Dioctahedral Smectite Research Articles

Development of Fe-rich clay minerals in a weathering profile derived from serpentinized ultramafic rock in Nui Nua massif, Vietnam

We investigated the chemical and mineralogical characteristics of a weathering profile of the Nui Nua serpentinized ultramafic-mafic massif (Thanh Hoa Province, Vietnam) using TEM-EDX, XRD, and FT-IR methods to understand the effects of monsoon climate conditions on weathering processes and clay minerals, especially transformation and neoformation of clay minerals. Clay minerals of this profile were characterized by a temporariness of serpentine clay minerals though chlorite seemed to be stable and increased from the bottom layer to the top layer. The nature of Fe changed rapidly from ferrous Fe (close to parent rocks) to ferric Fe and additionally it was incorporated into the octahedral sheet of dioctahedral smectite. Consequently, a Fe-rich smectite formed at the beginning of the weathering process and was detected as a dominant phase in the clay fraction of the profile. This phase was later transformed progressively to Al-rich smectite because of Fe-oxidation and corresponding Si-mitigation. Chemically, the contents of Fe and Mg reduced from the bottom layer to the top layer but Al developed vice versa. Magnetite in top layer was formed by bacterial activities. The transformations of clay minerals were controlled strongly by leaching conditions in the weathering system and by the effects of oxidation of Fe (Fe2+→Fe3+) on clay minerals paragenesis.

RHEOLOGICAL PROPERTIES OF PALYGORSKITE-SMECTITE SUSPENSIONS FROM THE VENTZIA BASIN, W. MACEDONIA, GREECE

In this contribution we examine the rheological properties of palygorskite rich clays from the Ventzia Basin, W. Macedonia, Greece. The clays consist of palygorskite and/or dioctahedral Fe-rich smectite (nontronite) and quartz as main components, and serpentine, amphibole and sepiolite as minor constituents. The apparent and plastic viscosity and yield point increase with increasing concentration of clay in the suspension. Flow is Newtonian for 1% suspensions, becoming gradually Bingham plastic (3% clay suspensions) and then pseudoplastic with yield point described by the Herschel Bulckley flow model. In the case of suspensions cf smectite-free clays Bingham plastic flow behaviour was not observed. Addition of 1M NaCl electrolyte deteriorates the rheological behaviour of the smectite-bearing palygorskite clays but it does not affect significantly the smectite-free, palygorskite rich clays. The different rheological properties of the palygorskite compared to smectite is due to the different morphological and crystal-chemical properties of these two minerals. It is suggested that the palygorskite-rich clays can be used successfully as drilling muds in seawater based drilling fluids, in which smectite-based drilling muds tend to flocculate.

Origin of clay minerals in Early Eocene volcanic paleosols on King George Island, Maritime Antarctica

The paleoclimate during the Early Eocene in Maritime Antarctica is characterized by cool conditions without a pronounced dry season. Soils formed on volcanic material under such climate conditions in modern analogue environments are usually Andosols rich in nanocrystalline minerals without pedogenic smectite. The paleosols formed on volcanic material on King Georges Island are covered by basalts, dated by 6 new 40Ar/39Ar datings to 51–48 Ma, and are rich in smectite. A pedogenic origin of the smectites would suggest a semi-arid rather than a wet non-seasonal humid paleoclimate. To investigate the origin of the smectites in these paleosols we used X-ray diffraction and microscopic techniques. Minor mineralogical changes between the volcanic parent material and the paleosols and a homogenous distribution of smectites throughout the paleosol horizons indicate that these smectites were mainly inherited from the pyroclastic parent material, which was altered prior to surficial weathering. Nevertheless, the mineralogical properties, such as degree of crystallinity and octahedral site occupancy, of these smectites were modified during the ancient soil formation. Our findings highlight that trioctahedral smectites were a product of deuteric alteration of pyroclastic rocks and were progressively transformed to dioctahedral smectites during weathering in a soil environment on King George Island.

