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In laboratory experiments, the precipitation of dolomite at ambient temperature is virtually impossible due to strong solvation shells of magnesium ions in aqueous media and probably also due to the existence of a more intrinsic crystallization barrier that prevents the formation of long-range ordered crystallographic structures at ambient surface conditions. Conversely, dolomite can easily form at high temperature (>100 °C), but its precipitation and growth requires several days or weeks depending on experimental conditions. In the present study, experiments were performed to assess how a single heat-ageing step promotes the formation of dolomite under high-carbonate alkaline conditions via dissolution-precipitation reactions. This reaction pathway is relevant for the so-called hydrothermal dolomite frequently observed in carbonate platforms, but still ill-defined and understood. Our precipitation route is summarized by two main sequential reactions: (1) precipitation of Mg-calcite at low temperature (∼20 °C) by aqueous carbonation of synthetic portlandite (Ca(OH) 2 ) in a highly alkaline medium (1 M of NaOH and 1 M of MgCl 2 ), leading to precipitation of oriented nanoparticles of low- and high-Mg calcite (∼79 wt%) coexisting with aragonite (∼18 wt%) and brucite (∼3 wt%) after 24 h; (2) fast dolomitization process starting from 1 h of reaction by a single heat-ageing step from ∼20 to 200, 250 and 300 °C. Here, the Mg-calcite acts as a precursor that lowers the overall kinetics barrier for dolomite formation. Moreover, it is an important component in some bio-minerals (e.g. corals and seashells). Quantitative Rietveld refinements of XRD patterns, FESEM observations and FTIR measurements on the sequentially collected samples suggest fast dolomite precipitation coupled with dissolution of transient mineral phases such as low-Mg calcite (Mg < 4 mol%), high-Mg calcite (Mg > 4 mol%), proto-dolomite (or disordered dolomite; Mg > 40 mol%) and Ca-magnesite. In this case, the dolomite formation rate and the time-dependent mineral composition strongly depend on reaction temperature. For example, high-purity dolomitic material (87 wt% of dolomite mixed with 13 wt% of magnesite) was obtained at 300 °C after 48 h of reaction. Conversely, a lower proportion of dolomite (37 wt%), mixed with proto-dolomite (43 wt%), Ca-magnesite (16 wt%) and high-Mg calcite (4 wt%), was obtained at 200 °C after 72 h. The present experiments provide an additional mechanism for the massive dolomite formation in sedimentary environments (ex. deep sea organic-rich carbonate-sediments) if such sediments are subjected to significant temperature variations, for example by hot fluid circulations related to volcanic activity. In such systems, organic degradation increases the carbonate alkalinity (HCO 3 − ) necessary to induce the dolomitization process at low and high temperature. [ABSTRACT FROM AUTHOR]

Potassium (K) is an essential plant nutrient mainly present in the crystal structure of K-bearing soil minerals (K-feldspars, micas). To assess the evolution of (clay) mineralogy due to K+ release from these minerals in continuously cultivated soils, s amples were collected from long-term (1904–2014) field experiments submitted to contrasting crop rotations and amendments, with and without fertilisation. Soil samples were size-fractionated and mineralogy of the silt fraction and of clay subfractions was determined quantitatively by modelling X-ray diffraction (XRD) patterns. Clay subfractions were also analysed for their cation exchange capacity (CEC). Mineralogical data indicate the stability of clay mineralogy and the increased abundance of the finest clay subfraction at the expense of coarser ones, regardless of agronomic practices or amount of K removed by plants. This increase is accompanied by an increase in the bulk clay CEC owing to the major contribution of the finest clay subfraction to this parameter. Mineral dissolution, rather than alteration of mineral phases from coarse clay subfractions, most likely supplements K for plant nutrition in this soil. Dissolution is favoured over cation exchange owing to the dioctahedral character of micas and to their fine-grained granulometry. The observed long-term mineralogical resilience also indicates that part of plant nutrition is obtained from subsoil in a non K-limited context, whereas K supply through K-feldspar dissolution appears marginal. Unlabelled Image • Evolution of clay mineralogy was quantified in long-term experimental plots. • Different agronomical practices lead to contrasting K balances. • Clay mineralogy is stable over time in all subplots independent of K balance. • K is supplied by the dissolution of illite-rich coarse clay fractions and by subsoil. • Dioctahedral character of fined-grained micas favours dissolution over K-exchange. [ABSTRACT FROM AUTHOR]

In the French north‐western Alps, several lakes of glacial origin, such as Aiguebelette and La Thuile, present some mass‐transport deposits within their sedimentary sequences. These event layers can result from lake sediment destabilization eventually triggered by earthquakes. On Lake Aiguebelette, based on sedimentological, geochemical and magnetic analyses, and high‐resolution seismic and bathymetric surveys a ca 1 m thick event layer was identified in the deepest lake basin and a synchronous ca 2 cm thick deposit in the shallow basin. Age‐depth models based on radiocarbon ages reveal that both mass‐transport deposits in Lake Aiguebelette occurred around the Younger Dryas – Early Holocene climatic transition (i.e. 11 700 cal bp) time range. In Lake La Thuile, located 30 km away, unique mass‐transport deposits (translational slide type) were recorded at the same time range in sedimentary records. Additionally, high‐resolution seismic profiles previously acquired in Lake Annecy and Lake Bourget support the hypothesis of significant mass‐transport deposits occurring at the Younger Dryas – Early Holocene time range. These outcomes on four north‐western peri‐Alpine and Alpine lakes highlight the regional occurrence of mass‐transport deposits in the Younger Dryas – Early Holocene time range. Seismic and rockfall events are discussed as potential sources of these significant and similarly aged mass‐transport deposits. Based on this study and a literature review, the authors suggest that mechanisms induced by rapid climate change and glacial retreat, such as crustal rebound and erosional unloading, could favour the triggering of earthquakes and rockfall events. In the case of mass‐transport deposits archived in north‐western Alpine lakes during this time period, this study favours the hypothesis of increased seismicity as the primary source driving process involved. [ABSTRACT FROM AUTHOR]

