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The aim of this study was to further our understanding of the pedogenic and lacustrine modification of clay minerals. Some of these modifications are of special interest because they constitute reverse weathering reactions, rare in surface environments, and because there is not yet an accurate assessment of their global relevance in mineralogical and geochemical cycles. For this study, two sections from the Central System in Spain were selected. Both are sections through the Uppper Cenomanian-Turonian mixed clastic and carbonate succession, containing both calcite and dolomite, in the Sierra de Guadarrama. Mid-Turonian sea level fall resulted in the formation of a coastal plain environment in which extensive pedogenesis occurred around saline lagoons. The mineralogical changes that have occurred as a result of sedimentation in saline lagoons and as a consequence of pedogenesis are described. Textural relationships indicate that the dolomite cement pre-dates the calcite. Silicate minerals are represented by quartz, kaolinite, illite-smectite, illite, minor plagioclase and alkali feldspar, and trace chlorite and palygorskite. There is a positive correlation between the intensity of pedogenesis and the proportion of illite in the clay assemblage in one of the sections, indicating pedogenic illitisation. In this section, the intensity of the illitisation process increases up, reaching a maximum where pedogenesis is most intense in the middle part, and then decreases as marine influence increases towards the top of the Alcorlo Formation and the overlying marine Tranquera Formation. The clay assemblages are consistent with a slow transformation process from kaolinite to illite by way of illite-smectite, taking place under surface conditions. The illitisation process has resulted in a less Fe-rich, more Mg-, and Al-rich illite than the majority of previously documented cases in the near surface. Formation of Al-rich illite is not therefore restricted to the deep subsurface. The mechanism for low temperature illitisation involves enhanced layer charge resulting from Mg 2 + substitution for Al 3 + (or Fe 3 + ) and Fe 3 + to Fe 2 + reduction. Mg 2 + enrichment may have occurred principally in saline lagoons or lakes, while Fe 3 + to Fe 2 + reduction occurred as a result of wetting and drying in a pedogenic environment. So far as it has been possible to establish, this dual mechanism has not previously been documented. This study indicates clearly that the dolomite and calcite are authigenic cements that precipitated in a clastic sediment, probably soon after deposition. Dolomitisation and Mg enrichment of the clay may have occurred at the same time. Seawater is the most probable source of Mg. [ABSTRACT FROM AUTHOR]

This study investigated soil clay mineralogy of mangrove forests along the Brazilian coast in eight regions of different environmental characteristics, with a focus on the crystallochemical features and genesis of 2:1 phyllosilicates. Samples from two different depths (0-30 and 60-90 cm) and two clay size fractions (<2 and <0.2 mm) were studied. The analytical tools used were x-ray diffraction, Fourier-transform infrared spectroscopy, and transmission electron microscopy-energy dispersive spectroscopy (TEM-EDS). A complex assemblage was found in both fractions, including high proportions of kaolinite, illite (Fe-illite/glauconite), and smectite (Fe-beidellite, Fe-montmorillonite), minor amounts of gibbsite and quartz, and traces of K-feldspar, halloysite, and amphibole crystals (TEM-EDS data). Evidence of illite-smectite and possibly kaolinite-smectite mixed-layer phases was found in the <0.2-mm fraction. An authigenic transformation process likely taking place, from kaolinite to Fe-illite/glauconite, through transitory kaolinite- smectite and illite-smectite phases. The reaction is triggered by high Fe activity in solution generated by dissolution of Fe oxides and pyrite present in the sediment, at approximately neutral pH and high salt content in the water. Smectite illitization takes place by substitution of Al3+ for Si4+ in the tetrahedral sheet, of Fe and Mg2+ for Al3+ in the octahedral sheet, and progressive Fe reduction, all of which produces an increase in the layer charge and K uptake. The sequential nature of the transformation (kaolinite-smectite-illite) suggests a solid-state transformation process. Two unusual kaolinite-rich mangroves were found, indicating low reactivity or high deposition rate of continental sediments of soils derived from the Barreiras Group, dominated by kaolinite. [ABSTRACT FROM AUTHOR]