Mobility and reactivity of sulphide in bentonite clays – Implications for engineered bentonite barriers in geological repositories for radioactive wastes

Bentonite clays will be used as barriers in geological repositories for radioactive wastes. Anoxic conditions will prevail in such repositories, and the presence of sulphide-producing bacteria in commercial bentonites and deep groundwater environments is well established. In this study, sulphide was found to reduce ferric iron in bentonites denoted Asha, MX-80 and Calcigel under the formation of elemental sulphur, ferrous iron and iron sulphide. These reactions rendered an immobilisation capacity of the clays that was 40μmole sulphide (g clay)−1 or more, depending on the load of sulphide, and type of clay. In addition, the effective diffusion coefficients for sulphide in Asha bentonite, compacted to saturated wet densities of 1750kg m−3 and 2000kg m−3, were determined to 2.74×10−11m2s−1 and 6.60×10−12m2s−1, respectively. The found immobilisation effect can reduce the mass of sulphide that corrode metal canisters over repository life times, but the concomitant reduction of ferric iron may be problematic due to the destabilizing effect of ferrous iron on dioctahedral smectites such as montmorillonites.

Tetrahedral charge and Fe content in dioctahedral smectites

AbstractNatural aluminosilicates can contain Fe in tetrahedral or octahedral coordination. Amongst smectites, tetrahedral iron is known to occur in Fe-rich nontronites but few indications exist for the presence of tetrahedral Fe in smectites of the montmorillonite–beidellite series. A set of 38 different bentonites showed a correlation of tetrahedral charge and Fe content in their smectites. All materials with large tetrahedral charge were rich in Fe. This could be explained by a general tendency of Fe to enter the tetrahedral sheet. To investigate this correlation, nine materials were selected and investigated by Mössbauer, UV-Vis, Fe K pre-edge and EXAFS spectroscopy with respect to tetrahedral Fe (Fe[IV]). The latter two methods were at the detection limit but Mössbauer and UV-Vis spectroscopy provided consistent results indicating the significance of both methods in spite of some scatter caused by the overall small amount of tetrahedral Fe. The results indicate the absence of any relation between Fe content and tetrahedral Fe. Tetrahedral Fe can be present in Fe-poor smectites and absent in the case of Fe-rich materials. This means that Fe-rich montmorillonites have a larger tetrahedral charge which is not caused by Fe[IV]but by Al[IV]. A possible explanation for this indirect relation is based on: the coordination of Al3+in the weathering/smectite-forming solutions determines the coordination in the precipitates; and the Al[IV/VI]ratio increases with increasing pH. The correlation could thus be explained if the pH of weathering solutions generally was higher in Fe-rich parent smectite rocks than in more acidic smectite parent rocks. The relation between tetrahedral charge and Fe content can probably be explained by different geochemical contexts throughout the formation of smectites which affect the coordination of dissolved Al.

New insights into microbial smectite illitization in the Permo-Triassic boundary K-bentonites, South China

