Layer Charge Of Smectites Research Articles

New laboratory procedure for distinguishing smectites with varying permanent layer charge based on the combination of Cu-trien and ethylene glycol solvation methods and X-ray diffraction

Permanent layer charge of smectites (or smectite layers in mixed-layer minerals) is important for their properties and materials' applicationss. However, distinguishing low- and high-charge smectites by traditional laboratory methods is time-consuming and instrumentally demanding and it can also produce equivocal results. In addition, the use of structural formulae based on chemical analyses combined with knowledge on crystallochemistry is challenging in arbitrary mineral mixtures. The aim of this work was to test a novel laboratory procedure for distinguishing actual multi-mineral specimens of smectites according to their permanent layer charge. This is conducted on interlayer cation exchange by the Cu-trien complex and subsequent solvation with ethylene glycol. This procedure returned more straightforward results than the traditional routine of ethylene glycol-saturated K+-exchanged specimens by producing better-resolved X-ray diffraction (XRD) patterns. The practical application was demonstrated by analysing sauconite samples from Skorpion in Namibia, which was shown to contain more sauconite phases of distinct crystallographic properties present in variable proportions. XRD of the oriented ethylene glycol-saturated specimens was insufficient for this purpose. The procedure was tested by reference clay minerals of the smectite group with varying structural parameters. It was distinguished more smectite phases with distinct layer charges and estimated the layer charge by combining interlayer cation exchange with the Cu-trien complex and ethylene glycol solvation. This was followed by XRD analyses and full-profile diffraction pattern modeling. Further investigation is required for comparison with other methods. Possible reasons for the formation of two populations of sauconites were discussed.

Adsorption of ammonium ions onto the external surface of smectite: Effects of layer charge, concentration, anion and comparison with interlayer adsorption

The residual and release of ammonium in ion-adsorption rare earth tailings and soil are related to the interaction of NH4+ with clay minerals. Molecular dynamics simulations were used to study the adsorption and diffusion of NH4+ on the external surface of smectite. The factors, such as layer charge position (octahedral and tetrahedral substitution) and charge density (0.5, 1.0 and 1.5 e⋅uc−1, where uc denotes unit cell), initial NH4+ concentration (from 0.64 to 1.60 mol⋅L−1), and anion types (Cl−, NO3− and SO42−) were considered. The results of NH4+ adsorption on the external surface were also compared with that in the interlayer. The layer charge of smectite significantly affected the interaction between NH4+ and smectite. With increasing charge density, the major NH4+ surface complexes were changed from outer-sphere to inner-sphere, resulting in increased adsorption density and stability, which was verified by calculated hydration structure and adsorption free energies. The inner-sphere NH4+ complexes were located above the centers of the hexagonal rings and the Si tetrahedras for montmorillonite at a charge density of 1.0 e⋅uc−1, while only located in the vicinity of tetrahedral substitutions for beidellite. NO3− promoted the adsorption of NH4+ as inner-sphere complexes relative to Cl− and SO42−. As the water content in the interlayer increased, the confinement effect weakened and the mobility of NH4+ increased, and the NH4+ adsorption in the interlayer exhibited similar characteristics to that on the external surface at a water content of 10 H2O⋅uc−1.

In Situ Measurements of the Hydration Behavior of Compacted Milos (SD80) Bentonite by Wet-Cell X-ray Diffraction in an Opalinus Clay Pore Water and a Diluted Cap Rock Brine