Ceria (CeO 2 ) crystalline compound is widely used as a catalyst or catalyst support and many other applications. However, the studies are continuing with a view to improving existing methods and/or developing innovative routes to obtain well-controlled shapes and sizes of ceria nanoparticles. In the present study, we report an original method to synthesize ceria nanoparticles (CeO 2 and CeO 2−x ) by using two independent stages: (1) the precipitation of bastnäsite-rich material under hydrothermal conditions (90 and 300 °C) and (2) the calcination of powdered basnäsite-rich material at different temperatures (500, 1000 and 1600 °C) and under different atmospheres (air, Ar, N 2 and secondary vacuum). In addition, simultaneous thermal analyses (TGA/DSC) coupled with gas chromatography (μGC/MS) were performed in order to investigate on the ceria formation during bastnäsite de-carbonation and its thermal behavior at high temperature (until 1600 °C) under three different gas atmospheres (air, Ar and N 2 ). Herein, ceria was in-situ formed independently on the gas investigated atmosphere. This means that sufficient oxygen was also available in the so-called inert atmospheres (Ar and N 2 ) to oxidize the Ce(III) contained in Ce-carbonates to Ce(IV) constituting ceria cubic structure. Moreover, μGC/MS measurements confirm that in-situ formed ceria CeO 2 at about 600 °C is then partially reduced to CeO 2−x at high temperature after 1000 °C because an increase in oxygen was clearly detected in expelled gas during heating process. This fundamental experimental study provides the hydrothermal conditions to which Ce fluorocarbonates could be formed in natural environments. In addition, new experimental conditions to produce ceria with oxygen vacancies CeO 2−x without reducing gas agent and at lower temperature are also provided in this study. [ABSTRACT FROM AUTHOR]

Kimberlites carry a dense cargo of olivine. Although normally referred to as macrocrysts, much of this olivine is present as multigrain xenoliths of dunite. Because of their well-rounded outlines, we refer to these objects as 'nodules' and apply the term to both multigranular and monogranular varieties. Although compositions within each nodule are very uniform, the entire population displays an enormous variation in Fo content (100*Mg/(Mg + Fe), cation proportions), from close to Fo95 to about Fo82. The change is continuous, and there is no indication of separate Fo-rich and Fo-poor populations. Likewise, the sizes, internal textures and habits of Fo-rich and Fo-poor olivines are indistinguishable. Fo-poor nodules have compositions like those of olivines of the megacryst suite, and they share similar sizes and internal structures. We propose that the dunite nodules in kimberlites and olivines of the megacryst suite have a common origin. In our interpretation, almost all olivines in kimberlites, except for the marginal rims and rinds, are fragments of dunite that was produced during interaction between mantle peridotite and CO2-rich fluid. Analogues of this process can be found in ophiolites where dunite is the product of reaction between basaltic magma and peridotite. In situ reactive crystallization eliminated pyroxene, leaving only olivine with Fo contents ranging from higher to lower Fo than in the host peridotite. This interaction occurred near the base of the lithosphere and not during transit of kimberlite towards the surface. [ABSTRACT FROM AUTHOR]

In volcanic environments, the presence of smectite may indicate recent hydrothermal circulations. Smectite is also responsible for enhanced rock electrical conductivity, as well as mechanical weakening. Therefore, quantifying smectite is important in geothermal exploration. Smectite identification requires X-ray diffraction (XRD) but quantification based on XRD is time-consuming and not always accurate. In the present study, we investigate the use of an optimized unbuffered Cation Exchange Capacity (CEC) determination, by back-titration of the Copper-triethylenetetramine(II) "Cu-trien" molecule, to quantify the smectite content of altered volcanic rock samples. We establish that a satisfying trade-off between the instrument uncertainty and an independant systematic error is theoretically reached for a fraction of reactants consumed of about 30% at the end of the exchange reaction. We suggest a modification to classical protocols to fall in that range. Finally, we show that optimized CEC determination by Cu-trien are a direct measure of the smectite weight fraction in altered volcanic samples, with an average CEC of pure smectite of 90 ± 5 meq/100 g. [ABSTRACT FROM AUTHOR]