Abstract: Aluminous clay deposits on Mars are recognized from remotely sensed infrared spectral features similar to those of montmorillonite, beidellite, and/or kaolinite. The nature of aluminous clay deposits on Mars is of interest because they likely indicate a different formation mechanism than that of Fe–Mg clays, which are widespread on Mars and likely alteration products of the Fe–Mg-rich basaltic crust. The near-infrared reflectance spectra of aluminous martian clay deposits frequently display characteristics typical of both montmorillonite and kaolinite. The question arises whether such mixed character is due to the existence of end-member phases or to kaolinite–smectite mixed-layer (K–S). The issue is relevant because K–S implies the existence of a smectite precursor that alters into kaolinite, and thus constrains the timing and intensity of the alteration processes that generates it. A mixture of kaolinite and smectite end-members may indicate locally heterogeneous alteration processes, or alternatively, could result from the physical mixing of altered materials of different provenance. A group of natural K–S samples and synthetic kaolinite/smectite mixtures of known proportion, all of which had been thoroughly characterized in previous work using several analytical techniques, were investigated here using near-infrared (NIR) spectroscopy. The NIR spectral features correlate well with their kaolinite–smectite relative proportions. The shape of spectral features attributed to Al–OH in K–S is subtly different from those in physical mixtures of kaolinite and smectite. Based on qualitative comparison, some regions on Mars appear to have spectral signatures similar to K–S. We also applied a quantitative technique using the second derivative of spectra. In this technique, plots of the height of the features at (λ=) 2.21μm (band present in kaolinite and montmorillonite) and 2.17μm (kaolinite only) were able to discriminate between K–S and kaolinite–smectite physical mixtures, as they generated correlations with different slopes. The method of discrimination was applied to Mars spectra, which resulted in reasonable evidence for the existence of K–S in Nili Fossae and Mawrth Vallis, and mixtures of end-members in Mawrth Vallis and Leighton Crater. This is one of the first times that evidence for mixed-layer clay minerals, and particularly K–S, on Mars has been gathered. The ability to detect mixed-layer clays is an important step forward for further development of our understanding of the processes that generated clay on Mars. [Copyright &y& Elsevier]

The hydrothermal reactivity of the <1 µm fraction, K-saturated SWy-2 Wyoming low-charge montmorillonite was studied in the 250-400 °C temperature range with reactions lasting between 5 and 120 days, with a solid/solution mass ratio of 1:10, and in 1 mol/L KCl solution. From X-ray diffraction (XRD) profile modeling results on K-saturated and ethylene-glycol solvated samples the illitization process appears to occur as a progressive replacement of expandable layers by layers with illitic behavior, in a single illite-smectite phase. However this treatment overestimates the amount of illite layers because of the presence of smectitic non-expandable layers. This was revealed by calcium exchange of the products, which causes re-expansion of the apparent illite layers. The illitization model then obtained consists of four phases with increasing illite content: discrete smectite, a randomly interstratified mixed-layered mineral (MLM) and two ordered MLMs that progressively replace the phases with less illite content. This polyphase model is consistent with results from transmission electron microscopy and chemical microanalysis, which show several types of particle morphology with different interlayer K content and Al-for-Si substitution that seem to correspond to the several phases detected by XRD. Thus, the actual illitization mechanism of smectite in our experiments is not a progressive reaction but a dissolution-precipitation process following the Ostwald step rule in which metastable smectite transforms into illite through a series of metastable illite-smectite phases. [ABSTRACT FROM AUTHOR]

Abstract: Amino acid–smectite interaction may have catalyzed prebiotic reactions essential for the emergence of life. Lysine solutions (0.05 M) were reacted with Na-smectite in adsorption–desorption experiments. The lysine–smectite complexes were heated at 80 °C for 10 days to investigate (1) possible slow processes taking place at surface temperature that would be accelerated at higher temperature and (2) processes taking place in hydrothermal systems. Three sets of experiments were performed: thermal treatment in closed tubes and water added regularly; thermal treatment in closed tubes without adding water; and thermal treatment in open tubes and no added water. After lysine desorption (displacement with 0.1 M CaCl2), the solutions were investigated using circular dichroism (CD) and the smectite samples using FTIR and CHN elemental analysis. CD spectra were dependent on the solution pH, which was controlled by lysine protonation state. The lysine protonation state was altered by the adsorption–desorption process, with a higher Lys+/Lys± ratio after desorption. The CD and CHN analyses show that the thermal treatment in a moist state causes stronger smectite–lysine binding. FTIR data suggest that the stronger binding is caused by more or stronger H bonds between –NH+ 3 lysine groups and smectite basal O atoms. [Copyright &y& Elsevier]