Microbially mediated smectite illitization has been extensively demonstrated in laboratory experiments. Despite occasional observations in geological environments, the transformation mechanism is still poorly understood. Here we investigated a series of volcanic ash (tephra) samples from the Permo-Triassic boundary (PTB) successions of South China, using short-wavelength infrared (SWIR) spectroscopy, in combination with XRD, MIR, EPMA, SEM, TEM, and 27Al NMR analyses, in order to establish a better understanding of microbial illitization, and to test the applicability of SWIR as a monitor for this process. Mixed-layer illite-smectite (I-Sm) clays are the predominant clay minerals, with illite percentages ranging from 32 to 93%. The (060) value of I-Sm varies from 1.4956 to 1.4996Å, indicating a dioctahedral precursor smectite. The precursor smectite of I-Sm is nearly free of Fe, and is probably montmorillonite. The b-cell dimension (b0 parameter) exhibits a positive correlation with the illite percentage of I-Sm, suggesting that the illitization occurs with significant cation substitutions. The different spectroscopic results all indicate that tetrahedral Al substitutes for Si, and octahedral Al is mainly substituted by Mg and/or Fe with illitization, consistent with the findings of XRD analysis. Chemical compositions and trends in their variation show typical features of a dissolution-precipitation (DP) mechanism. The SEM images reveal abrupt changes in morphology and the TEM analyses suggest progressive changes in polytype with illitization, also supporting the DP process. The transformation mechanism of I-Sm contradicts the conventional wisdom concerning diagenetic bentonite I-Sm, similar to that of hydrothermal origin, probably as the result of to the involvement of microorganisms. The SWIR parameters, such as position, depth, area and asymmetry of the absorption bands, versus illite percentage in I-Sm, generally give good correlations. SWIR spectroscopy is demonstrated as a valuable method to identify I-Sm with different percentages of illite, and with a capability as a sensitive monitor for I-Sm structure ordering and crystal-chemistry. Thus it can be used as an alternative or complementary method in conventional I-Sm studies, as well as in epithermal and hydrocarbon-reservoir explorations and astrobiological investigations.

Comparative efficacy of different schemes for the therapy of rotavirus gastroenteritis in children. Case–control study

Objective research : compare the efficacy of different types of therapy for rotavirus gastroenteritis in children. Materials and methods. A study was carried out, the design of which corresponded to the case–control type. The main group is children hospitalized in a hospital with laboratory confirmed rotavirus infection (35 cases), who were prescribed a modern regimen of therapy that included a low-carbohydrate diet, rehydration with low osmolar oral solutions, dioctahedral smectite, probiotic containing Saccharomyces boulardii. For each child of the main group was selected another patient (control), the corresponding sex, age, the period from the onset of the disease to admission to hospital, and the degree of dehydration. Patients of the control group (n=35) were assigned a traditional «old» therapy regimen, including a diet corresponding to Table No. 4 in Pevznur, oral rehydration solutions with osmolality of 282 mOsm/l, anthibacterial therapy (ceftriaxone), dioctahedral smectite and pancreatin at age-dependent doses. The duration of the observation was 5 days. Results : By the 5th day of treatment, the normalization of the stool was recorded in 91.4% [95% CI 82.1–100.7], 32/35 in the main group and 71.4% [95% CI 56.5–86.3], 25/35 children in the control (p <0.05, OR 4.27 [95% CI 1.06–17.2]). The timing of relief of excess meteorism also had statistically significant differences and was shorter in patients receiving a «new» therapy regimen (OR 2.8 [95% CI 1.01–8])). The positive effect of the proposed scheme was also observed when assessing the duration of dehydration symptoms and the timing of hospitalization. Conclusions : The conducted study case–control showed the advantage of modern schemes of therapy of rotavirus infections in children.

Soil Mineralogy of a Vernal Pool Catena in Southern California

Core Ideas Soil mineralogy, as with morphology, varies with slope position in the vernal pool watershed. Vernal pool soil mineralogy is determined by parent material, weathering, relative solubility of elements, and hydrology. Kaolin dominates on upper slope positions; smectite dominates on lower slope positions. Vernal pools are seasonal wetlands that are noted for their unique flora and fauna. Soils are critical to the hydrologic function of the vernal pools, but few studies have examined the mineralogy of vernal pool soils in detail. In this study, the soil mineralogy of a vernal pool catena on basalt in southern California was investigated to identify the mineral components, their origin, and their relationship to the soil hydrologic environment. Kaolin and smectite are found in the soil clay fraction throughout the catena. Kaolin, derived from plagioclase weathering, is the predominant clay mineral in the loamy upper slope soils. Excess silica and other soluble cations from the weathering of plagioclase and pyroxene are carried in throughflow to the lower slope positions where they precipitate primarily as smectite, producing clayey Vertisols. Minor amounts of quartz and biotite are present in the soils, apparently from eolian deposition. The biotite is interpreted to weather to vermiculite, and subsequently to a trioctahedral smectite, which is found in both upper and lower slope soils. Slope wash has transported soil material, including trioctahedral smectite and kaolin, from the upper slope to lower slope soils. Two dioctahedral smectite species are distinguished in the basin soils based on d(060) spacings and are interpreted to have neoformed and transformed from biotite or pyroxene. The mineralogy of the vernal pool catena is differentiated by pedogenic processes related to mineral weathering, relative solubility of released elements, slope wash, and soil hydrology.