Global warming leads to drastic glaciers shrinkage worldwide, hence affecting the global water balance. Small mountain glaciers are the most widespread types of glaciers but have received less attention compared to larger ones, despite their importance for the regional hydrological cycle. To better understand the forcing underlying their dynamics, we investigated the glacial activity in the Hengduan Mountains in the SE Tibetan Plateau over the last 2000 years by analysing sediments from the Yunzhu proglacial lake. Five sediment cores were retrieved and dated using short-lived radionuclides, 14C, and analyses of Earth's magnetic field palaeosecular variations. A multiproxy analysis was performed on sediment cores, including geochemical, sedimentological, and hyperspectral analyses. As the sediment was relatively homogeneous and poor in organic material, geochemical ratios and pigment analysis were used to track the evolution of terrigenous inputs and lake photosynthetic activity. Sedimentation is dominated by clastic inputs, which are traced by K flux, while the authigenic components are traced by chlorophyll pigments. During the Little Ice Age (LIA), anoxic conditions prevailed at the bottom of the lake, leading to the preservation of millimetric laminae in the sediment and a bloom of purple sulphur bacteria, visible through abundant bacteriochlorophyll a pigment. We attribute this to persisting ice cover preventing water column mixing over most of the year. During cold periods such as the LIA or the Dark Ages, the relative amount of terrigenous inputs increased at the expense of lake bioproductivity. We interpret the observed erosion increases during cold periods as the consequence of glacial activity, consistent with other sparse glacial records available on the Tibetan Plateau. We show that glacial extent was mainly controlled by temperature in the past two millennia rather than by precipitation, yielding potential widespread melting in the next decades as well as more irregular water supply downstream. • Increase of lake's terrigenous flux during glacial advances. • We document glacial advances during the Dark Ages and an early onset of the Little Ice Age. • Temperature and solar irradiance controlled small glacier's activity on the Tibetan Plateau. • During cold periods, ice cover led to anoxic conditions at the lake bottom. [ABSTRACT FROM AUTHOR]

Layered double hydroxides (LDHs) are anion exchangers with a strong potential to scavenge anionic contaminants in aquatic environments. Here, the uptake of selenite (SeO32-) by Ca-Al LDHs was investigated as a function of Se concentration. Thermodynamic modeling of batch sorption isotherms shows that the formation of SeO32--intercalated AFm (hydrated calcium aluminate monosubstituent) phase, AFm-SeO3, is the dominant mechanism controlling the retention of Se at medium loadings. AFm-Cl2 shows much stronger affinity and larger distribution ratio (Rd ~ 17800 L kg-1) toward SeO32- than AFm-SO4 (Rd ~ 705 L kg-1). At stoichiometric SeO32- loading for anion exchange, the newly formed AFm-SeO3 phase results in two basal spacing, i.e., 9.93 ± 0.06 Å and ~11.03 ± 0.03 Å. Extended X-ray absorption fine structure (EXAFS) spectra indicate that the intercalated SeO32- forms inner-sphere complexes with the Ca-Al-O layers. In situ X-ray diffraction (XRD) shows that basal spacing of Ca-Al LDHs have a remarkable linear relationship with the size of hydrated intercalated anions (i.e., Cl-, SO42-, MoO42-, and SeO32-). Contrary to AFm-SeO3 with inner-sphere SeO32- complexes in the interlayer, the phase with hydrogen-bonded inner-sphere complexed SeO32- is kinetically favored but thermodynamically unstable. This work offers new insights about the determination of intercalated anion coordination geometries via XRD analyses. [ABSTRACT FROM AUTHOR]

Layered double hydroxides (LDHs) have been considered as effective phases for the remediation of aquatic environments, to remove anionic contaminants mainly through anion exchange mechanisms. Here, a combination of batch isotherm experiments and X-ray techniques was used to examine molybdate (MoO42-) sorption mechanisms on CaAl LDHs with increasing loadings of molybdate. Advanced modeling of aqueous data shows that the sorption isotherm can be interpreted by three retention mechanisms, including two types of edge sites complexes, interlayer anion exchange, and CaMoO4 precipitation. Meanwhile, Mo geometry evolves from tetrahedral to octahedral on the edge, and back to tetrahedral coordination at higher Mo loadings, indicated by Mo K-edge X-ray absorption spectra. Moreover, an anion exchange process on both CaAl LDHs was followed by in situ time-resolved synchrotron-based X-ray diffraction, remarkably agreeing with the sorption isotherm. This detailed molecular view shows that different uptake mechanisms edge sorption, interfacial dissolution-reprecipitation are at play and control anion uptake under environmentally relevant conditions, which is contrast to the classical view of anion exchange as the primary retention mechanism. This work puts all these mechanisms in perspective, offering a new insight into the complex interplay of anion uptake mechanisms by LDH phases, by using changes in Mo geometry as powerful molecular-scale probe. [ABSTRACT FROM AUTHOR]

