Octahedral Charge Research Articles

Illitization of montmorillonite in ammonium solutions under hydrothermal conditions

In diagenetic solutions, ammonium may be incorporated into smectites as an exchangeable cation and become fixed within the interlayer space of illites and other white micas. To study the potential impact of NH4+ on the smectite-to-illite transformation reaction, a series of hydrothermal experiments were carried out at 100, 150 and 200 °C, spanning reaction duration from 15 to 90 days, and two NH4+ concentrations (0.1 and 0.2 M). The solids resulting from these alteration experiments were characterized using X-ray diffraction (XRD), Fourier-transform infrared spectroscopy (FT-IR), and transmission and analytical electron microscopy (TEM and AEM). The XRD analysis revealed that, under the specified experimental conditions, smectite incorporates NH4+ in the structure, leading to the formation of non-swelling layers, resulting in partial transformation to illite layers and producing packets of disordered illite/smectite (I/S). In addition, a minor XRD peak at ∼10 Å suggests the formation of discrete illite crystals. The FTIR spectra demonstrated the uptake of NH4+, with deformation bands observed at 1400 and 1430 cm−1, corresponding to exchangeable NH4+ in smectite and fixed NH4+ in high-charge layers, respectively. TEM analysis revealed that smectite particles exhibited wavy stacks comprising a few layers with abundant defects and lateral discontinuities. The interlayer spacing in these particles ranged from 12 to 15 Å and became thinner and more plate-like with increasing temperature and time. Moreover, they contained inclusions of 10–10.3 Å layers, either as discrete layers or in packets of several layers, indicating the formation of disordered mixed-layer illite-smectite. Lateral transitions from 12 to 15 Å to 10 Å layers were frequently observed and interpreted as reaction fronts due to local rearrangement. At 150 and 200 °C, isolated packets of 10 Å layers were identified as discrete illite crystals that precipitated directly from solution. Analysis of the chemical composition of individual particles revealed an increase in octahedral charge (MgVI for AlVI substitution) in smectite particles, followed by increase in tetrahedral charge in I/S particles. Interlayer NH4+ played a stabilizing role in the high-charge layers and favored the smectite-to-illite conversion process.

Adsorption of Capsaicin into the Nanoconfined Interlayer Space of Montmorillonite by DFT Calculations

Capsaicin is the main compound responsible of the hot sense of the chili fruits. This compound has interesting therapeutic properties including anticancer, anti-inflammatory effects, and analgesic. However, its use has several secondary effects, such as skin irritation and allergies. Then, new therapeutic strategies are searched in order to overcome these problems. Montmorillonite has proved to be an excellent excipient for the release of pharmaceutical drugs. In this work, the molecular structure and crystal structure of capsaicin, and the adsorption of this molecule into the interlayer space of montmorillonite have been studied using quantum mechanical calculations based on Density Functional Theory (DFT) level of theory and molecular dynamics simulations. The crystal structure has been predicted with these calculations and the intermolecular interactions have been determined with a higher resolution than the previous experimental data. The adsorption of capsaicin into the confined interlayer space of montmorillonite is energetically favourable with low and high octahedral charge. This adsorption can be monitored by IR spectroscopy observing frequency shifts in some bands during the adsorption. This enhances the use of these clay minerals for capsaicin therapeutic formulations.