Compacted bentonite is currently being considered as a suitable backfill material for sealing underground repositories for radioactive waste as part of a multi-barrier concept. Although showing favorable properties for this purpose (swelling capability, low permeability, and high adsorption capacity), the best choice of material remains unclear. The goal of this study was to examine and compare the hydration behavior of a Milos (Greek) Ca-bentonite sample (SD80) in two types of simulated ground water: (i) Opalinus clay pore water, and (ii) a diluted saline cap rock brine using a confined volume, flow-through reaction cell adapted for in situ monitoring by X-ray diffraction. Based on wet-cell X-ray diffractometry (XRD) and calculations with the software CALCMIX of the smectite d(001) reflection, it was possible to quantify the abundance of water layers (WL) in the interlayer spaces and the amount of non-interlayer water uptake during hydration using the two types of solutions. This was done by varying WL distributions to fit the CALCMIX-simulated XRD model to the observed data. Hydrating SD80 bentonite with Opalinus clay pore water resulted in the formation of a dominant mixture of 3- and 4-WLs. The preservation of ca. 10% 1-WLs and the apparent disappearance of 2-WLs in this hydrated sample are attributed to small quantities of interlayer K (ca. 8% of exchangeable cations). The SD80 bentonite of equivalent packing density that was hydrated in diluted cap rock brine also contained ca. 15% 1-WLs, associated with a slightly higher concentration of interlayer K. However, this sample showed notable suppression of WL thickness with 2- and 3-WLs dominating in the steady-state condition. This effect is to be expected for the higher salt content of the brine but the observed generation of CO2 gas in this experiment, derived from enhanced dissolution of calcite, may have contributed to the suppression of WL thickness. Based on a comparison with all published wet-cell bentonite hydration experiments, the ratio of packing density to the total layer charge of smectite is suggested as a useful proxy for predicting the relative amounts of interlayer and non-interlayer water incorporated during hydration. Such information is important for assessing the subsequent rates of chemical transport through the bentonite barrier.

Mineralogy Class of Indian Cracking Clay Soils (Vertisols and intergrades) in the US Soil Taxonomy: A Critical Appraisal

Soil Taxonomy (USDA) places cracking clay soils (CCS) (Vertisols and vertic intergrades) into the smectitic mineralogy class. Smectite is a group name consisting of montmorillonite, beidellite and nontronite.Numerous studies, especially undertaken at the ICAR-NBSS&LUP, Nagpur over the last few decades, established that the layer charge of smectites in reality reflect that of specimen bentonite (montmorillonite). Therefore, it would be prudent to revise the mineralogy class of the CCS as ‘montmorillonitic’ instead of the present class as ‘smectitic’. The presence ofpalygorskite as one of the other minerals, is quite common in the Deccan Basalt derived alluvial parent materials of the CCS. Despite the problem in identification of small amounts of this mineral by X-ray diffraction analysis, simple method, based on the unique depth distribution of exchangeable Ca, Mg, Na, Ca/Mg, and base saturation > 100, is in place to ascertain its presence. Remarkable reduction in hydraulic properties in the subsoils, observed due to the presence of palygorskite, is expected to impair the productivity of the CCS under rain-fed conditions of the semi-arid climates (SAT). This important edaphological issue can be well highlighted if the mineralogy class of the CCS containing palygorskite is revised as ‘montmorillonitic-palygorskitic’instead of only‘montmorillonitic’. Such revision will helppedologists/soil mappers to guide the SATfarming communities to raise agricultural crops. This critical appraisal is prepared for its acceptance in the US Soil Taxonomy, which is a dynamic and an open-ended system, and has provision to accommodate any new concepts developed over time through concerted global research efforts to make the system more rational, understandable and meaningful for the soils of tropical India.

Tunable High-Pressure Field Operating on a Cationic Biphenyl Derivative Intercalated in Clay Minerals

We propose a methodology for applying a pseudo uniaxial pressure to an organic molecule under ordinary temperature and pressure, namely by intercalation into smectites. The pseudo pressure on a biphenyl derivative (BP) was estimated from the averaged dihedral angle around the central bond of BP. In a high hydrostatic pressure field, biphenyl takes a planar conformation. In the interlayer space of synthetic saponite (SSA), the averaged dihedral angle of BP at a loading level of 27% versus the cation exchange capacity was ~26.3°, which indicates that the pseudo pressure applied to BP in the SSA interlayer space corresponds to 0.99 GPa. The high pseudo-pressure field in the interlayer space of SSA was also confirmed by absorption measurements. The dihedral angle around the central bond of the biphenyl moiety decreased to enhance the planarity of the molecule, mainly in response to the electrostatic force that operates between the negatively charged SSA layer and the interlayer cation. The pseudo pressure operating on BP in the smectite interlayer space could be controlled by varying the smectite layer charge density and/or the BP loading level. By using this methodology, controllable pseudo high-pressure properties of organic molecules can be obtained at ordinary temperatures and pressures.