Ferroan brucite, (Mg,Fe)(OH) 2 , is a common mineral product of serpentinization reactions. The alteration of ferroan brucite under subsurface conditions is expected to form magnetite and hydrogen (H 2). Ferroan brucite, (Mg 0.8 Fe 0.2)(OH) 2 , with a grain size of 20–100 nm, was synthesized by precipitation of iron salts in the presence of NaOH under sub-anoxic conditions. Both composition and grain size of the synthetic product are similar to those of ferroan brucite found in serpentinized peridotites collected during the Oman Drilling Project. Synthetic ferroan brucite was then reacted in aqueous solutions at 378 and 403 K during 1 to 36 days either in PTFE-lined reactors or in gold capsules placed in externally heated pressure vessels. In gold capsules, ferroan brucite barely reacted and minor magnetite and H 2 were produced. In PTFE-lined reactors, reaction progress over 75% could be achieved with reaction products composed of magnetite, pyroaurite [Mg 6 Fe 2 (OH) 16 (CO 3)•4.5H 2 O], and a new phase, identified as ferrian brucite [(Mg 0.8 FeIII 0.2)O 0.2 (OH) 1.8 ] based on the Rietveld refinement of its X-ray powder diffraction data, FeIII/Fe tot colorimetric determination and thermogravimetric analysis. Ferrian brucite is isostructural to ferroan brucite and displays the same iron/magnesium ratio. However, all iron is trivalent. X-ray diffraction and thermogravimetric data support deprotonation as the mechanism responsible for charge compensating FeII oxidation (Fe2+ + OH−=Fe3+ + O2−). The difference in reaction products obtained with the two types of reactors is attributed to the higher permeability to H 2 (and/or O 2) of the PTFE-lined reactors compared to gold capsules. Ferroan brucite conversion into ferrian brucite was unexpectedly fast and proceeded in a few days. The thermodynamic stability of ferrian brucite is discussed as a function of oxygen fugacity and pH. • Reaction of synthetic ferroan brucite, a serpentinization product, at temperatures below 403 K. • Ferrian brucite is formed from ferroan brucite oxidation under intermediate oxygen fugacity. • Ferrian brucite is isostructural to ferroan brucite and has the same Fe/Mg ratio. • Deprotonation of ferrian brucite layer is responsible for charge compensating FeII oxidation. • In the presence of common anions (Cl−, CO 3 2−), LDHs are formed from ferroan brucite oxidation. [ABSTRACT FROM AUTHOR]

Heterosite FePO 4 is synthesized for the first time by direct thermal oxidation of sarcopside Fe 3 (PO 4 ) 2 . Both FePO 4 and Fe 3 (PO 4 ) 2 have a pseudo olivine structure. Complete isostructural conversion of sarcopside into FePO 4 is achieved at a temperature of 450 °C within 3 days according to the reaction Fe 3 (PO 4 ) 2 + ¾ O 2 → 2 FePO 4 + ½ Fe 2 O 3 which leads to the extraction of iron from the sarcopside structure. Appropriate heating ramp must be applied in order to avoid the crystallization of Fe 7 (PO 4 ) 6 . Electrochemical performances of the oxidation product are consistent with those of olivine FePO 4 . [ABSTRACT FROM AUTHOR]

The kinetics of the irreversible reaction Ca(OH)2+MgCO3 → CaCO3+Mg(OH)2 were investigated at high pressures and temperatures relevant to metamorphic petrology, using both in situ synchrotron X-ray diffraction and post-mortem analysis of reaction rim growth on recovered samples. Reaction kinetics are found to strongly depend on water content; comparable bulk-reaction kinetics are obtained under water-saturated (excess water, c. 10 wt %) and under intermediate (0.1-1 wt % water) conditions when temperature is increased by c. 300 K. In contrast, similar reaction kinetics were observed at ~673 K and 823 K between intermediate and dry experiments, respectively, where dry refers to a set of experiments with water activity below 1.0 (no free water), as buffered by the CaO-Ca(OH)2 assemblage. Given the activation energies at play, this gap-- corresponding to the loss of no more than 1 wt % of water by the assemblage--leads to a difference of several orders of magnitude in reaction kinetics at a given temperature. Further analysis, at the microscopic scale, of the intermediate and dry condition samples, shows that intergranular transport of calcium controls the reaction progress. Grain boundary diffusivities could be retrieved from the classic treatment of reaction rim growth rate. In turn, once modeled, this rate was used to fit the bulk kinetic data derived from X-ray powder diffraction, offering an alternative means to derive calcium diffusivity data. Based on a comparison with effective grain boundary data for Ca and Mg from the literature, it is inferred that both dry and intermediate datasets are consistent with a water-saturated intergranular medium with different levels of connectivity. The very high diffusivity of Ca in the CaCO3+Mg(OH)2 rims, in comparison with that of Mg in enstatite rims found by earlier workers, emphasizes the prominent role of the interactions between diffusing species and mineral surfaces in diffusion kinetics. Furthermore, we show that the addition of water is likely to change the relative diffusivity of Mg and Ca in carbonate aggregates. From a qualitative point of view, we confirm, in a carbonate-bearing system, that small water content variations within the 0-1wt % range have tremendous effects on both intergranular transport mechanisms and kinetics. We also propose that the water content dependent diffusivity of major species (Mg, Ca) in low-porosity metamorphic rocks is strongly dependent on the interaction between diffusing species and mineral surfaces. This parameter, which will vary from one rock-type to another, needs also to considered when extrapolating (P, T, t, xH2O) laboratory diffusion data to metamorphic processes. [ABSTRACT FROM AUTHOR]