New insights into the nature of glauconite

AbstractGlauconite must be assessed as mica-rich mica-smectite R3 interstratified mineral, with the pure end-member mica also having intrinsic K-deficient chemical characteristics (K+ ~ 0.8 apfu). This assertion is in accordance with our X-ray diffraction (XRD) and high-resolution tranmission electron microscopy (HRTEM) studies and chemical analyses by electron probe microanalysis (EPMA) of mature glauconites in Cenozoic Antarctic sediments that indicate that: (1) It consists of a glauconite-smectite (R3 ordered) mixed-layer silicate, composed mainly of mica-type layers (>90%), but displaying slightly different proportions of Fe(III)-smectite layers (<10%). (2) More mature glaucony grains are characterized by major K+ and VIFe2+ (mica layers) and minor VIFe3+ (smectite layers) content in the interstratified glauconite-smectite. (3) Potassium is stabilized at the interlayer site by the octahedrally coordinated Fe2+. (4) Microtexture of the glauconite crystals are comparable with those of other micas and illite minerals, with straight, defect-free lattice fringes of ~10 Å spacings glauconite packets characteristic of mica with minor interstratified poorly crystalline smectite layers. In addition, our new findings give insights into the glauconitization process and at the same time investigate the potassium-deficient character of the dioctahedral mica “glauconite.” These findings show that glauconite crystallizes by a layer-growth mechanism at the expense of a poorly crystalline smectite precursor and that smectiteto-glauconite transformations are accompanied by a gradually higher octahedral charge deficiency (Fe2+/Fe3+) stabilized by K+ uptake into the interlayer sheet.

Effects of charge density on the hydration of siloxane surface of montmorillonite: A molecular dynamics simulation study

Abstract The physicochemical properties of clay minerals strongly depend on their hydration characteristics which therefore have drawn great concerns from different research communities. In the present work, the effects of charge density of montmorillonite (Mt) on the hydration characteristics of its interlayer spaces, particularly the siloxane surface, were studied using classical molecular dynamics (MD) simulations. Four Mt. models with various octahedral charges are established, and these charges are compensated with tetramethylammonium cation (TMA). The simulation results showed that water molecules within the hydration layer of siloxane surface will donate hydrogen atoms to form H-bond with the surface oxygen atoms, while those surrounding TMA only slightly have their oxygen atoms point towards TMA. In addition, water molecules prefer to first hydrate the siloxane surface and then the TMA as the water content increases. These findings indicate that water molecules have stronger hydration interaction with siloxane surface than with TMA, and TMA can be ideal counterbalance cation in terms of studying the hydration characteristics of siloxane surface. Charge density can significantly influence the hydration of TMA-Mt. Although increasing charge density will not lead to the formation of stronger H-bond (i.e., no obvious reduction of H-bond length) between water molecules and siloxane surface, water molecules are more likely to be drawn to the siloxane surface and form more H-bonds between them. Subsequently, the hydration energy increases and the mobility of water molecules decreases as the charge density rises. These findings show that charge density can evidently influence the hydrophobicity of siloxane surface, which may further influence its interaction with organic species, e.g., the adsorption of organic contaminants.

Influence of layer charge origin and layer charge density of smectites on their aflatoxin adsorption

The aflatoxin adsorption efficiency of a bentonite depends strongly on the physical, chemical, and mineralogical properties of the smectite, the dominant clay mineral in bentonites. The objective of this study was to determine the effects of the layer charge origin, octahedral cation composition, and layer charge density on the smectites' selectivity and adsorption capacity for aflatoxin.Six smectite samples with different layer charge densities and octahedral cation compositions were evaluated: a montmorillonite (4TX) and a hectorite, which represent smectites with octahedral charge origin; and a beidellite, two saponites from Australia and Spain, and a nontronite, which represent smectites with tetrahedral charge origin. The clay fraction of the smectites was saturated with Na, Ca, and Ba, respectively. Aflatoxin adsorption isotherms were conducted for these smectites. The CEC of one high-charge montmorillonite (5OK) and the nontronite were reduced by Li-saturation and heating.Similar effect of the exchangeable cations was observed in all smectites, but significant differences in the aflatoxin adsorption capacity were observed among the smectites. The octahedrally charged smectites showed higher aflatoxin adsorption than the tetrahedrally charged smectites. The aflatoxin-smectite bonding mechanism was not affected by the layer charge origin or density. Optimal layer charge density was required to have sufficient non-polar sites on the smectite for aflatoxin molecules. High-charge-density smectites had low adsorption capacity and binding affinity for aflatoxin, but aflatoxin adsorption on these smectites can be improved by reducing the layer charge density of the clay mineral. Smectites with octahedral charge increased the interlayer accessibility of aflatoxin molecules. The octahedral cation composition had a negligible effect on the aflatoxin adsorption capacity of smectites.