Tightly bound water in smectites

Abstract Smectites are able to retain molecular tightly bound water (TBW) at temperatures above 100 °C, even after prolonged drying. The presence of TBW affects the stable isotope ratios, the dehydroxylation behavior of smectites and smectite-rich samples and also has implications in measuring various properties of clay-rich rocks. Five reference smectites, in Mg-, Ca-, Na-, and Cs-exchanged forms were subjected to different drying protocols followed by the determination of TBW contents using precise thermogravimetric (TG) analysis. Activation energies (Ea) of the removal of different water fractions at temperatures up to 1000 °C were determined in non-isothermal TG experiments using model-independent methods. Additionally, 4A and 13X zeolites were examined in both cases as apparent OH-free references. After drying at 110 °C, all smectites still contained up to 3 water molecules per interlayer cation. The TBW contents in smectites were found to be primarily dependent on the isothermal drying temperature. For a given temperature, TBW contents decreased with respect to the type of interlayer cation in the following order: Mg > Ca > Na > Cs. The influence of the time of drying and the smectite layer charge were found to be negligible. The Ea of dehydration below 100 °C, as determined by the Friedman method, was quite constant within the 45–60 kJ/mol range. The Ea of TBW removal increased along with the degree of reaction from 90 to 180 kJ/mol, while the Ea of dehydroxylation was found in the 159–249 kJ/mol range, highly depending on the sample’s octahedral sheet structure and the interlayer cation. The Mg2+ cation can hold H2O molecules even beyond 550 °C, making it available during dehydroxylation or—for geologic-scale reactions—pass H2O to metamorphic conditions. High similarities between the TBW contents and the Ea of dehydration for smectites and cationic (low Si/Al-) zeolites lead to the conclusion that TBW in smectites is remarkably similar to zeolitic water in terms of cation bonding and diffusion characteristics. The optimal drying protocol for smectites is to substitute interlayer cations with cations of a low-hydration enthalpy, such as Cs, and to dry a sample at 300 °C, provided that the sample is Fe-poor. Fe-rich smectites should be dried at 200 °C to avoid dehydroxylation that occurs below 300 °C.

Ethylene glycol intercalation in smectites. Molecular dynamics simulation studies

Molecular dynamics simulations were performed in order to study the interactions of ethylene glycol (EG) with smectite. The simulations have also taken into account that EG–smectite complex contains, as a rule, some adsorbed water molecules. The simulations results show that in the two-layer glycolate the content of water is about 1.0 H2O per half of the smectite unit cell. For a typical smectite a clear thermodynamic preference for one- or two-layer structure of the complexes was observed. The calculated radial distribution functions and running coordination numbers indicate that the H2O and EG molecules compete for the coordination sites near the calcium ions in the clay interlayer spaces. The EG and H2O packing in the interlayer space is controlled by the differences in the total smectite layer charge, charge distribution, and the type of the interlayer cation, strongly affecting the basal spacing and the structure of the complex. Varying amounts of EG and water and the ratio EG/H2O are, however, the most important factors influencing the extent of the smectite expansion.A comparison of the two-layer structure obtained from MD simulations with previous models leads to the conclusion that the arrangement of EG molecules in the interlayer spaces, typically used in simulations of clay mineral X-ray diffractograms, can be modified. In contrast to the earlier Reynolds model (1965), the main difference is that the interlayer ions tend to change their positions depending on the specific distribution of the clay mineral charge. In the case of montmorillonite, Ca2+ ions are located in the middle of the interlayer space, while for beidellite they are located much closer to the clay mineral surface. Water molecules in this structure do not form distinct layers but are instead spread out with a tendency to be concentrated closer to the interlayer ions and to the smectite surface. One-layer structure of EG/water–smectite complex, characteristic of vermiculite is also proposed.

