results of the tests show that for all of the soils, zones exist in the compaction plane (ie, dry unit weight vs water content) where the hydraulic conductivity is similar These zones fall roughly parallel to contours of constant initial saturation (degree ofsaturation at compaction), with lower hydraulic conductivities generally occurring for conditions corresponding to higher initial saturation Wet of the line of optimums, lower hydraulic conductivity is also attained for soils that are more plastic and have a greater quantity of fines A regression equation was developed from the data to estimate hydraulic conductivity given the initial saturation, compactive effort, plasticity index, and clay content

An improved model for structural transformations of heat-treated aluminous dioctahedral 2:1 layer silicates

Abstract: An improved model for the interpretation of thermal effects during dehydroxylation in aluminous dioctahedral 2:1 layer phyllosilicates considers trans-vacant (tv) and cis-vacant (cv) 2:1 layers and leads to very different temperatures of dehydroxylation for these tv and cv vacant modifications. In particular, smectites and illites consisting of cv 2:1 layers are characterized by dehydroxylated temperatures which are higher by 150°C to 200°C than those for the same minerals consisting of the tv 2:1 layers. A considerable lengthening of the OH-OH edges in cv 2:1 layers in comparison with the OH-OH edges in the tv 2:1 layers is postulated as the reason for the higher dehydroxylation. Dehydroxylation in aluminous cv 2:1 layer silicates should occur in two stages. Initially, each two adjacent OH groups are replaced by a residual oxygen atom and the Al cations, which originally occupied cis -and trans-sites, become 5- and 6-coordinated, respectively. The structure of 2:1 layers corresponding to this stage of the dehydroxylation is unstable. Thus the Al cations migrate from the former trani-sites to vacant pentagonal prisms. The resulting dehydroxylated structure of the original cv 2:1 layers is similar to that of the former tv 2:1 layers. Diffraction and structural features of the cv dehydroxylates predicted by the model are in agreement with X-ray diffraction effects observed for cv illite, illite-smectite and montmorillonite samples heated to different temperatures. In particular, the diffusion of Al cations to empty five-fold prisms during dehydroxylation of the tv 2:1 layers explains why dehydroxylation of reheated cv montmorillonites occurs at temperatures lower by 150°C to 200°C than samples that were not recycled.

Decarbonation Behavior of Mg-Al-CO3 Hydrotalcite-like Compounds during Heat Treatment

Abstract: Historically, the decarbonation of Mg-A1-CO3 hydrotalcite-like compounds (HTlc) has been thought to occur between 400* and 500"C. The present work demonstrates that when HTlcs having the maximum AI content, AU(A1 + Mg) = 0.33, are heated to 5000C, 20-30% of the carbonates remain. The evolution of the remaining carbonates was observed as two maxima, at 600 and 900~ At these tem- peratures, AI ions go into MgO, and spinel (MgA1204) forms. Therefore, the carbonates are released as the A1 ions migrate. At a lower A1 content, AI/(A1 + Mg) = 0.25, CO2 evolution is almost complete at 500"12. This HTlc has no maximum of CO2 evolution above 5000C. Lower charge densities, due to lower A1 contents, lead to lower amounts of remaining carbonate anions.

Fourier Transform Raman Spectroscopy of Kaolinite, Dickite and Halloysite

Abstract: The vibrational modes of clay minerals are uniquely accessible to FT Raman spectroscopy, but this potentially powerful technique has found limited application to the study of clay mineral structure. Raman spectra in the 50 to 3800 cm-t region were obtained for a number of kandite clays. The kandite clay minerals are characterised by relatively intense bands centred at 142.7 cm -I for kaolinite, 143 cm -~ for halloysite and 131.2 cm-I for dickite with prominent shoulders at 129, 127, and 120 cm- 1 respectively. These vibrational modes are attributed to the O-A1-O and O-Si-O symmetric bends. Differences in the lattice modes for the kandite clay minerals in the 200 to t200 cm -I were obtained. Four OH bands were obtained for kaolinite 3621, 3652, 3668, and 3695 cm-1; three OH bands were found for a selection of dickites and halloysites. The San Juan Dickite and the Eureka Halloysite show further resolution of the low frequency 3620 cm -1 hydroxyl band. This splitting is attributed to variation in the position of the inner hydroxyls. Variation in band intensity and position was found to be sample dependent.