Influence of pH, layer charge location and crystal thickness distribution on U(VI) sorption onto heterogeneous dioctahedral smectite

The UO22+ adsorption on smectite (samples BA1, PS2 and PS3) with a heterogeneous structure was investigated at pH 4 (I=0.02M) and pH 6 (I=0.2M) in batch experiments, with the aim to evaluate the influence of pH, layer charge location and crystal thickness distribution. Mean crystal thickness distribution of smectite crystallite used in sorption experiments range from 4.8nm (sample PS2), to 5.1nm (sample PS3) and, to 7.4nm (sample BA1). Smaller crystallites have higher total surface area and sorption capacity. Octahedral charge location favor higher sorption capacity. The sorption isotherms of Freundlich, Langmuir and SIPS were used to model the sorption experiments. The surface complexation and cation exchange reactions were modeled using PHREEQC-code to describe the UO22+ sorption on smectite. The amount of UO22+ adsorbed on smectite samples decreased significantly at pH 6 and higher ionic strength, where the sorption mechanism was restricted to the edge sites of smectite. Two binding energy components at 380.8±0.3 and 382.2±0.3eV, assigned to hydrated UO22+ adsorbed by cation exchange and by inner-sphere complexation on the external sites at pH 4, were identified after the U4f7/2 peak deconvolution by X-photoelectron spectroscopy. Also, two new binding energy components at 380.3±0.3 and 381.8±0.3eV assigned to AlOUO2+ and SiOUO2+ surface species were observed at pH 6.

Influence of hydration on23Na,27Al, and29Si MAS-NMR spectra of sodium saponites and sodium micas

Synthetic sodium saponites, Na x Mg 3 (Si 4-x Al x )O 10 (OH) 2 ·nH 2 O, with 0.33 ≤ x ≤ 1, and trioctahedral sodium micas series, Na(Mg 3-y Al y )(Si 3-y Al 1+y )O 10 (OH) 2 ·nH 2 O, with 0 ≤ y ≤ 1, have been investigated by MAS-NMR spectroscopy. The presence of anhydrous, one-layer and two-layer hydrates, deduced by X‑ray diffraction, has been associated with specific lines detected in 23 Na MAS-NMR spectra. In these phyllosilicates, the location of tetra- and octahedral charge has been analyzed by 27 Al MAS-NMR spectroscopy. The salient result is the major effect of the interlayer charge on 29 Si chemical shift of the four NMR components ascribed to Si 3 , Si 2 Al, SiAl 2 , and Al 3 environments. This effect is much more important than the most commonly accepted contribution of the ditrigonal distortion of tetrahedral sheets. In saponites, 29 Si MAS-NMR spectra change considerably with the sodium hydration. In dehydrated samples, where Na cations are engaged in two pseudo-hexagonal cavities, 29 Si MAS-NMR components split as a consequence of the partial sodium occupancy of three neighboring hexagonal rings that surrounds a particular tetrahedron. In hydrated samples, where Na + cations interact with water, chemical shifts of resolved components are averaged as a consequence of interlayer water and cation mobilities.