Thermal behaviour of stevensite at temperatures up to 800°C.

Stevensite is a Mg-trioctahedral smectite with layer charge stemming from vacancies in the octahedral sheet. In the present work we studied the thermal behavior of Jbel Ghassoul stevensite from Morocco, known as Ghassoulite or Rhassoulite, free of talc layers. The clay fraction of the material was separated by sedimentation, it was subsequently heated from 250° to 800° C and the end products were examined with X-ray diffraction (XRD) and Fourier Transform Infrared (FTIR) spectroscopy. The influence of heating on the stevensite structure begins at 400°C and is completed at 500°C. It involves irreversible collapse of the layers at ~10Å, which do not re-expand in ethylene glycol (EG) vapors. In contrast, heating at lower temperatures does not affect the stevensite layers, which expand completely in EG. The FTIR spectra indicate the formation of talc-like (kerolite) layers after heating at temperatures exceeding 400°C. Within the current experimental setup, the transition to kerolite layers takes place without the formation of an intermediate mixed-layer stevensite/talc phase. Heating at higher temperatures does not change the transformation pattern, until 800°C where complete dehydroxylation of the 2:1 layer takes place, which is associated with the formation of enstatite. The results of this study clearly demonstrate that opposite to common trioctahedral and dioctahedral smectites, stevensite converts to another layer silicate prior to dehydroxylation.

Simulating the Dehydroxylation Reaction in Smectite Models by Car–Parrinello-like–Born–Oppenheimer Molecular Dynamics and Metadynamics

The first step of a dehydroxylation reaction of a medium-charged dioctahedral smectite model is studied by means of ab initio molecular dynamics simulations. Combining the Car–Parrinello-like–Born–Oppenheimer molecular dynamics method with metadynamics, we find two previously proposed mechanisms (dubbed on-site and cross). We confirm the existence of an intermediate state where the tetrahedral structure is deformed, a feature that is also present in the final (semidehydroxylate) state of this reaction. This shows that the thermal transformations of phyllosilicates are more complex than often assumed, which will be relevant when elucidating the mechanisms of other transformations of these materials.

Dehydroxylation of dioctahedral smectites in water vapor atmosphere

Due to dehydration and dehydroxylation, it has to be considered, that thermal treatment of large quantities of bentonites in industrial applications may result in a local water vapor atmosphere above the sample. This study investigates the consequences of a local water vapor atmosphere during thermal treatment of bentonites with emphasis on the influence of water vapor on the DHX of dioctahedral smectites with Na+-saturated interlayers.The dehydroxylation behavior of Na+-exchanged montmorillonites in water vapor atmosphere was compared to dehydroxylation in dry synthetic air. Water vapor at atmospheric pressure delays dehydroxylation of montmorillonites by 50°C. The temperature boundary for dehydroxylation of trans-vacant and cis-vacant layers shifts by 50 to 75°C to higher temperatures. Interlayer Na+ acts as flux and partial melting results in hollow spheres and bubbles that delayed dehydroxylation and entrapped water after dehydroxylation, which is suddenly discharged at higher temperatures. The formation of high temperature phases after heating to 1000°C is controlled by the montmorillonite composition.