Dehydroxylation of Aluminous Goethite: Unit Cell Dimensions, Crystal Size and Surface Area

Abstract: This work investigates unit cell dimensions, crystal size and specific surface area of aluminous goethite that was progressively dehydroxylated to form hematite. Goethite synthesized from the ferrous system altered to hematite with DTGA maximum increasing from 236° to 273°C for 0 to 30.1 mole % Al-substitution. Unit cell dimensions of goethite and hematite decreased as Al-substitution increased and increased as excess OH increased. The crystallographically equivalent a axis of goethite and c axis of hematite were more sensitive than other axes to the presence of excess structural OH associated with Al-substitution. Specific surface area increased from 147 to 288 m2/g for goethite and from 171 to 230 m2/g for hematite as Al-substitution increased. An increase in specific surface area on heating goethite at temperatures between 200° and 240°C is related to a decrease in the size of coherently diffracting domains of goethite crystals and to the development of pore and structural defects associated with the formation of hematite. The decrease in specific surface area for heating temperatures above 240°C is attributed to the growth of hematite crystals by diffusion.

A technique for maintaining texture and permanent expansion of smectite interlayers for TEM observations

Abstract: A process for treating smectite-bearing rock samples that utilizes LR White resin; allows TEM observation of expanded smectite interlayers and therefore preservation of original rock textures. Examples of several lattice fringe images are shown, including: (1) Dioctahedral smectite layers from a shale (1388.9 meter depth, Texas Gulf Coast) give fringes that consistently have spacings of 1.2–1.3 nm, yet duplicate other features previously observed in collapsed samples. (2) Packets of illite layers give fringes with 1.0-nm spacings coexisting with packets of (dominantly) R1 I/S having 2.1-nm lattice fringe spacings in a Gulf Coast shale from 4742.1 m. (3) Rectorite from Garland Co., Arkansas gives 2.3-nm lattice fringes. Samples with wide ranges of I/S ratios and lithologies have been found to be permanently expanded with retention of original textures, commonly leading to unambiguous identification of illite and smectite interlayers in lattice fringe images.

Effects of exchangeable cations on hydraulic conductivity of a marine clay

Abstract: 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 defloccalation.

Origin of the bentonite deposits of eastern Milos, Aegean, Greece; geological, mineralogical and geochemical evidence

Abstract: The Lower Pleistocene bentonite deposits of Eastern Milos, Greece have been formed at the expense of volcaniclastic rocks under submarine conditions. Systematic variation of the major chemical elements reveals that the deposits were formed from different precursors which were erupted from different volcanic centers belonging to at least two separate volcanic provinces. The volcanic eruptions were probably subaqueous. The major authigenic phases are smectite, K-feldspar, opal-CT and the zeolites mordenite and clinoptilolite. The deposits have a complex history and have been affected by hydrothermal alteration. The geological features of bentonites coupled by the presence of abundant authigenic K-feldspar indicate that alteration of the parent volcanoclastic rocks took place under low temperatures and is probably not related to hydrothermal alteration, which is a separate event. Hydrothermal alteration has modified both the mineralogical characteristics and the properties of bentonites. Alteration of the parent rocks to bentonites was favoured by high water: wall rock ratios and fluid flow and is associated with leaching and subsequent removal of Na, K and Ca. The source of Mg was the parent rocks and only small scale Mg-uptake from the sea water has probably taken place. The formation of authigenic K-feldspar has probably been favoured by a high K+/H+ activity ratio and high Si activity of the pore fluid. Such conditions might have been favoured by the pH conditions and the cooling history of the parent rocks.