The model of Fe3+ distribution between octahedra and tetrahedra in dioctahedral smectites by Decarreau and Petit (2014) used data from infrared analysis. From their own and other general evidence, the resulting data are likely to be affected by significant uncertainty. This aside, their model has limited application because it is based on synthetic smectites containing only Si, Al, and Fe3+. [ABSTRACT FROM AUTHOR]

Clay samples of hydrothermal origin from several oceanic spreading centers were studied using XRD, microprobe, infrared, and thermal analysis. They are talc, smectite, and mixed-layer talc-smectite (T-S) where the talc layers have several degrees of crystalline order. The smectite is trioctahedral in most cases, but there is also dioctahedral smectite both as mixed-layer and as a separate phase. All specimens contain Fe3+, some of them in moderate amounts (up to 17% Fe2O3) distributed between the tetrahedral and octahedral sheets (maximum values: IVFe = 0.32, VIFe = 0.68, per O10[OH]2). Octahedral Fe abundance correlates with the presence of molecular water that is lost in a well-defined dehydration event above 200 °C. This water does not cause layer expansion and is interpreted to be present within the pseudo-hexagonal cavity, next to Fe3+ ions that generate a local charge imbalance. The presence of octahedral Fe3+ is accompanied by vacancies in the octahedral sheet to balance the excess positive charge. An infrared band at ~790 cm-1 is assigned to OH bending in the group Fe-Mg-〉-OH. Analysis of this band suggests a range of short-range Fe-〉 distributions, from random to ordered. Our sequence talc, T-S, trioctahedral smectite is defined by an increasing Al for Si substitution in the tetrahedra and increasing crystal disorder. The presence of Fe also causes crystal defects. This mixed-layer series can be considered as a continuum generated by the combination of chemical and crystal defect variability. Kerolite was used to designate disordered, hydrated talc. We find that there is no clear line delimiting talc from kerolite as a single phase or in mixed-layer minerals and that it is better to use a descriptive term for the latter such as "disordered talc." Dioctahedral smectite is also a possible end-member of the mixed-layer sequence, which implies an Al + 〉 for Mg substitution in the octahedral sheet. If T-S consists of polar TOT layers, the existence of dioctahedral smectite in T-S raises the question of the actual composition of the octahedral sheets within polar TOT layers. [ABSTRACT FROM AUTHOR]

Nuclear waste from thermal plants poses a lasting risk to the biosphere because of its long radioactive life. The planned definitive storage place for it is in deeply buried repositories. Such repositories would need to be both impermeable to water, and plastic during deformation, in order to avoid the formation of cracks that may allow water in. One of the clay minerals, smectite, has these two properties and is an ideal candidate as a sealing material or even host rock for nuclear waste repositories. The chemical stability of smectite in the repository environment is sufficient to maintain good sealing properties during the active life of the relevant radionuclides. [ABSTRACT FROM AUTHOR]