Rheological properties of palygorskite–bentonite and sepiolite–bentonite mixed clay suspensions

This study examines the influence of layer charge and smectite content on the rheological properties of mixed bentonite–palygorskite suspensions. Eight bentonite samples containing smectite with layer charge ranging between 0.38 and 0.54e/huc were used to prepare 5% w/v mixed palygorskite–bentonite aqueous suspension at pH7. The influence of smectite on the apparent viscosity and yield point of palygorskite suspensions depends on the proportion of the smectite in the mixture and the layer charge and charge localization of the smectite. The interaction between the two end member clay minerals is maximum at low percentage (10 to 20%) of bentonite in the mixture. Low charge and high charge smectite tended to improve and deteriorate, respectively, the yield point and apparent viscosity of the suspension. This behaviour seemed to be related to the influence of layer charge of smectite on the degree of delamination and the thickness of smectite quasicrystals, which consequently determine the number of particles involved in the interaction and the strength of particle–particle linkages.

The Charge of Component Layers of Illite-Smectite in Bentonites and the Nature of End-Member Illite

AbstractThe nature of component layers of mixed-layer illite-smectite and their possible evolution in the course of illitization have been debated since the 1960s. The present study is a new attempt to solve these problems, using samples collected from diverse geological formations around the world. Twenty three purified illite-smectites from bentonites and hydrothermal rocks, covering the complete range of expandability, were analyzed chemically, including NH4+\$\end{document} determination, and their structural formulae were calculated. The exchangeable cations (EXCH) were plotted vs. the fixed cations (FIX) yielding the following experimental regression: EXCH=−0.43×FIX+0.41(R2=0.98)\$\end{document}FIX and EXCH depend on the charge of the illite (Qi) and smectite (Qs) interlayers, and the fractions of these interlayers in the bulk clay leading to: EXCH=−Qs/Qi×FIX+Qs\$\end{document}Analysis of these relations and independent measurements of the total specific surface area (TSSA) indicate that the layer charges of both types do not change in the course of illitization. The smectite layer charge is equal to 0.41 and the illite layer charge is equal to 0.95 per O10(OH)2. End-member illite has a well defined composition that is close to intermediate between muscovite and phengite, with fixed cations content greater than that specified in the AIPEA classification of layer silicates: FIX0.95(Si3.25Al0.75)(Al1.81Fe0.01Mg0.19)O10(OH)2\$\end{document}The established relationship allows the calculation of the mean number (N) of 2:1 layers in all fundamental particles and also the fraction of smectitic layers (fs) from FIX: N=Qi/(Qi−FIX)fs=(Qi−FIX)/Qi\$\end{document}N and fs can be used to calculate TSSA, and all three parameters can also be calculated from cation exchange capacity and from X-ray diffraction peak positions, utilizing the regressions established here.

Interaction of montmorillonite with phenothiazine dyes and pyronin in aqueous dispersions: A visible spectroscopy study

Abstract Layer charge is one of the key parameters used for the characterisation of expandable clay minerals, smectites. It determines most significant properties of the material which are important from the industrial application point of view. This work is related to a novel method introduced to characterize the layer charge of smectites, based on using cationic organic dyes as molecular sensors. One xanthene and four phenothiazine cationic dyes were tested using reduced charge montmorillonites (RCMs) and compared with methylene blue, which has been used most frequently. The characterization of the charge was based on the formation of molecular assemblies (H- and J-aggregates) composed by dye cations, which were easily detectable using absorption spectroscopy in the UV/VIS spectrum. More detailed characterization of the spectra required calculations of second-derivative curves. For all of the reaction systems tested in this work, the molecular aggregation increased with the layer charge of RCMs. Slight to moderate differences in the formation of dye assemblies related to the differences in the molecular structures of the individual dye cations. For example, the molecular asymmetry of azure A brought about the formation of coexistent species of similar structures. The structure of the heteroaromatic skeleton affected the extent of the aggregation and spectral changes with time. The presence of reactive, non-substituted amino groups in thionine cations probably partially decomposed in the clay mineral colloids based on high-charge RCMs. Any of the tested dyes could be used as molecular sensors for empirical characterization of the layer charge of clays taking into account the differences mentioned above.