Ion exchange reactions of major inorganic cations (H+, Na+, Ca2+, Mg2+ and K+) on beidellite: Experimental results and new thermodynamic database. Toward a better prediction of contaminant mobility in natural environments

To our knowledge, no thermodynamic database is available in the literature concerning ion-exchange reactions occurring in low-charge smectite with tetrahedral charge (beidellite). The lack of this information makes it difficult to predict the mobility of contaminants in environments where beidellite and major cations, which act as competitors with contaminants for sorption on the clay phase, are present. The present study proposes a multi-site ion exchange model able to describe experimental data obtained for H+ and the four major cations (Na+, Ca2+, Mg2+ and K+) found in natural waters interacting with a <0.3μm size fraction of Na-beidellite. The nature of the sites involved in the sorption processes is assessed using qualitative structural data. Moreover, the effect of the charge location in the smectite on the selectivity coefficient values is discussed by comparison with the results reported in the literature for smectite characterized by octahedral charge (montmorillonite). The new thermodynamic database proposed in this study is based on the same total sorption site density and distribution of sites regardless of the cations investigated. This database is valid for a large range of physico-chemical conditions: a [1–7] pH range, a total normality higher than 5×10−3mol/L corresponding to a flocculated state for water/clay systems, and when sorption of ions pairs can be neglected. Note that this study provides evidence that a thermodynamic database describing ion exchange reactions between H+ and the four major cations of natural water for smectite cannot be valid irrespective of the total normality of the aqueous phase because smectite can form different phases (e.g., floc, gel) when immersed in aqueous system. The model proposed can be easily included in reactive transport software and is applied in the present study to predict the evolution of beidellite surface composition toward major inorganic cations in the context of mining site remediation. Finally, the new thermodynamic database proposed could be used for a better understanding of ion-exchange reactions occurring in many environments where beidellite is present, from soils to smectite-bearing sedimentary formations.

Physico - Chemical and Mineralogical Characterization of a Moroccan Bentonite (Azzouzet) and Determination of its Nature and its Chemical Structure

This study concerns the results of Physicochemical and mineralogical characterization of a Moroccan bentonite (bentonite Azzouzet) and determination of its nature and its chemical structure. Several techniques were used; in particular X-ray diffraction (XRD), scanning electron microscopy coupled with EDX microanalysis (SEM-EDX), differential thermal and gravimetric analyses (DTA-TGA) and finally infrared Fourier transform (FTIR) and X-ray fluorescence (XRF). The bentonite fine fraction (< 2 µm in size), was characterized by BET method for specific surface area, and a cation- exchange capacity.The X-ray diffraction analysis supplemented by gravimetric analysis showed that Azzouzet bentonite is primarily (smectite) (49 wt %) with the presence of feldspar (34 wt %), Quartz (5 wt %), magnetite (7 wt %) and amorphous iron oxide (3 wt %). It has a cation- exchange capacity of 73,3 meq/100g and a specific surface area by using surface area analyser and N2 gaz was used as adsorbate under atmosphere of 31 m²/g.the structural formula calculated for bentonite has shown that the montmorillonite has a high charge density. SEM-EDX, X-ray, chemical analysis and Infrared spectroscopy demonstrated that this montmorillonite contains iron Fe3 + in the octahedral sheet. The origin of the charge sheets is tetraedral (Si4) (Al2.12 Mg0.15), with the montmorillonite character of the bentonite (15%) is present (less than 50% octahedral charge) with a phase rich in iron.

Structural Characterization of Reduced-Charge Montmorillonites. Evidence Based on FTIR Spectroscopy, Thermal Behavior, and Layer-Charge Systematics