Reduction and reoxidation of nontronite; questions of reversibility

Abstract: Redox cycles are common in nature and likely have a profound effect on the behavior of soils and sediments. This study examined a key component of redox cycles in smectites, namely, the reoxidation process, which has received little attention compared to the reduction process. Unaltered (oxidized) and reoxidized ferruginous smectites (nontronites) were compared using infrared and Mossbauer spectroscopies, and thermal gravimetric analysis. The infrared and thermal gravimetric data revealed that the structural OH content of reduced-reoxidized clay is about 15 to 20% less than in the original (oxidized) sample, indicating that the structure remains partially dehydroxylated even after reoxidation. Mossbaner spectra of reoxidized samples consisted of larger quadrupole splitting for Fe(III) doublets than in the unaltered samples, suggesting that the environment of Fe(III) is more distorted after the reduction-reoxidation treatment.

Influence of Nonstoichiometry and the Presence of Maghemite on the Mössbauer Spectrum of Magnetite

Abstract: Several samples of large- and small-particle magnetite (Fe3O4), as well as its thermal decomposition products formed at different temperatures and atmospheres, have been studied extensively by Mossbauer spectroscopy (MS), both with and without an applied field of 6T. Synthetic mixtures of magnetite and poorly- or well-crystallized maghemite have also been studied. Large-particle magnetite (MCD > 200 nm), when heated in air for 12 hours at T < 400°C, transforms to a mixture of well-crystallized hematite and magnetite, the latter one remaining stoichiometric, according to the relative area-ratios obtained from MS. Thermal treatment at 1300°C in a controlled O2 partial pressure, produced a mixture of stoichiometric and nonstoichiometric magnetite, but the latter component seems to be composed of particles with different degrees of nonstoichiometry. The Mossbauer spectra of the decomposition products at T < 200°C in air of small-particle magnetite (MCD ~ 80 nm) could be successfully interpreted as a mixture of magnetite and maghemite, rather than nonstoichiometric magnetite. This suggestion is further supported by the experiments with the synthetic mixtures. It is clearly demonstrated that is not possible, even by applying a strong external field, to separate the contribution of the A-site of magnetite from that of maghemite.

Tetramethylphosphonium- and Tetramethylammonium-Smectites as Adsorbents of Aromatic and Chlorinated Hydrocarbons: Effect of Water on Adsorption Efficiency

Abstract: Tetramethylphosphonium-smectite (TMP-clay) and tetramethylammonium-smectite (TMA-clay), were prepared and characterized as adsorbents for a series of aromatic and chlorinated hydrocarbons. The sorption of benzene, alkylbenzenes, and carbon tetrachloride as vapors and as solutes from water was studied to evaluate the effect of water on adsorption efficiency. Adsorption of organic vapors depended on the N2 BET surface area. TMA-clay was a slightly better adsorbent than TMP-clay, due to its somewhat higher surface area. The Langumir isotherms obtained indicated that adsorption occurred predominantly in the interlayer micropores, apparently on mineral surfaces between onium ions. Adsorption efficiency of both organo-clays decreased, compared to vapor sorptions, in presence of water. Lower sorption was apparently due to shrinkage of the interlayer pore or cavity sizes by hydration of interlayer TMA and TMP cations. Although sorption efficiencies of both organo-clays was reduced in presence of bulk water, the extent of reduction was much less for TMP-clay. Thus, TMP-clay was a better adsorbent than TMA-clay in presence of water, despite its lower surface area, in direct contrast to vapor sorption. The Langumir isotherms indicated interlayer sorption of benzene, alkylbenzenes and carbon tetrachloride from water by TMP-clay. The absence of Langumir isotherms for toluene, ethylbenzene and p-xylene uptake from water by TMA-clay indicated that these bulkier solutes were not adsorbed in the interlayers. These results indicate that hydration of TMA cations causes shrinkage of the interlayer pores to dimensions that exclude these solutes. The lower degree of hydration of TMP cations enables TMP-clay to maintain interlayer pores large enough to accommodate the bulkier alkylbenzenes.

Illite/smectite diagenesis and its variable correlation with vitrinite reflectance in the pannonian basin

Abstract: —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: $$-dS/dt=e^{log(A)-E/RT}\cdot\;S$$ where S = fraction of smectite layers in I/S, t = time (Ma), e = exponential function, log(A) = frequency factor = 7.5 (Ma−1), E = activation energy = 31.0 kJ mol−1, R = universal gas constant, and T is temperature in Kelvin. This result also suggests an important role for time. By combining the kinetics of the smectite-to-illite reaction with a kinetic model of vitrinite maturation it is possible to define a domain within which all ‘normal’ (burial diagenesis) correlations between Ro and I/S diagenesis should lie. Such diagrams can be used to identify different thermal histories related to different geotectonic settings and ‘anomalous’ data such as that affected by igneous intrusions.