The objective of this work was to investigate the fate of silver nanoparticles (Ag-NPs) in a sludge-amended soil cultivated with monocot (Wheat) and dicot (Rape) crop species. A pot experiment was performed with sludges produced in a pilot wastewater treatment plant containing realistic Ag concentrations (18 and 400 mg kg-1, 14 mg kg-1 for the control). Investigations focused on the highest dose treatment. X-ray absorption spectroscopy (XAS) showed that Ag2S was the main species in the sludge and amended soil before and after plant culture. The second most abundant species was an organic and/or amorphous Ag--S phase whose proportion slightly varied (from 24% to 36%) depending on the conditions. Micro and nano X-ray fluorescence (XRF) showed that Ag was preferentially associated with S-rich particles, including organic fragments, of the sludge and amended soils. Ag was distributed as heteroaggregates with soil components (size ranging from ≤0.5 to 1-3 μm) and as diffused zones likely corresponding to sorbed/complexed Ag species. Nano-XRF evidenced the presence of mixed metallic sulfides. Ag was weakly exchangeable and labile. However, micronutrient mobilization by plant roots and organic matter turnover may induce Ag species interconversion eventually leading to Ag release on longer time scales. Together, these data provide valuable information for risk assessment of sewage sludge application on agricultural soils. [ABSTRACT FROM AUTHOR]

Layer charge deficit is a key crystal-chemical parameter for the classification and nomenclature of 2:1 phyllosilicates that also controls some of their fundamental properties such as cation exchange capacity, expandability, water content, or rheology. The clay community has thus devoted significant efforts to determine this parameter either from the smectite/vermiculite chemical composition (the structural formula method) or experimentally using the alkylammonium method, that theoretically allows the determination of the mean layer charge density and of its distribution. In the present study density functional theory (DFT) and molecular dynamics (MD) simulations are compared to experimental X-ray diffraction data for a series of synthetic saponites with layer charge ranging from medium-charge smectite to medium-charge vermiculite. Consistency of computed and measured layer-to-layer distances confirms the ability of computational approaches to accurately predict the organization of alkylammonium cations in smectite interlayers. Thermalization of DFT-optimized structure models through MD simulations allows probing possible alternative interlayer configurations such as the electrostatically-favored location of ammonium heads above/below Al-substituted tetrahedra. X-ray diffraction results showed that layer-to-layer distances intermediate between those corresponding to the stable h1, h2, etc. interlayer configurations described in the literature result from the interstratification of different stable configurations. The overall consistency of computational and experimental results confirms also the ability of the alkylammonium method to accurately determine a mean value of layer charge density consistent with smectite structural formula when using the revised equations proposed by Laird et al. (1989). The validity of the method appears however limited to smectite-group minerals [layer charge density ranging from ~0.5 to ≤1.2 per O 20 (OH) 4 ] most likely owing to the coexistence of both stable "layered" and paraffin-like interlayer configurations for higher values of layer charge. In addition, this consistency challenges the ability of the method to quantitatively describe distributions of layer charge in expandable phyllosilicates even when using the complete series of alkylammonium cations. • Layer charge of synthetic saponites is assessed using the alkyl ammonium method (AAM). • Variation of layer-to-layer distances with alkyl chain length is compared with simulations. • Consistency of experimental and computational data confirms the robustness of the AAM. • The AAM appears unable to determine distribution of layer charge. • The AAM appears unable to differenciate high values of layer charge (vermiculites). [ABSTRACT FROM AUTHOR]

Fossil mesofauna and bacteria recovered from a paleosol in a moraine situated adjacent to the inland ice, Antarctica, and dating to the earliest glacial event in the Antarctic Dry Valleys opens several questions. The most important relates to understanding of the mineralogy and chemistry of the weathered substrate habitat in which Coleoptera apparently thrived at some point in the Early/Middle Miocene and perhaps earlier. Here, Coleoptera remains are only located in one of six horizons in a paleosol formed in moraine deposited during the alpine glacial event (>15 Ma). A tendency for quartz to decrease upward in the section may be a detrital effect or a product of dissolution in the early stage of profile morphogenesis when climate was presumably milder and the depositing glacier of temperate type. Discontinuous distributions of smectite, laumontite, and hexahydrite may have provided nutrients and water to mesofauna and bacteria during the early stage of biotic colonization of the profile. Because the mesofauna were members of burrowing Coleoptera species, future work should assess the degree to which the organisms occupied other sites in the Dry Valleys in the past. Whereas there is no reasonable expectations of finding Coleoptera/insect remains on Mars, the chemistry and mineralogy of the paleosol is within a life expectancy window for the presence of microorganisms, principally bacteria and fungi. Thus, parameters discussed here within this Antarctic paleosol could provide an analogue to identifying similar fossil or life-bearing weathered regolith on Mars. [ABSTRACT FROM AUTHOR]