AbstractIn the present study, the gradual layer-charge reduction of two Li-saturated smectites, SAz-1 from Arizona, USA, and FEO-G from Troodos, Cyprus, with octahedral charge of 0.54 electrons per half unit cell (e/huc) and 0.39 e/huc, respectively, was monitored by X-ray diffraction of K-saturated, ethylene glycol-solvated samples, by thermogravimetry-differential thermogravimetry, and by mid- and near-Fourier transform infrared spectroscopy after heating at 80–300ºC. With increasing heating temperature, the layer charge and cation exchange capacity (CEC) of both smectites decreased gradually due to Li fixation. At temperatures &gt;200ºC, ~25% residual CEC was observed, suggesting incomplete Li fixation due to kinetic constraints. Dehydration of the original Li-smectites occurred in two steps, one peaking at ~100ºC and another at 175–180ºC. The latter decreased upon progressive Li fixation and vanished from smectites treated above ~125ºC. Dehydroxylation occurred at 635–640ºC in both smectites and was not affected by Li fixation. The second derivative analysis of the infrared spectra showed that Li fixation was manifested in both smectites by the growth of two new sharp OH-stretching fundamentals at ~3640 and 3670 cm−1 and their overtones at ~7115 and 7170 cm−1. The new bands constitute pairs of fixed energy and relative intensity which grow simultaneously at the expense of the broad OH-stretching and overtone features of the original smectites. Based on this result, Li fixation is suggested to be accompanied by the simultaneous formation of two distinct trioctahedral-like structural OH species, which is compatible with Li+ occupying trans-octahedral vacancies in both smectites.

Hydration of methane intercalated in Na-smectites with distinct layer charge: Insights from molecular simulations

Molecular dynamic simulations have been carried out to investigate the behavior of methane hydration in Na-smectite interlayers with different layer-charge distributions and water contents under certain pressure–temperature (P–T) conditions, which is analogous to the methane hydrate-bearing marine sediments. It was found that sufficient interlayer water is necessary for coordinating with methane and forming hydrate-like structures. Methane molecules are solvated by nearly 12–13 water molecules and coordinated with six oxygen atoms from the clay surface in the interlayer of nontronite as well as in montmorillonite. The mobility of the interlayer water of smectite, which is determined by the layer-charge amount and distribution of smectite, also influences the stability of hydrate methane complexes. The tetrahedral negative charge site is closer to the surface than the octahedral charge site and is more effective in confining water than methane water molecules.

Mössbauer characteristics of glauconitisation

57Fe Mossbauer spectroscopy, X-ray diffraction and local (WDX, EDX) and bulk chemical methods (ICP-AES complemented with Fe2 + titration) were used to study glauconites originating from three sandstone samples (NY1, NY3 and EWT) of the Upper Oligocene Eger Formation (Northern Hungary). The, during glauconitisation, increasing aggregate density and magnetic susceptibility offered a unique possibility to separate grains of different evolutionary stage from the same glauconite population. 57Fe Mossbauer spectra of both structurally and chemically well characterised glauconite fractions were decomposed into four doublets. Two-two of the doublets were assigned to octahedral Fe2 + and octahedral Fe3 + . Fe2 + /Fe values determined by Mossbauer measurements were used for the calculation of the chemical formula of glauconite samples [including octahedral occupancy (BO) and octahedral charge (XO) as well]. The gradual increase of Fe2 + /Fe was documented with the increase of aggregate density in correlation with the foregoing of glauconitisation. Our results give a quantitative demonstration that octahedral occupancy (BO) decrease and the substitution of octahedral Al by Fe2 + are the major processes allowing the interlayer K incorporation during glauconitisation. The systematic crystalchemical evolution as reflected in the systematic changes of the Mossbauer data are discussed in terms of the subsequent steps of glauconitisation.

Effects of layer-charge distribution on the thermodynamic and microscopic properties of Cs-smectite