The Rheological and Colloidal Properties of Bentonite Dispersions in the Presence of Organic Compounds V. Bentonite and Sodium Montmorillonite and Surfactants

Abstract: The influence of surfactants on the flow behavior of sodium montmorillonite dispersions (2% w/w) was studied for a cationic (cetylpyridinium chloride, CPCl) and an anionic surfactant (sodium dodecylsulfate, SDS). When the dispersion pHs were >3.5 and 10−4 M increased the shear stress but the Bingham yield value remained virtually unchanged (τo ≈ 100 mPa). At pH ≈ 7, the shear stress and yield point decreased with increasing CPCl concentration (τo from 430 to 100 mPa). The flow properties of sodium calcium bentonite dispersions were independent of pH and CPCl concentrations ≤ 10−4 M; they increased modestly at higher concentrations. At pH 4, the flow values decreased to a minimum value at 10−2 M SDS (τo from 430 to 50 mPa). The flow of the sodium calcium bentonite dispersions at pH > 4 was independent of SDS concentrations ≤ 10−3 mole/liter; at higher SDS concentrations, the flow values increased more strongly in sodium calcium bentonite than in sodium montmorillonite dispersions. Surfactants influence the flow behavior of sodium montmorillonite dispersions by their action on the card-house networks in strongly acidic medium and, at higher pH, by the electroviscous effect. At the highest surfactant concentrations without flocculation, the shear stress and yield value are increased by interacting chains of opposed particles. Addition of the surfactants increases the salt (NaCl) stability of the dispersions because the adsorbed surface active agents influence the counterion distribution between the Stern and the diffuse ionic layer.

Evolution of Hematite Surface Microtopography Upon Dissolution by Simple Organic Acids

Abstract: The surface microtopography of hematite over the course of dissolution in oxalic and citric acids was examined by in-situ and ex-situ atomic-force microscopy, ln-situ imaging of the basal-plane surface of a centimeter-scale natural hematite sample immersed in 2 mM citric acid demonstrated that the basal-plane surface was relatively unreactive; rather, dissolution occurred along step edges and via etch-pit formation. Ex-situ imaging of synthetic hematite particles following batch dissolution in 1 mM oxalic acid showed similar dissolution features on basal-plane surfaces; in addition, etching along particle edges was apparent. The presence of etch features is consistent with a surface-controlled dissolution reaction. The results are in agreement with previous investigations suggesting that the basal-plane surface is relatively unreactive with respect to ligand exchange. Both in-situ and ex-situ imaging of particle surfaces can provide valuable information on the roles of surface structures and microtopographic features in mineral dissolution.

Effects of iron and aluminium oxides on the colloidal and surface properties of kaolin

Abstract: 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% A1 (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 A1. 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 A1 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 A1 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.

Crystal chemistry of hydrous iron silicate scale deposits at the Salton Sea geothermal field

Abstract: The crystal chemistry of Fe-Si scales deposited from geothermal brines at Salton Sea, California, was studied by powder X-ray diffraction and spectroscopic techniques including infrared, 5VFe M6ssbauer, 27A1 and 29Si nuclear magnetic resonance (NMR), and Fe and Si K-edge extended X-ray absorption fine structure (EXAFS). Scales precipitated at near 250~ from dissolved ferrous iron and silicic acid are composed of hisingerite. This phase is shown to possess the same local structure as nontronite and is a poorly-crystallized precursor of the ferric smectite. A clear distinction can be made at the local scale between hisingerite and 2-line ferrihydrite because, even in their most disordered states, the former possesses a two-dimensional and the latter a three-dimensional anionic framework. At temperature near 100*C Fe-Si scales are a mix of Al-containing opal and hydrous ferrous silicate, whose local structure resembles minnesotaite and greenalite. This hydrous ferrous silicate is very well ordered at the local scale with an average Fe coordination about Fe atoms of 6 +_ 1. The difference in crystallinity between the ferrous and ferric silicate scales was related to variations of growth rates of clay particles precipitated from ferrous and ferric salt solutions. The low crystallinity of the ferric smectite suggests that the oxidation of ferrous iron occurs before polymerization with silica.