Abstract: Talc, kerolite–smectite, smectite, chlorite–smectite and chlorite samples from sediments, chimneys and massive sulfides from six seafloor hydrothermal areas have been analyzed for mineralogy, chemistry and oxygen isotopes. Samples are from both peridotite- and basalt-hosted hydrothermal systems, and basaltic systems include sediment-free and sediment-covered sites. Mg-phyllosilicates at seafloor hydrothermal sites have previously been described as talc, stevensite or saponite. In contrast, new data show tri-octahedral Mg-phyllosilicates ranging from pure talc and Fe-rich talc, through kerolite-rich kerolite–smectite to smectite-rich kerolite–smectite and tri-octahedral smectite. The most common occurrence is mixed-layer kerolite–smectite, which shows an almost complete interstratification series with 5 to 85% smectitic layers. The smectite interstratified with kerolite is mostly tri-octahedral. The degree of crystal perfection of the clay sequence decreases generally from talc to kerolite–smectite with lower crystalline perfection as the proportion of smectite layers in kerolite–smectite increases. Our studies do not support any dependence of the precipitated minerals on the type/subtype of hydrothermal system. Oxygen isotope geothermometry demonstrates that talc and kerolite–smectite precipitated in chimneys, massive sulfide mounds, at the sediment surface and in open cracks in the sediment near seafloor are high-temperature (>250 °C) phases that are most probably the result of focused fluid discharge. The other end-member of this tri-octahedral Mg-phyllosilicate sequence, smectite, is a moderate-temperature (200–250 °C) phase forming deep within the sediment (∼0.8 m). Chlorite and chlorite–smectite, which constitute the alteration sediment matrix around the hydrothermal mounds, are lower-temperature (150–200 °C) phases produced by diffuse fluid discharge through the sediment around the hydrothermal conduits. In addition to temperature, other two controls on the precipitation of this sequence are the silica activity and Mg/Al ratio (i.e. the degree of mixing of seawater with hydrothermal fluid). Higher silica activity favors the formation of talc relative to tri-octahedral smectite. Vent structures and sedimentary cover preclude complete mixing of hydrothermal fluid and ambient seawater, resulting in lower Mg/Al ratios in the interior parts of the chimneys and deeper in the sediment which leads to the precipitation of phyllosilicates with lower Mg contents. Talc and kerolite–smectite have very low trace- and rare earth element contents. Some exhibit a negative or flat Eu anomaly, which suggests Eu depletion in the original hydrothermal fluid. Such Eu depletion could be caused by precipitation of anhydrite or barite (sinks for Eu2+) deeper in the system. REE abundances and distribution patterns indicate that chlorite and chlorite–smectite are hydrothermal alteration products of the background turbiditic sediment. [Copyright &y& Elsevier]

Abstract: The kinetics of smectite illitization in shale samples from eight wells have been modeled using the equation −dS/dt = kK 0.25 S 5 [Cuadros, J., Linares J., 1996. Experimental kinetic study of the smectite-to-illite transformation. Geochim. Cosmochim. Acta 60, 439–453], where S is the proportion of smectite layers in illite-smectite (I–S), t is time, k is the rate constant and K is the concentration of potassium. The I–S compositions and the physical parameters were obtained from the literature. Potassium concentration is the only unknown parameter and it was chosen to fit the experimental data. I propose that this variable is termed “effective K concentration,” as it is affected not by the actual K concentration alone but also by those of other cations that compete with K for the smectite interlayer sites, especially Ca, by ion mobility in the sediments and possibly by other variables. The kinetic equation allows an accurate fit to the experimental data in all the wells, reproducing both the illitization onset and the patterns of the plots of % smectite vs. depth. The relative changes of the values of “effective K concentration” compare well with K/Ca equivalent ratios in the whole rock. Three of the modeled wells, located in near proximity, produced similar “effective K concentration” patterns. These results support the validity of the equation used in the models. They also suggest that this equation can be used to assess K concentrations in sediments and K mobility within basins. [Copyright &y& Elsevier]

Investigates the formation of smectite under hydrothermal conditions in the aluminum-magnesium series. Types of smectite clays based on their charges; Models rationalizing kaolinite and smectite growth; Chemical analysis of gels; Difference in the growth rate of trioctahedral and dioctahedral smectite; Precursors to the growth of smectite.

Analyzes iron segregation in the octahedral sheet of bentonitic illite-smectite (I/S). Cation arrangement and order-disorder in minerals presenting isomorphous distributions; Existence of medium- or long-range ordering of cations in the octahedral sheets of the most illitic I/S samples.

Highlights a study which investigated the smectite-to-illite transformation mechanism through an analysis of mixed-layer illite-smectite from different locations in North America and Europe. Significance of this transformation mechanism; Details on the samples which were studied; Details on optical microscopy, scanning electron microscopy and X-ray diffraction of bulk rocks; Information on atomic force microscope analysis; Results of the study.