To explore the effects of layer-charge distribution on the thermodynamic and microscopic properties of Cs-smectites, classical molecular dynamic simulations are performed to derive the swelling curves, distributions and mobility of interlayer species, and Cs binding structures. Three representative smectites with distinct layer-charge distributions are used as model clay frameworks and interlayer water content is set within a wide range from 0 to 380 mg water/g clay. All the three smectites swell in a similar way, presenting the characteristic swelling plateaus and similar trends of swelling energetic profiles. The full-monolayer hydrate, corresponding to the global minima of the immersion energy, is the most stable hydrated state of Cs-smectites. The calculated diffusion coefficients of interlayer species disclose the confining effects in all smectites: both water molecules and ions diffuse slower than corresponding bulk cases and they are much more mobile in the direction parallel to the clay surfaces than perpendicular to them. The formed inner-sphere complex structures are very similar in different smectites: ions bind on the H-sites or T-sites and water molecules form cage-like caps covering the ions. Layer-charge distribution is found to have significant influences on the mobility of interlayer species and preference of ion binding sites. A general sequence is proposed to elucidate the preferences of various hexagonal sites (H-sites) and triangular sites (T-sites), that is, tetrahedrally substituted H-sites > nonsubstituted H-sites > tetrahedrally substituted T-sites > nonsubstituted T-sites, but the influence of octahedral substitutions on the preference of the neighboring sites is not obvious. Analysis of mobility indicates that H-sites are more stable Cs-fixation positions than T-sites and smectite with higher fraction of octahedral charges seems to be the most effective barrier material no matter how water content varies although all smectites can immobilize Cs ions in relatively dry conditions. These findings will not only facilitate basic research in geochemistry and material sciences, but also promote the barrier material designing.

Growth of thermophilic and hyperthermophilic Fe(III)-reducing microorganisms on a ferruginous smectite as the sole electron acceptor.

Recent studies have suggested that the structural Fe(III) within phyllosilicate minerals, including smectite and illite, is an important electron acceptor for Fe(III)-reducing microorganisms in sedimentary environments at moderate temperatures. The reduction of structural Fe(III) by thermophiles, however, has not previously been described. A wide range of thermophilic and hyperthermophilic Archaea and Bacteria from marine and freshwater environments that are known to reduce poorly crystalline Fe(III) oxides were tested for their ability to reduce structural (octahedrally coordinated) Fe(III) in smectite (SWa-1) as the sole electron acceptor. Two out of the 10 organisms tested, Geoglobus ahangari and Geothermobacterium ferrireducens, were not able to conserve energy to support growth by reduction of Fe(III) in SWa-1 despite the fact that both organisms were originally isolated with solid-phase Fe(III) as the electron acceptor. The other organisms tested were able to grow on SWa-1 and reduced 6.3 to 15.1% of the Fe(III). This is 20 to 50% less than the reported amounts of Fe(III) reduced in the same smectite (SWa-1) by mesophilic Fe(III) reducers. Two organisms, Geothermobacter ehrlichii and archaeal strain 140, produced copious amounts of an exopolysaccharide material, which may have played an active role in the dissolution of the structural iron in SWa-1 smectite. The reduction of structural Fe(III) in SWa-1 by archaeal strain 140 was studied in detail. Microbial Fe(III) reduction was accompanied by an increase in interlayer and octahedral charges and some incorporation of potassium and magnesium into the smectite structure. However, these changes in the major element chemistry of SWa-1 smectite did not result in the formation of an illite-like structure, as reported for a mesophilic Fe(III) reducer. These results suggest that thermophilic Fe(III)-reducing organisms differ in their ability to reduce and solubilize structural Fe(III) in SWa-1 smectite and that SWa-1 is not easily transformed to illite by these organisms.

Hydrothermal synthesis and characterization of dioctahedral smectites: A montmorillonites series