Geochemistry and Mineralogy of a Modern Buserite Deposit from a Hot Spring in Hokkaido, Japan

Abstract: The hot spring water discharging from a flank of an active volcano is precipitating unique monomineralic manganese deposits over volcanic terrain. The major and trace element chemistry, XRD mineralogy, DTA, and SEM observations indicate that the deposits consist of 10 A phyllomanganate (buserite) accommodating inter-layer Ca and Mg with negligible amounts of detrital minerals. Other metallic elements can be accommodated by buserite, but concentrations are negligible ranging less than 10 ppm to 500 ppm. Abundance and pattern of REE (less than 100 ppm in total) are similar to those from hydrothermal manganese deposits. The buserite is enriched in Ca and Mg but depleted in Na in comparison with those in the spring water. The distribution coefficients for Ca, Mg and Na between the buserite and the host water were calculated assuming an ion-exchange equilibrium in the Yuno-Taki Fails, which proved applicable to other manganese deposits from surficial environments on land and oceans.

Rotationally Disordered Illite/Smectite in Paleozoic K-Bentonites

Abstract: The three-dimensional crystal structure of rotationally disordered illite/smectite (I/S) in K-ben- tonite samples from the Appalachian basin and neighboring areas is described using the parameters of 1) Po, the proportion of zero-degree layer stacking rotations, such as in the polytype series IMd-IM; 2) Pcv, the proportion of 2:1 layers with cis-vacant (cv) octahedral sites that are randomly interstratified with trans-vacant (tv) layers; and 3) Pro, the proportion of layers with n- 60 ~ rotations (as opposed to n. 120 ~ in the rotated layers. These parameters were determined by computer modeling of experimental randomly oriented powder X-ray diffraction patterns. The proportion of cv interstratification in the I/S increases with A1 and decreases with Mg and Fe content. The proportion of n. 60 ~ rotations in the rotated layers increases with Mg and Fe content. The cv 120 ~ disordered structure correlates with tetrahedral A 1 for Si substitution and increasing tetrahedral charge. The tv n.60 ~ disordered structures correlate with octahedral Mg for A1 substitution. The data indicate that the type of unit cell and nature of rotational disorder in I/S is controlled by the octahedral Mg content. The three-dimensional structures do not show any systematic correlation with Reichweite and percent expandability as determined from diffraction patterns of oriented sample preparations.

Thermal Behavior and Decomposition of Intercalated Kaolinite

Abstract: Intercalation complexes of a Hungarian kaolinite were prepared with hydrazine and potassium acetate The thermal behavior and decomposition of the kaolinite-potassium acetate complex was studied by simultaneous TA-EGA, XRD, and FTIR methods The intercalation complex is stable up to 300°C, and decomposition takes place in two stages after melting of potassium acetate intercalated in the interlayer spaces Dehydroxylation occurred, in the presence of a molten phase, at a lower temperature than for the pure kaolinite FTIR studies revealed that there is a sequence of dehydroxylation for the various OH groups of intercalated kaolinite The reaction mechanism was followed up to 1000°C via identification of the gaseous and solid decomposition products formed: H2O, CO2, CO, C3H6O, intercalated phases with basal spacings of 141 A, 115 A, and 85 A as well as elemental carbon, K4H2(CO3)3 · 15H2O, K2CO3 · 15H2O, and KAlSiO4

Sepiolite-palygorskite from the Hekimhan region (Turkey)