Abstract: The present study reports original experiments in order to investigate the simultaneous serpentinization and carbonation of olivine with relevance in Earth systems (e.g. functioning of hydrothermal fields) or in engineered systems (e.g. ex-situ and in-situ mineral sequestration of CO2). For this case, specific experimental conditions were examined (200°C, saturated vapor pressure≈16bar, solution/solid weight ratio=15, olivine grain size<30μm and high-carbonate alkalinity≈1M NaHCO3). Under these conditions, competitive precipitation of magnesite and serpentine (preferentially lizardite type) was clearly determined by using conventional analytic tools (XRD, FESEM, FTIR and TGA); excluding the fate of the iron initially contained in olivine, the alteration reaction for olivine under high-carbonate alkalinity can be expressed as follows: This reaction mechanism implied a dissolution process, releasing Mg and Si ions into solution until supersaturation of solution with respect to magnesite and/or serpentine. The released iron contained in the olivine has not implied any precipitation of iron oxides or (oxy)hydroxides; in fact, the released iron was partially oxidized (about 50%) via a simple reduction of water (2Fe2+ +2H2O→2Fe3+ +H2 +2OH−). In this way, the released iron was incorporated in serpentine (Fe(II) and Fe(III)) and in magnesite (Fe(II). The latter was clearly determined by FESEM/EDS chemical analysis on the single magnesite crystals. The nucleation and epitaxial growth processes at the olivine–fluid interfaces cannot be excluded in our investigated system. The experimental kinetic data fitted by using a kinetic pseudo-second-order model have revealed a retarding process of serpentine formation with respect to magnesite (about three times slower); in fact, the magnesite seems to reach an apparent stabilization after about 20days of reaction while the serpentine follows a progressive slower evolution. We assumed that the magnesite has reached a fast apparent equilibrium with solution because the available carbonate species are not renewed from fluid phase as typically constrained in aqueous carbonation experiments where a given CO2 pressure is imposed in the system. On the other hand, the reactivity of serpentinized olivine (chrysotile+brucite+small amount of residual olivine) and high-purity chrysotile at the same above investigated conditions; and the olivine serpentinization in initial acid pH≈0.66 are also reported as complementary information in this study. These novel experimental results concerning simultaneous serpentinization and aqueous carbonation of olivine expand the thermodynamic conditions where serpentine and magnesite can simultaneously precipitate; this could contribute to a better understanding of fluid–rock interactions in natural active hydrothermal fields on Earth. [Copyright &y& Elsevier]

The study of paleofluvial dynamics is crucial to understand the role of rivers as biogeographic boundaries in Amazonia during the Cenozoic. In central Amazonia, Mesozoic and Cenozoic fluvial deposits - Alter do Chão, Iranduba and Novo Remanso Formations - host supergene iron oxides and record changes in the distribution of flooded and non-flooded (upland) environments. Geochronological data on these deposits are still scarce to constrain past landscape changes. Therefore, in this study we investigate iron oxides precipitated within weathering profiles developed on ancient fluvial terraces to access the interplay between flooded and non-flooded environments in central Amazonia. We aimed to trace the history of abandonment of alluvial plains and the subsequent weathering of lowland sediment deposits during the Cenozoic. We identified at least two main periods of iron oxide precipitation: (1) one starting before ~42 Ma and ending at ~18 Ma; and (2) a well-defined humid and weathering prone phase between ~8 and 1 Ma. Dominant goethite precipitation marks a major climatic shift towards more humid conditions from ~3 Ma. The increase in water discharge of the Negro and Solimões rivers possibly promoted fluvial incision and conversion of floodplains into long-lasting upland terrains as indicated by the development of lateritic weathering profiles. This major phase of upland expansion corroborates upland birds phylogenetic data, which indicate the emergence of a major biogeographic barrier in central Amazonia during late Pliocene/Pleistocene. • Refining the age of the fluvial sedimentary units in central Amazonia • Multiple Cenozoic discrete weathering phases in central Amazonia • Linking weathering phases with fluvial landscape changes • Constraining the emergence of a major biogeographic boundary [ABSTRACT FROM AUTHOR]

Geological storage of CO2 in mafic and ultramafic rocks relies on the dissolution of their silicate components, followed by the precipitation of carbonates, the overall process being commonly referred to as carbonation. To gain a better understanding of the rate- and yield-controlling factors of mineral carbonation, three batch experiments were conducted in CO2-saturated water, at (a) 90°C, (b) 120°C and (c) 170°C, respectively (with citrate ligands added to the 90°C experiment) in a Ti-reactor at pCO2 =280bar, using San Carlos olivine (Fo88) as a model mineral of (ultra)mafic environments. Those physicochemical conditions were purposely chosen so as to promote the dissolution rate of olivine. (a) At 90°C, ~40wt.% of olivine were dissolved within 3weeks, a result which suggests that the passivation barrier evidenced in previous studies carried out in citrate-free media had been overcome. However, while saturation with respect to magnesite was overstepped in this experiment, the carbonation yield remained below ~1wt.%, which we attributed to the slow kinetics of magnesite precipitation. (b) At 120°C, the concentration of Mg and Si monitored by aqueous fluid sampling reached an apparent plateau for over 4weeks, at conditions close to saturation with respect to amorphous silica, and with Fe concentration being below the detection limit. A resumption of the dissolution process was subsequently observed while the concentration of Fe suddenly increased in solution. The concentration plateau was attributed either to the formation of a protective Si–Fe(III) layer, or to the occurrence of Fe(III) phases clogging the porosity of a secondary silica layer. The resumption of olivine dissolution was ascribed to the increasingly reducing conditions that led to the breakdown of the protective phases. This late resumption led to a carbonation yield slightly above 2wt.%. (c) At 170°C, Fe was incorporated into a presumably permeable Fe-rich phyllosilicate layer, so that the dissolution process was likely less affected by such an interfacial layer. Nevertheless, carbonate minerals still formed in low quantity (~6wt.%), while other amorphous phases represented the main sink for Mg cations. Overall, this study emphasizes that achieving carbonation reactions in reducing environments and circum-neutral aqueous solutions may represent a necessary requirement for making olivine carbonation a viable process in these temperature ranges. It also insists on the importance of transitory phases, which form both at the fluid-silicate interface and from the bulk solution. While they may not appear in overall carbonation equations or in thermodynamic databases, arguably they can drive the global kinetics of olivine dissolution and its transformation towards magnesite. [ABSTRACT FROM AUTHOR]