The aim of this study is to synthesize and finely characterize montmorillonite samples, dioctahedral smectites without tetrahedral charges (structural formulae Nax(Al(2−x)Mgx)Si4O10(OH)2), to allow their use as reference samples in clay science. The montmorillonites synthesis under hydrothermal conditions at different pressures and with various layer charge deficit has been attempted. The temperature was fixed at 320 °C, the pressure parameter values were 20 MPa, 80 MPa, 120 MPa and 200 MPa. The Mg content varied from 0.25 to 0.60 per half unit cell. The reaction products have been characterized with multi-technique analyses (ICP-AES, EMP, CEC, XRD, FTIR, NMR and TGA).Montmorillonite phase was only produced at 120 and 200 MPa.At 20 and 80 MPa, the results suggest that a 0.33 and 0.16-tetrahedral charge deficit exist in the formed samples. Moreover, the octahedral occupancies are higher than two (2.15 and 2.07 at 20 and 80 MPa respectively). In these experimental conditions, the synthetic smectites are mixtures between montmorillonite, beidellite and saponite.At 120 MPa and for a Mg content of 0.25 or higher than 0.33, the synthetic products were also mixtures of smectites. Tetrahedral charge deficits of 0.11, 0.11 and 0.15 were found for Mg contents of 0.25, 0.50 and 0.60 respectively. The octahedral occupancy was also higher than 2.00.A montmorillonite phase with only octahedral charges and an octahedral occupancy near 2.00 was synthesized for a Mg content of 0.33 and at pressures equal to or higher than 120 MPa. This low charge reference smectite shows a very low amount of accessory minerals and an octahedral charge deficit only created by the presence of magnesium in the structure. This montmorillonite can be compared structurally to the most studied natural one: the montmorillonite SWy-2 from Wyoming.

Infrared spectroscopy of NH 4+-bearing and saturated clay minerals: A review of the study of layer charge

Infrared (IR) spectroscopy study of the layer charge of clay minerals demonstrates that the υ 4 NH 4 + band, assigned to the deformation vibrations of the NH 4 + ion can be used to evaluate the charge characteristics of NH 4 +-bearing clay minerals. Total accessible charge, equivalent to cation exchange capacity (CEC), as well as tetrahedral and octahedral charge distribution in both dioctahedral and trioctahedral smectites are determined by quantifying the υ 4 NH 4 + band before and after lithium treatment. When using the KBr pressed pellets technique, the position and the shape of the υ 4 NH 4 + bands can distinguish exchangeable and fixed NH 4 + in clay minerals.

Influence of charge location on 29Si NMR chemical shift of 2:1 phyllosilicates

A series of synthetic sodium-rich saponites and trioctahedral Na, K, and Ba mica solid solutions have been investigated by 29 Si MAS-NMR spectroscopy. The salient result is the major effect of the interlayer charge on 29 Si chemical shift variations detected in NMR components ascribed to tetrahedral Si 3 , Si 2 Al, SiAl 2 , and Al 3 environments. In this analysis, the effect of the octahedral charge is considerably lower. The contribution of the interlayer charge is much more important than the previously reported contribution of the ditrigonal distortion of tetrahedral layers, generated by the misfit between tetrahedral and octahedral layers. This observation rests on the peculiar disposition of tetrahedra in phyllosilicates, with three out of four oxygen atoms exposed to the interlamellar space. A similar effect could be operative in other silicates, like zeolites, in which most of tetrahedra interact with charge compensating cations.

Genesis and compositional heterogeneity of smectites. Part III: Alteration of basic pyroclastic rocks--A case study from the Troodos Ophiolite Complex, Cyprus

Upper Cretaceous basic pyroclastic rocks, which overlie the Upper Pillow Lavas of the Troodos ophiolitic complex, Cyprus have been altered to bentonites. The resulting smectite is Fe-rich montmorillonite and Fe-rich beidellite, with moderate Mg contents. The smectite is trans-vacant and contains abundant exchangeable K. The presence of K is linked with hydrothermal alteration, which affected the higher members of the Troodos ophiolitic suite. The smectite displays significant compositional heterogeneity, which involves substitution of Fe for Al and to a lesser degree substitution of Mg for Al, and that reflects the influence of microenvironmental conditions on smectite formation. The layer charge of the smectite is controlled mainly by the tetrahedral charge, whereas the influence of octahedral charge is of lesser importance, because of Fe for Al substitutions, which does not create a charge deficit. Although the parent pyroclastic rocks were basic, the bentonites contain abundant Sipolymorphs and Si-rich zeolites, from dissolution of abundant radiolarian frustules, which increased the Si-activity of the pore waters, and also produced the partial replacement of smectite by palygorskite at a later stage. Dissolution of frustules was facilitated by the high heat flow from the ocean floor and by the circulation of hydrothermal fluids. The crystal chemistry of smectite and the bulk mineralogy of the bentonites influence the physical properties and industrial applications of the Cyprus bentonites, as well as their response to acid treatment.