Surface properties of illite-smectite minerals as detected by interactions with rhodamine 6G dye

AbstractInteractions between smectite clay minerals and various organic dyes have been studied extensively, but little information has accumulated from dye interactions with mixed-layer illite-smectite (I-S) minerals, especially regarding relationships with clay layer expandability, layer charge, particle size/shape, and molecular aggregation of organic dye molecules. The purpose of this study was to investigate the surface interactions of a set of mixed-layer illite-smectites from different geological environments with Rhodamine 6G dye. The samples used have different amounts of expandable smectite interlayers, different particle size and/or shape, and different layer-charge density at the surface. Five smectites with differences in layer charge and some non-expandable layer silicates were also tested. The interactions detected by UV-vis spectroscopy show no reaction between R6G and non-expandable minerals (kaolinite, mica), and intense reactions forming H-aggregates and monomers with smectites and illite-smectites. The intensity of H-aggregate formation increases with increase in the layer charge of smectites. Mixed-layer illite-smectites interact with R6G more intensely than do smectites. H-aggregate and monomer formation increases with the illitization process for randomly ordered illite-smectites (R = 0) and decreases in the course of illitization for the ordered illite-smectites (R > 0).

Do bentonites have contradictory characteristics? An attempt to answer unanswered questions

AbstractThe formation of bentonites via alteration of volcanic glassy rocks is associated with apparently contradictory mineralogical, geological and geochemical characteristics, which remain poorly understood. This contribution attempts to answer some of the questions based on the work carried out in our laboratory over the past 15 y. The first question concerns the formation of opal-CT-free bentonites from acidic precursors. It is suggested that the formation of opal-CT-free bentonites is closely related to the mechanism of eruption and emplacement of parent volcanic rocks if the temperature of the parent glass after emplacement is high and/or the establishment of hydraulic gradients coupled with high permeability after deposition of volcanic glass (i.e. diagenetic alteration) if the temperature of the parent glass is low. The second question pertains to the behaviour of Fe during bentonite formation. Based on mineralogical and geological characteristics of different bentonite deposits, it is suggested that Fe is present mainly as Fe2+ and hence may be mobilized during alteration. Oxidation of Fe may take place when bentonites are exposed to more oxidizing conditions close to the surface. Assignment of Fe as Fe2+ increases the layer charge of smectites and decreases significantly the range of layer charge. Thus the smectites may have considerably different properties when formed compared with their counterparts on the surface. The third question considers the possible influence of parent-rock chemistry on the crystal chemistry of smectites. It is proposed that the parent rock may affect the chemistry of the neoformed smectites to some degree; therefore the smectite chemistry may provide indications about the geochemical affinities of the parent rock.

Surface area and layer charge of smectite from CEC and EGME/H2O-retention measurements

AbstractThe total specific surface area (TSSA) and smectitic layer charge (Qs) calculated from the structural formulae and unit-cell dimensions of 12 pure smectite samples were used as a reference in the design and evaluation of TSSA and Qs measurement techniques based on cation exchange capacity (CEC), H2O retention at 47% RH, and ethylene glycol monoethyl ether (EGME) retention. A thermogravimetric analysis-mass spectrometry (TGA-MS) technique was used to study the release of H2O from smectite on heating, and to introduce a correction for H2O remaining in the smectite after heating to 110°C, because the sample weight at this temperature has been used routinely as a reference in CEC and EGME sorption measurements. A temperature of 200°C was found to be the optimum reference for such measurements.A good agreement between Qs from the structural formula and from CEC was obtained when this correction was applied. The TSSA of smectite was measured with similar accuracy (mean error of ±5–7%) by three techniques: (1) using mean H2O coverage; (2) using mean EGME coverage; and (3) using a combination of H2O coverage and CEC. A reduction of the mean error from 5–7% to 4% can be obtained by averaging these measurements, and a further reduction to 3% by introducing corrections for the dependence of H2O and EGME coverage on layer charge. The study demonstrates that Ca2+-smectite samples at 47% RH have H2O contents corresponding to 88–107% of the theoretical mass of a monolayer and offers an explanation of this variation.