Interaction of Magnesium Cations with Dioctahedral Smectites under HLRW Repository Conditions

AbstractIn some real and up-scale tests using high-level radioactive waste (HLRW), Mg accumulation was observed in smectites at the contact of heated Fe or Cu metal tubes. It is important to understand why Mg accumulated in order to model the long term performance of bentonites in HLRW systems. In some of these tests, an increased number of trioctahedral domains was measured in the smectites using X-ray diffraction (XRD) and infrared spectroscopy (IR). The trioctahedral domains either formed by the dissolution/precipitation of smectites or by the addition of Mg through a solid-state reaction similar to the Hofmann-Klemen effect. The Hofmann-Klemen effect is used in the Greene-Kelly test to distinguish montmorillonites from beidellites. Many studies have been carried out about Li-uptake by smectites, but Mg was rarely taken into account. The present study was, therefore, undertaken to compare the interactions of different bentonites with Li and Mg under various conditions. A significant CEC decrease was found for Li- and Mg-saturated bentonite samples after heating at 250°C under dry conditions. The extent of this CEC reduction depended on the octahedral to tetrahedral charge ratio and was smaller for Mg-saturated samples than Li-saturated samples. This finding proved that it is much more difficult for Mg to enter octahedral vacancies than Li, which probably can be explained by the larger hydration energy and/or slightly larger radius of Mg. The relationship between CEC reduction and the octahedral/tetrahedral charge ratio of both Li- and Mg-saturated samples, however, suggests a similar process. The Mg that can reside at the bottom of the pseudohexagonal holes would not explain this relationship. The important result with respect to understanding HLRW bentonite performance, on the other hand, is that Mg fixation only occurs under dry conditions and that Mg fixation acts as a sink for Mg and, hence, leads Mg to diffuse towards the heated metal surface.

Clay mineral diagenesis in Cretaceous clastic reservoirs from West African passive margins (the South Gabon Basin) and its impact on regional geology and basin evolution history

The Gabon coastal region located along the western African margin hosts several sedimentary basins that developed prior, during and after the opening of the South Atlantic Ocean. A range of diagenetic processes controlled the distribution of clay paragenesis shown to be of high importance for the understanding of the basin's burial history and geotectonic development. Materials acquired for this study stem from the siliciclastic fluvio-lacustrine-deltaic, petroleum bearing, Early Cretaceous Dentale and Gamba formations cored by two respective wells at depths of ~1500m. Sampled materials were analysed by X-ray diffraction, automated electron microscopy, and inductively coupled plasma mass spectrometry in order to reconstruct eogenetic and mesogenetic variations directing the formation of clay assemblages in the basin. The clay contents in both cores consist of authigenic mixed-layer minerals like illite-smectite, chlorite-smectite and berthierine-chlorite, and some minor detrital illite/mica and chlorite. I-Sm and C-Sm phase chemistry implied that the original dioctahedral (montmorillonite to beidellite) and trioctahedral (saponite) smectite precursors formed out of acid volcanic feedstock during eogenesis. Different magmatic fractionation degrees, from rhyodacite to trachyandesite, reflected in the uniform REE curves of volcanic glass conform to an active geotectonic development of the Cretaceous margins of Africa. Mesogenesis led to the decrease of smectite and formation of mixed-layered phases; I-Sm composition showed maximal burial depths of sediments to be ~1000m and ~500m deeper than today for Gamba and Dentale sediments, respectively. Besides, temperature, a major role in the formation of mixed-layer minerals had the porosity of sediments and geochemistry of smectitic precursors. Thus, Fe-Mg smectite showed higher thermal stability and lower rates of transformations in non-expanding clays during burial compared to Al-rich smectite. Different burial histories of sediments from the two cores can be attributed to rift-related normal faulting and subsequent differential denudation.