Abstract: Upper Cretaceous-Tertiary marine clayey-calcareous rocks of the Hekimhan basin contain fibrous clay minerals in significant amounts. Ophiolitic rocks in the provenance area have contributed the elements to form the clay minerals. XRD, SEM, major, trace and REE analyses were applied to samples taken from several stratigraphic sections. Diagenetic minerals such as smectite, dolomite, calcite, gypsum, celestite and quartz/chalcedony are associated with sepiolite-palygorskite group clays. Trace and rare earth elements (REE) are more abundant in palygorskite than sepiolite. REE abundances in the sepiolite-palygorskite are characterized by negative Eu and positive Nd anomalies when normalized with respect to chondrite and shale. Sepiolites with sharp XRD peaks are formed by diagenetic replacement of dolomite and diagenetic transformation of palygorskite, or by direct crystallization from solution. The average structural formula of the sepiolite is: (Mg7.15Al0.13Fe0.31Cr0.06Ni0.04)(Si11.98Al0.02)O30(OH)4(OH2)4Cao0.03Na0.02K0.02.8H2O Palygorskite appears to be authigenic by direct precipitation from solution. It exists in both monoclinic and orthorhombic forms with the mean structural formula given below (Mg2.22Al1.00Ti0.04Fe0.77Cr0.01Ni0.02)(Si7.68Al0.32)O20(OH)2(OH2)4Cao0.07Na0.05K0.10.4H2O

Nature of Structural Disorder in Natural Kaolinites: A New Model Based on Computer Simulation of Powder Diffraction Data and Electrostatic Energy Calculation

Abstract: A new model for the description of the structural disorder in natural kaolinite materials is proposed, based on the stacking of two 1:1 layers and their enantiomorphs, and encompassing previously proposed models. The layers, where randomly stacked along the c axis (using probabilistic functions nested in recursive algorithms), correctly describe the observed powder diffraction patterns of natural kaolinites with any density of structural faults. The proposed model was evaluated using electrostatic energy calculations against earlier models of disorder based on layer shift, layer rotation, statistical occupancy of the Al octahedra, or enantiomorphic layers. The present 4-layer model has a minimum of potential energy with respect to the previous models. As expected, the fully ordered triclinic structure of kaolinite possesses the absolute minimum of potential energy.

Mossbauer spectroscopic evidence for citrate-bicarbonate-dithionite extraction of maghemite from soils

Abstract: In a previous paper, we used powder X-ray diffraction and changes in magnetic susceptibility to argue the importance of pedogenic maghemite to soils and the efficacy of the chemical extractant citrate-bicarbonate-dithionite (CBD) to preferentially remove pedogenic maghemite from soil samples while not removing coarse-grained magnetite. Although X-ray diffraction provides strong support for this contention, M6ssbauer spectroscopy is the method of choice for determining the oxidation state of iron in minerals and for inferring mineralogy of the iron oxide phases. Our objective in this work was to seek confirming evidence of the importance of maghemite as a pedogenic mineral and the usefulness of the CBD procedure in separating pedogenic maghemite from lithogenic magnetite. We present M6ssbauer data on magnetic fractions from pre- and post-CBD treated soil samples. Six of the l0 samples had only maghemite as the sextet component and after CBD treatment, four lost between 96 and 100% of the magnetic susceptibility. Two samples were interpreted as highly oxidized magnetite or a mixture of magnetite and maghemite. We cannot distinguish between these with M6ssbauer spectroscopy. In the remaining two samples, iron existed as hematite, ilmenite, magnetite and minor (< 10%) amounts of maghemite. Our results provide additional support for pedogenic maghemite in soils and for the pref- erential removal of maghemite by the CBD procedure.

Formation of aluminum hydroxides as influenced by aluminum salts and bases

Abstract: The influence of different ions in the formation of AI(OH)3 polymorphs has been studied experimentally by promoting stoichiometric reactions between the aluminum salts (A1C13, Al(NO3)3, A12(504)3) and bases (NaOH, KOH, NH4OH). In all cases the polymorphs obtained were a mixture of gibbsite, bayerite and nordstrandite or pseudoboehmite with the exception of the reaction between KOH, or NH4OH and A12(504) 3 which produced amorphous gels. Ageing of these gels at ambient temperature and pressure for 180 days or at 60*(2 for 20 days resulted in crystalline structure. Specifically, pseudo- boehmite was crystallized from the reaction between A12(SO4)3 and NH4OH. Significantly, of all ions present in solution in the present experiment, only the sulphate ones were observed to have a marked influence in the precipitation of A1 oxyhydroxides.