The dehydration dynamics of the of the Ca-saturated <1 µm size fraction of SWy-1 (low-charge montmorillonite) were studied at the crystal scale under isothermal conditions using X-ray diffraction with a position-sensitive detector (XRD-PSD) in the 30.170 °C temperature range. A total of 630 XRD patterns were modeled between 30 and 125 °C using a trial-and-error approach based on the direct comparison of experimental and calculated XRD patterns. The proportion of layers with different hydration states (bihydrated, mono-hydrated, and dehydrated) were determined in the temperature-time space as well as small variations of layer thickness within each hydration state. The results showed that dehydration produces complex structures with heterogeneous hydration states, some of which are stable (not transient) and remain at the end of the experiment. The evolution of other structural parameters (interlayer water content, layer thickness fluctuation) was consistent with previous reports of smectite hydration. For bihydrated layers, the amount of water molecules per interlayer cation indicated the presence of water molecules both coordinated and non-coordinated to the interlayer cation. The transition from bi- to mono-hydrated layers produced the maximum structural heterogeneity, with (1) strong interlayer thickness fluctuation (in individual layers), and (2) the presence of several elementary mixed-layer structures. In contrast, the transition from mono-hydrated to dehydrated layers occurs homogeneously within layers. Finally, the decrease in thickness of mono-hydrated layers only implied the removal of some water molecules forming the hydration shell of the interlayer cation. [ABSTRACT FROM AUTHOR]

A kinetic study of Ca-montmorillonite dehydration was performed based on information derived from X-ray diffraction (Ferrage et al. 2007, this issue) and, thus, focusing on interlayer water only. The dehydration was quantified following the two processes that were observed in the X-ray pattern modeling: the transitions between the different hydration states and small thickness decrease observed in the bi- and mono-hydrated layers. The thickness decrease of bihydrated layers with dehydration (activation energy Ea = 16 kJ/mol) was found to be controlled by a mechanism of two-dimensional diffusion of water molecules through the interlayer space, whereas for mono-hydrated layers the variation of thickness (Ea = 18 kJ/mol) occurred as a mechanism of slight local layer collapse and collapse propagation, attributed to a rearrangement of the configuration of the interlayer cation hydration shell. For the transition between the bi- and mono-hydrated state (Ea = 84 kJ/mol), the mechanism of reaction was found to evolve gradually with increasing temperature from local layer collapse and collapse propagation to a two-dimensional diffusion mechanism, as the forced diffusion of water molecules produced by the layer collapse transfers the control of the process to diffusion mechanism. This phenomenon causes the coexistence of two hydration states in a given interlayer. Finally, the transition between mono-hydrated and dehydrated layers (Ea = 132 kJ/mol) indicated the concomitance of water diffusion and local layer collapse and propagation mechanisms, although the structures were found to be homogeneous during this transition. The determination of both mechanisms and the activation energy for these processes were used to establish a model of smectite dehydration at the crystal scale. This model can be used to calculate crystal shrinkage and interlayer water content upon dehydration, and to predict the evolution of the system. [ABSTRACT FROM AUTHOR]

Abstract: The preparation of ultra-thin clay mineral films promises to open novel applications for more sensitive sensors, stronger plastics and other unexpected functionalities. However, most natural and synthetic clay particles necessarily need often tedious preparation procedures to remove crystalline impurities and to obtain completely exfoliated forms. Furthermore, the wide range of particle sizes characteristic of commonly available synthetic and natural clays can lead to variable properties and unexpected reactions. Here we report the synthesis of a homogenous monolayer-thick Zn-smectite that will benefit specific applications requiring ultra-thin and monodispersed layered nanomaterials. Furthermore, the fast and easy preparation overcomes the problems associated with the processing of both currently available synthetic and natural smectites. XRD, FE-SEM, and TEM analyses show the synthetic Zn-smectites to be relatively impurity free. Electron microscopy imaging illustrates its nanometer-sized dimension and very thin sheet-like morphology. Based on light scattering and small angle X-ray scattering measurements, it has a narrow size distribution (ave. ∼70nm) and a molecular-level thickness (∼0.7nm), respectively. This nanostructure combined with the interesting properties of Zn is anticipated to yield novel applications as a clay-based nanomaterial. [Copyright &y& Elsevier]