Traprock Transformation into Clayey Materials in Soil Environments of the Central Siberian Plateau, Russia

AbstractThe study of hard rock conversion into fine earths and clayey materials in the pedosphere is important in understanding the relative proportions of recent soil features to features that were inherited from ancient epochs. Cold environments are widely thought to be areas of physical weathering, but the coexistence of physical and chemical processes have also been shown. To further examine mafic rock (dolerite) weathering in soil environments and the conversion into clayey materials, Entic Podzols formed in the cold continental climate were studied. The key study was located in the central part of the flood basalt complex, or traps (traprocks), of the Central Siberian Plateau (Russia). The qualitative mineralogy was studied using X-ray diffraction and the quantitative mineral composition was determined using X-ray diffraction and subsequent Rietveld analysis. The micromorphological characteristics of the soils were studied in thin sections. Dolerite fragments and fine earths were sampled from soil profiles underlain by dolerite. XRD analyses indicated that pyroxene and especially plagioclase contents in the dolerite fragments and fine earths decreased from the bottom to the top soil horizons mostly in the mature soil profiles that were affected by chemical weathering of dolerite. The dioctahedral and trioctahedral smectites in the soils were inherited from a dolerite previously subjected to chemical weathering. The smectite was conserved in the inherited aggregates and protected against dissolution even in acidic soil horizons. Recent pedogenesis processes fractured individual fragments, converted it into soil micromass, and slightly decreased the total smectite content of the <1 µm soil fraction. However, in soil samples collected from the bottom to the top horizons of a mature soil profile, trioctahedral smectite contents decreased as dioctahedral smectite contents increased. This suggests that dioctahedral smectites formed by pedogenic alteration of inherited trioctahedral smectites.

Revisiting the cation mass-valence sum approach to assigning infrared OH-bands in dioctahedral smectites in the light of new data from synthetic Ga-Fe3+ smectites

A series of pure Ga-Fe3+ smectites were synthesized at 180°C in order to complete IR data in the Al-Ga-Fe3+ system. MIR and NIR data from natural and synthetic smectites in the Al-Fe3+, Al-Ga and Fe3+-Ga chemical series exemplified a case where some ν OH, δ OH and (ν+δ) OH band positions did not follow the relation: ν OH (cm−1)=(1/2π c)*(kOH/μOH)1/2. This relation is often used to approximate the position of the OH vibration bands from the reduced masses of octahedral cations linked to hydroxyls in clay minerals. In this study, better relations were established between the wavenumbers of the OH absorption band of smectites and the average ionic radius of neighboring octahedral cations. Similar trends were established previously for M-(OH)2 hydroxides (G.W. Brindley, C. Kao, Phys. Chem. Miner. 10 (1984) 187-191) suggesting a possible generalization of a link between OH frequencies in IR spectra and the average ionic radii of neighboring octahedral cations for hydroxylated minerals.

Alteration minerals, fluids, and gases on early Mars: Predictions from 1‐D flow geochemical modeling of mineral assemblages in meteorite ALH 84001

AbstractClay minerals, although ubiquitous on the ancient terrains of Mars, have not been observed in Martian meteorite Allan Hills (ALH) 84001, which is an orthopyroxenite sample of the early Martian crust with a secondary carbonate assemblage. We used a low‐temperature (20 °C) one‐dimensional (1‐D) transport thermochemical model to investigate the possible aqueous alteration processes that produced the carbonate assemblage of ALH 84001 while avoiding the coprecipitation of clay minerals. We found that the carbonate in ALH 84001 could have been produced in a process, whereby a low‐temperature (~20 °C) fluid, initially equilibrated with the early Martian atmosphere, moved through surficial clay mineral and silica‐rich layers, percolated through the parent rock of the meteorite, and precipitated carbonates (thereby decreasing the partial pressure of CO2) as it evaporated. This finding requires that before encountering the unweathered orthopyroxenite host of ALH 84001, the fluid permeated rock that became weathered during the process. We were able to predict the composition of the clay minerals formed during weathering, which included the dioctahedral smectite nontronite, kaolinite, and chlorite, all of which have been previously detected on Mars. We also calculated host rock replacement in local equilibrium conditions by the hydrated silicate talc, which is typically considered to be a higher temperature hydrothermal phase on Earth, but may have been a common constituent in the formation of Martian soils through pervasive aqueous alteration. Finally, goethite and magnetite were also found to precipitate in the secondary alteration assemblage, the latter associated with the generation of H2. Apparently, despite the limited water–rock interaction that must have led to the formation of the carbonates ~ 3.9 Ga ago, in the vicinity of the ALH 84001 source rocks, clay formation would have been widespread.