Abstract: A novel method for producing hydrogen with water and BOF steel slag was developed. The steel slag was reacted with water during 2–57 days at 50 MPa for temperatures ranging from 473 to 673 K. The quantitative evolution of the slag and gas compositions indicated that the main H2 producing reaction is: 3FeO + H2O → Fe3O4 + H2 At 523 K, approximately 5 NL of H2 per kg of slag were produced in 3 days. The reaction rate was only 1.5 times faster when the slag was crushed down to an initial particle size below 50 μm. The H2 production has been also tested on slags carbonated beforehand at 0.142 ± 0.002 kg of CO2 per kg of slag. The reaction was found to be thermally activated. A high purity hydrogen (99.995%) is produced with non-carbonated steel slag below 573 K whereas CH4 production was measured in all the other experiments. [Copyright &y& Elsevier]

Herein, we report new insights into the nucleation and growth processes of chrysotile nanotubes by using batch and semi-continuous experiments. For the synthesis of this highly carcinogenic material, the influences of temperature (90, 200, and 300 °C), Si/Mg molar ratio, and reaction time were investigated. From the semi-continuous experiments (i.e., sampling of the reacting suspension over time) and solid-state characterization of the collected samples by XRPD, TGA, FTIR spectroscopy, and FESEM, three main reaction steps were identified for chrysotile nucleation and growth at 300 °C: 1) formation of the proto-serpentine precursor within the first 2 h of the reaction, accompanied by the formation of brucite and residual silica gel; 2) spontaneous nucleation and growth of chrysotile between about 3 and 8 h reaction time, through a progressive dissolution of the proto-serpentine, brucite, and residual silica gel; and 3) Ostwald ripening growth of chrysotile from 8 to 30 h reaction time, as attested to by BET and FESEM measurements. Complementary results from batch experiments confirmed a significant influence of the reaction temperature on the kinetics of chrysotile formation. However, FESEM observations revealed some formation of chrysotile nanotubes at low temperatures (90 °C) after 14 days of reaction. Finally, doubling the Si/Mg molar ratio promoted the precipitation of pure smectite (stevensite-type) under the same P (8.2 MPa)/ T (300 °C)/pH (13.5) conditions. [ABSTRACT FROM AUTHOR]

Abstract: Olivine mineral replacement by serpentine is one major alteration reaction of oceanic hydrothermalism. In the present experimental study, olivine grains were replaced by chrysotile and brucite under high alkaline conditions. In our study, olivine replacement implied a spatial and temporal coupling of dissolution and precipitation reactions at the interface between olivine and chrysotile–brucite minerals. Coupled dissolution–precipitation led to the alteration of starting olivine grains (so-called primary or parent mineral) to a porous mineral assemblage of chrysotile and brucite with preservation of the initial olivine morphology. This mineral replacement reaction of olivine (serpentinization) has been characterized using XRD, FESEM and FTIR measurements. Moreover, a simple and novel method is here proposed to quantify the mineral replacement rate (or serpentinization rate) of olivine using thermogravimetric (TG) and differential TG (DTG) analyses. Serpentinization extent depends on the grain size: it is complete after 30 days of reaction for the smallest olivine grains (<30μm), after 90 days of reaction for the intermediate olivine grains (30μm–56μm). For the largest fraction (56–150μm), 55% of serpentinization extent was reached after 90 days. Based on the fitting of the serpentinization extent (ξ t ) versus time (t) using a kinetic pseudo-second-order model, the serpentinization rates vary from 3.6×10−6 s−1 to 1.4×10−7 s−1 depending on the olivine grain size. An additional correlation between FTIR spectra analysis and TG measurements is proposed. The mineral replacement reactions frequently observed in natural alteration processes could be a powerful synthesis route to design new porous and/or nanostructured materials. [Copyright &y& Elsevier]

Summary: We explore the use of seismic noise produced by rivers to monitor the bed load transport in the case of a low-discharge braided river in the French Alps: the “torrent de St Pierre”. For this purpose, we deployed two dedicated seismic networks during summers 2007 and 2008, for which the characteristics of the recorded continuous signal are similar despite changes in the sensor locations. For dry weather conditions, only melting of nearby glaciers controls the supply of water to the stream. In these conditions, the river hydrology and the seismic energy in the 2–80Hz frequency band both follow a diurnal fluctuation similar to the thermal amplitude. In contrast during rainfall episodes, the temperature variation fails to explain the hydrodynamic changes. Dense cloud covers reduce glacier melting and the recorded seismic energy denotes bursts of high-frequency seismic noise well correlated with water level data. Comparisons between the recorded seismic signals and the collected hydrological and sediment load data indicate that a frequency band of 3–9Hz best explains the water level changes and thus the seismic waves coming from the flow turbulence. These analyses also reveal the presence of a seismic noise threshold that might be linked to the water shear stress exerted by the flowing water. Using the seismic energy in this frequency band as a proxy of the fluvial shear stress, the seismic–hydrologic relationship may be sensitive to variations in bed load transport. The spectral content of the seismic energy shows patterns consistent with the mobilization of sediment particles. From the interpretations of the seismic wave attenuation of river sources, we finally propose that stations at a distance from the stream less than 50m are able to record most sediment particles. Farther stations are still useful during extreme events when largest grain sizes are mobilized. More generally this study demonstrates the feasibility of using the river seismic signal to survey bed load transport in various river types from small braided mountain rivers like the “torrent de St Pierre” to the large entrenched Himalayan rivers. [Copyright &y& Elsevier]