Characterization and Origin of Fe3+-Montmorillonite in Deep-Water Calcareous Sediments (Pacific Ocean, Costa Rica Margin)

AbstractMillimetric to centimetric green grains widespread in pelagic calcareous sediments recovered at a water depth of3000 m near the Costa Rica margin were studied by X-ray diffraction, scanning electron microscopy (SEM), transmission electron microscopy (TEM) and Fourier transform infrared spectroscopy. Samples were collected, during the Ticoflux II expedition, from the upper bioturbated part of four sedimentary cores (0.13–3.75 m below seafloor). The sediments are calcareous and siliceous nanofossil oozes (coccoliths, diatoms, radiolarians, etc.).Green grains show generally a concentric zoning with a green rim in which smectite largely predominates over pyrite and a black core in which pyrite is prevalent. Observations by SEM indicate that this zoning results from a progressive inward alteration and replacement of the accumulations of pyrites by smectites. The high-resolution TEM observations of the smectite-pyrite interfaces suggest that the replacement of pyrites by smectite occurs through a dissolution-precipitation process with the formation of a gel. The pyrite matrix is composed of a huge number of very small (0.5–22 µm) pyrite octahedra, a typical texture resulting from the pyritization of organic material in early diagenetic environments.The accurate mineralogical and crystal chemical characterization of the smectites indicate that they are Fe3+-montmorillonites (Fe3+-rich smectite with a dominant octahedral charge, rarely recorded in the literature). The formation of such Fe3+-montmorillonites forming green grains could be explained by two successive diagenetic redox stages: (1) reducing stage: early pyritization of the organic matter by microbial reduction within reducing micro-environments; (2) oxidizing stage: Fe3+-montmorillonite crystallized in space liberated after dissolution of pyrite connected with the rebalancing of the redox conditions of the micro-environments with the oxidizing surrounding sediments.

Crystal structure and chemistry of trilithionite-2M2 and polylithionite-2M2

The crystal chemistry of three Li-bearing mica-2 M 2 crystals from pegmatites has been studied by chemical analyses and single crystal X-ray diffraction; their belonging to the trilithionite-polylithionite join is highlighted by the following compositional ranges in atoms per formula unit [based on O 12-(x+y) (OH) x F y ]: 3.198 ≤ Si ≤ 3.538, 0.462 ≤ [IV] Al ≤ 0.811, 1.195 ≤ [VI] Al ≤ 1.390, 0.031 ≤ (Fe+Mn) ≤ 0.072, 1.522 ≤ Li ≤ 1.757, 0.872 ≤ K ≤ 0.906, 0.030 ≤ Na ≤ 0.073, 0.000 ≤ (Cs+Rb) ≤ 0.099, 1.541 ≤ F ≤ 1.722. The correlation between F and Li content is confirmed, as observed in Li-rich micas. Crystal structure refinements were carried out in space group C 2/ c ( R values vary between 0.030 and 0.031). The crystal chemistry is mostly influenced by tetrahedral chemical composition. Increasing [IV] Al content, α and ψ M1 parameters increase; Si content involves a lowering of the interlayer separation and tetrahedral thickness. Li content affects octahedral thickness. The stability of 2 M 2 polytype seems to be induced by a relative increase of Δz tetrahedral parameter, which reduces the repulsion between basal tetrahedral oxygen atoms. Unlike Li-bearing muscovite, trioctahedral Li-bearing mica crystals show an octahedral occupancy not related to octahedral charge.