Reprint of Effect of plant growth on the occurrence and stability of palygorskite, sepiolite and saponite in salt-affected soils on limestone in South Australia

The Mg-rich clay minerals, palygorskite, sepiolite and also Mg-smectites are generally rare in soils. When they occur they are usually concentrated in subsurface horizons and become replaced by other minerals, e.g., dioctahedral smectites in near-surface horizons. Salt-affected soils on limestone in a flat landscape in southern Australia showed patchy pasture growth, including areas in which almost no growth occurred. Chemical and mineralogical analyses of deep profiles of the soils revealed two main types and that the extent of plant growth reflected their mineralogical composition. The soil type in which plants grow well (‘Chromosol’, an Alfisol) contains dioctahedral smectite (montmorillonite and/or beidellite) but only in substantial amounts at depth. The soil type in which little or no plants grew (‘Vertosol’, a Vertisol) has substantial concentrations of a trioctahedral Mg-smectite, saponite, in its surface horizons along with dioctahedral smectite in lower horizons. Both soil types contain sepiolite and palygorskite, largely in the lower horizons of the Chromosol, but also in near-surface horizons of the Vertosol. Both types of soil also contain illite and kaolinite, which are each concentrated in the upper horizons of the soils and are considered to be detrital. The genesis of the two main types of soils can be explained by the depositional history of the basin. On the eastern side of the basin, palustrine limestone formed when the sediments became exposed, possibly from uplift. The Chromosol formed on this material. On the western side, by contrast, lacustrine sediments on lowland adjacent to a stranded beach ridge, experienced prolonged seasonal inundation by water leading to the formation of the Vertosol. This soil is strongly sodic and strongly alkaline throughout and it is concluded that the high concentration of saponite in the surface swells and thereby ensures that the soil remains wet. Together with its high pH, this soil has very poor conditions for plant growth. The persistence of the Mg-rich clay minerals, especially saponite, at the surface of the Vertosol, suggests that the relative lack of plants has preserved these minerals throughout the profile.

Unraveling the Coupled Processes of (De)hydration and Structural Changes in Na+-Saturated Montmorillonite

The coupled processes of (de)hydration and structural changes of a well characterized Na+-exchanged dioctahedral smectite with a low layer charge (0.26 mol(+)/formula unit) were studied by in situ transmission Fourier transform infrared spectroscopy. Therefore, a sample preparation technique based on the tethering by aggregation and growth method is extended to clay minerals to obtain a thin film of montmorillonite on a silicon wafer. After equilibration of the sample at 53% relative humidity, infrared measurements were done with a custom-made, heatable sample holder in an oxygen free atmosphere under N2 purging which allowed subsequent FTIR measurements. Using a very thin film, deviation of the Si–O phonon modes could be simultaneously studied with the vibrational modes of water. The structural changes in the 2:1 layer of the montmorillonite during dehydration can be recognized from the shift of longitudinal optical Si–O phonon mode. The accompanied decrease and shift of the bending vibration of water δ(...

Clay minerals in sediments of the hydrothermally active southern trough in the Guaymas Basin (Gulf of California)