Traditional and novel methods for estimating the layer charge of smectites

Several methods for characterizing layer charge of smectites were compared. Ten samples of various Na +- and Li +-saturated smectites (four montmorillonites, one nontronite, two saponites and three hectorites) were used. The smectites included minerals isolated from bentonites, as well as some synthetic materials. Cation exchange capacities (CECs) were determined as a reference parameter. These data express the total charge of cations that balance the negative charge sites of smectites. Four CEC methods were used, based on ion exchange reactions with solutions of barium chloride, ammonium acetate, methylene blue and a chelate complex of Cu(II). The layer charge distribution in the octahedral and tetrahedral sheets was estimated from structural formulae that were calculated from chemical analyses. Charge calculation from a structural formula is sensitive to mineral impurities and depends on how the formula is calculated. Moreover, only trace deviations from an ideal structure, such as an inhomogeneous octahedral sheet may significantly affect charge values calculated from structural formulae. Layer charge distribution and average charge densities were determined using the alkylammonium method. After intercalation of n-alkylammonium ions, the interlayer distance depends on the length of alkyl chains and the layer charge of the mineral. The charge distributions of all smectite samples were heterogeneous. The layer charge determined by the alkylammonium method reflects the permanent charge. The estimated CECs calculated from alkylammonium layer charge values were systematically lower than measured CEC values, which include contributions from both the permanent and variable charge sites and some factors related to the alkylammonium method itself. The layer charge distribution was qualitatively estimated using the visible spectra of methylene blue/smectite suspensions. Trends in the spectra indicate that dye spectral properties that reflect molecular aggregation can potentially be used to qualitatively estimate or compare the layer charges of smectites. All the methods include some advantages and disadvantages and differ in their requirements for sample type, purity and composition, time and expenses needed for the analysis. In general, method selection depends on the specific application and whether the need is to characterize layer charge, charge density, charge distribution, or for comparing the charge of similar samples or samples after a chemical treatment or modification.

Layer charge of smectites: linking crystal structure with physical properties

Layer charge of smectites: linking crystal structure with physical properties

Structural Transformations of Clay Minerals in Soils of a Climosequence in an Italian Alpine Environment

AbstractClays of a soil sequence with five profiles in the Val Genova (northern Italy) along an elevation gradient with climate ranging from moderate to Alpine were investigated with XRD using several diagnostic treatments. Smectites developed in the surface horizons of podzolic soils either from chlorite through the removal of hydroxy interlayers or from mica, which weathers in a first step to regularly or irregularly interstratified clay minerals. Citrate treatment allowed the detection of low-charge expandable minerals in the Bhs or Bs horizons. Therefore, the reduction of the charge of 2:1 clay minerals occurred before the removal of hydroxy polymers by fulvic acids and low-weight organic acids. Due to the more intense podzolization process near the tree line, the d060 region showed a temporal evolution of trioctahedral to dioctahedral mineral structures in the well developed Podzols. The pedogenic smectites of the E or Bhs horizons generally included one or several populations with various charges. In most cases, smectite was a heterogeneous mixture of montmorillonite and interstratified beidellite-montmorillonite. A pure beidellite phase could not be detected. The soils near the tree line, where weathering processes were most intense, had two main components: one with a charge >0.75, representing vermiculite-like minerals, and the other with a charge near 0.25, representing smectite. The charges of the beidellitic component and montmorillonite were almost equal. The higher the weathering state of the investigated soils, the lower was the layer charge of smectites.

Visible Spectroscopy of Cationic Dyes in Dispersions with Reduced-Charge Montmorillonites

AbstractThe aggregation behavior of one azine and three triphenylmethane cationic dyes in dispersions of reduced-charge montmorillonites (RCMs) was investigated. The extent and types of dye aggregation were monitored using visible spectroscopy. Similar relationships between dye aggregation and layer charge were observed, independent of the structure of the dye cations. High charge-density surfaces induced the formation of H-aggregates, with a face-to-face association between the dye cations that then absorb light at relatively lower wavelengths. Moderate reductions in layer charge were reflected in the lowering of the H-aggregation in favor of monomers, dimers and less densely-packed J-aggregates, absorbing light at higher wavelengths. Dye spectra in the presence of the lowest-charge RCMs resembled those of dilute dye solutions, indicating the absence of dye aggregation of any type in this case. The relationship between dye spectral changes and layer-charge densities of smectites is a general phenomenon which can potentially be used to estimate the layer charge of smectites. However, applying this method to triphenylmethane dyes, which have structurally more complicated cations, may reduce the sensitivity of the probe.