The results of the study of clay mineral alterations in Upper Pleistocene sediments of the southern trough in the Guaymas Basin (Gulf of California) due to the influence of hydrothermal solutions and heat produced by sill intrusions are discussed. Core samples from DSDP Holes 477 and 477A were taken for the analysis of clay minerals. Application of the method of modeling X-ray diffraction patterns of oriented specimens of the finely dispersed particles made it possible to establish the phase composition of clay minerals, determine their structural parameters, and obtain reliable quantitative estimates of their contents in natural mixtures. The modeling data allowed us to characterize reliably the transformation of clay minerals in sediments of the hydrothermally active southern trough in the Guaymas Basin. In Upper Pleistocene sandy–clayey sediments of the southern trough, changes in the composition of clay minerals occurred under the influence of a long-living hydrothermal system. Its lower part (interval 170.0–257.5 m) with maximum temperatures (~300°C) was marked by the formation of chlorite. Terrigenous clay minerals are not preserved here. Saponite appears at a depth of 248 m in the chlorite formation zone. Higher in the sedimentary section, the interval 146–170 m is also barren of terrigenous clay minerals. Sediments of this interval yielded two newly formed clay minerals (chlorite and illite), which were formed at lower temperatures (above 180°C and below 300°C, approximately up to ~250°C), while the relatively low-temperature upper part (110–146 m) of the hydrothermal system (from ~140°C to ~180°C) includes the mixture of terrigenous and newly formed clay minerals. Terrigenous illite is preserved here. Illitization of the mixed-layer illite–smectite was subjected to illitization. The terrigenous montmorillonite disappeared, and chlorite–smectite with 5–10% of smectite layers were formed. In the upper interval (down to approximately 110 mbsf), the composition of terrigenous clay minerals remains unchanged. They are composed of the predominant mixed-layer illite–smectite and montmorillonite, the subordinate illite, mixed-layer chlorite–smectite with 5% of smectite layers, mixed-layer kaolinite–smectite with 30% of smectite layers, and kaolinite. This composition of clay minerals changed under the influence of sill intrusions into the sedimentary cover at 58–105 m in the section of Hole 477. The most significant changes are noted in the 8-m-thick member above the sill at 50–58 m. The upper part of this interval is barren of the terrigenous mixed-layer illite–smectite, which is replaced by the newly formed trioctahedral smectite (saponite). At the same time, the terrigenous dioctahedral smectite (montmorillonite) is preserved. The composition of terrigenous clay minerals remains unchanged at the top of the unit underlying the sill base.

Examining Structural and Related Spectral Change in Mars-relevant Phyllosilicates After Experimental Impacts Between 10–40 GPa

AbstractAccurate clay mineral identification is key to understanding past aqueous activity on Mars, but martian phyllosilicates are old (>3.5 Ga) and have been heavily bombarded by meteoroid impacts. Meteoroid impacts can alter clay mineral structures and spectral signatures, making accurate remote sensing identifications challenging. This paper uses nuclear magnetic resonance (NMR) spectroscopy to examine the short-range structural deformation induced in clay mineral samples of known composition by artificial impacts and calcination. Structural changes are then related to changes in the visible-near infrared (VNIR) and mid-infrared (MIR) spectra of these clay mineral samples. The susceptibility of phyllosilicates to structural deformation after experimental impacts varies by structure. Experimental results showed that trioctahedral, Mg(II)-rich saponite was structurally resilient up to peak pressures of 39.8 GPa and its unchanged post-impact spectra reflected this. Experimental data on kaolinite showed that this Al(III)-rich, dioctahedral phyllosilicate was susceptible to structural alteration at peak pressures ⩾ 25.1 GPa. This result is similar to previously reported experimental results on the Fe(III)-rich dioctahedral smectite nontronite, suggesting that dioctahedral phyllosilicates may be more susceptible to shock-induced structural deformation than trioctahedral phyllosilicates. The octahedral vacancies present in dioctahedral phyllosilicates may drive this increased susceptibility to deformation relative to trioctahedral phyllosilicates with fully occupied octahedral sheets. Thermal alteration accompanies shock in meteoroid impacts, but shock differs from thermal alteration. NMR spectroscopy showed that structural deformation in thermally altered phyllosilicates differs from that found in shocked phyllosilicates. Similar to shock, dioctahedral phyllosilicates are also more susceptible to thermal alteration. This differential susceptibility to impact-alteration may help explain generic smectite identifications from heavily bombarded terrains on Mars.