Layer charge characteristics of smectites from Vertosols (Vertisols) of New South Wales

Premature senescence in cotton has been attributed to K deficiency in the cotton soils of Australia. The availability, release, and fixation of K+ in soils are mainly dependent on the clay mineralogy and layer charge characteristics of 2 : 1 clay minerals. There is a little information on the mineralogy and charge characteristics of the cotton growing soils (Vertosols) of Australia. The aims of this study were to determine the clay mineralogy, the layer charge density, and layer charge distribution of some cotton growing soils by chemical and X-ray diffraction methods.Most soil clays contain abundant smectite associated with small amounts of mica, kaolinite, and an interstratified mineral. The total layer charge as determined by the alkylammonium method ranged between 0.55 and 0.67 mol(–)/(O10(OH)2), indicating a high interlayer charge density. The layer charge of smectites from different valleys and for different size fractions was similar. The Greene-Kelly test showed that most of the charge originated in the tetrahedral sheet. The chemical analysis indicates that the smectite is an iron-rich beidellite, which has possibly formed from the weathering of mica.

Reaction Mechanisms of Smectite Illitization Associated with Hydrothermal Alteration from Ponza Island, Italy

AbstractA hydrothermally altered rhyolitic hyaloclastite from Ponza island, Italy, has four alteration zones with unique clay assemblages: (1) a non-pervasive argillic zone characterized by smectite; (2) a propylitic zone with interstratified illite-smectite (I-S) containing 10–85% illite (I); (3) a silicic zone composed of I-S with ≥90% I and pure illite; and (4) a sericitic zone with I-S ranging from 66% I to pure illite. Atomic force microscopy reveals abrupt changes in particle morphology with illitization, including initial changes from anhedral plates to laths and then to euhedral plates and hexagonal plates. I-S particles progressively thicken with illitization and mean particle area (basal plane) remains constant from pure smectite to I-S with 80% I. However, particle area increases from 90 to 100% illite. Computer modeling of I-S structural forms indicates octahedral cation ordering progressively changes from cis- vacant smectite to interstratified cis- and tnuis-vacant I-S, and then to trans-vacant illite. In addition, polytypes progressively change from 1 Md to 1M, and then to 2M, illite. Electron-microprobe and X-ray fluoresence analyses show that I-S chemistry progressively changes during illitization, evolving toward a phengitic composition with ∼0.89 fixed interlayer K+ per O10(OH)2. Octahedral Mg2+ shows little change with illitization, varying from 0.3 to 0.5 cations per O,10(OH)2. The layer charge of smectite is ~0.38 equivalents per O10(OH)2.On the basis of abrupt changes in morphology and progressive changes in polytype and chemistry, smectite illitization on Ponza involved a dissolution and recrystallization mechanism with multiple stages of nucléation and crystal growth. In this multi-step model, temperature of alteration provided the major control for the layer composition, polytype, and morphology of I-S crystallites. Other factors that may play a secondary role include: K+ availability, water-rock ratio, and permeability. Alternatively, the mechanism of I-S and illite formation at Ponza and other hydrothermal environments may occur by direct precipitation of I-S crystallites from rhyolite glass and may not involve progressive reactions of smectite precursors.

Layer charge and electrophoretic mobility of smectites

The aim of this study is to determine the effect of the layer charge of smectites on their electrophoretic mobility (EM), using electrophoresis measurements. In order to cover the charge domain from 0 to 2 charges per unit structural cell, two clay series were used: thermally treated Cu-montmorillonite (0–0.7) and synthetic saponites (0.7–2). All the studied samples are negatively charged in a pH range from 2 to 12. Neither layer charge nor ionic strength influences the EM of smectites at neutral to alkaline pH. At acidic pH, the EM of smectites ranges from −3 to −0.4 10 −8 m 2 s −1 V −1 when the layer charge ranges from 0.8 to 2. High charge and low charge smectites, which are not expandable, tend to aggregate in aqueous suspension, and the measured EM corresponds then to the amphoteric sites of the edge faces of the layer stacks. These sites are negatively charged at pH values above 3.