Abstract: Carbonation reactions of wollastonite have been investigated in experiments conducted at conditions relevant to geologic CO2 sequestration in subsurface environments (T =90 °C and pCO2 =25 MPa). Experiments were performed in batch reactors on single-mineral powders of selected grain size, either in supercritical CO2 or in aqueous solutions with different alkalinities. The main reaction products were calcite and amorphous silica. Calcite occurred either as a compact, continuous coating of small crystals in the case of supercritical CO2 and in circum-neutral pH aqueous solutions, or as isolated larger crystals in acidic aqueous solutions. Measured extents of carbonation were faster in aqueous solutions than in supercritical CO2. Extents of carbonation in aqueous acidic solutions were modelled using a geochemical code incorporating kinetic parameters for wollastonite dissolution and calcite precipitation taken from the literature. Comparison with experimental results shows that, in aqueous solutions, wollastonite dissolution is the rate-limiting step of the carbonation reaction and that silica layers have only a minor passivation effect on the overall reaction rate. In acidic solutions, secondary calcite coating has only a minor effect on carbonation extent, whereas the experimental data in circum-neutral pH aqueous solutions are consistent with dissolution rates inhibited by the observed dense calcite coating. A geochemical code incorporating a modified reactive surface model successfully accounted for this passivation effect. The more efficient carbonation in acidic than in circum-neutral pH solutions is thus ascribed to differences in microstructures of the secondary calcite layers due to differences in supersaturation states of calcite when its precipitation begins. Inhibitory effects of dense calcite coating are also invoked for limiting carbonation extents of wollastonite in supercritical CO2. Secondary calcite precipitations are thus shown to play a major role both as coatings of reactive surfaces and by maintaining large undersaturation with respect to wollastonite dissolution. [Copyright &y& Elsevier]

Abstract: Carbonation reactions of wollastonite have been investigated in experiments conducted at conditions relevant to geologic CO2 sequestration in subsurface environments (T =90 °C and pCO2 =25 MPa). Experiments were performed in batch reactors on single-mineral powders of selected grain size, either in supercritical CO2 or in aqueous solutions with different alkalinities. The main reaction products were calcite and amorphous silica. Calcite occurred either as a compact, continuous coating of small crystals in the case of supercritical CO2 and in circum-neutral pH aqueous solutions, or as isolated larger crystals in acidic aqueous solutions. Measured extents of carbonation were faster in aqueous solutions than in supercritical CO2. Extents of carbonation in aqueous acidic solutions were modelled using a geochemical code incorporating kinetic parameters for wollastonite dissolution and calcite precipitation taken from the literature. Comparison with experimental results shows that, in aqueous solutions, wollastonite dissolution is the rate-limiting step of the carbonation reaction and that silica layers have only a minor passivation effect on the overall reaction rate. In acidic solutions, secondary calcite coating has only a minor effect on carbonation extent, whereas the experimental data in circum-neutral pH aqueous solutions are consistent with dissolution rates inhibited by the observed dense calcite coating. A geochemical code incorporating a modified reactive surface model successfully accounted for this passivation effect. The more efficient carbonation in acidic than in circum-neutral pH solutions is thus ascribed to differences in microstructures of the secondary calcite layers due to differences in supersaturation states of calcite when its precipitation begins. Inhibitory effects of dense calcite coating are also invoked for limiting carbonation extents of wollastonite in supercritical CO2. Secondary calcite precipitations are thus shown to play a major role both as coatings of reactive surfaces and by maintaining large undersaturation with respect to wollastonite dissolution. [Copyright &y& Elsevier]

In natural environments, Acid Volatile Sulfides (AVS) contained in anoxic waters or sediments, are composed of dissolved sulfides and neo-formed sulfides colloids or particles. Under acidic addition, AVS emit hydrogen sulfide gas and release the so-called simultaneously extracted metals (SEM). The measurement of AVS coupled with that of the SEM enables to evaluate the metal trapping capacity of sulfides in the environment. Because AVS are extremely reactive to oxidation, the most accurate methodology to quantify AVS and SEM requires to be able to process the samples extraction on-site, directly after sampling and avoiding oxygen exposure. However, most of available systems are based on glassware 'purge and trap' techniques developed for the laboratory and are not often adapted to field studies. In these systems, AVS extraction time can range from 30 min to 3 h with relative standard deviation from 7 to 44%. In this study, we developed a new 'purge and trap' system designed for both laboratory use and field AVS/SEM extractions. The system is optimized with a shortened extraction time, miniaturized, unbreakable, easy and reproducible to develop parallel extraction benches. Analytical yields, precision and stability have been improved, allowing to reduce the extraction time to 1 h with an absolute quantification limit of 0.12 μmol S(-II) with a relative standard deviation between 7 and 11% and under a complete extraction efficiency. [Display omitted] • A transportable and miniaturized AVS and SEM 'purge and trap' extraction system. • An AVS and SEM system unbreakable and easily duplicable. • AVS shortened extraction time. [ABSTRACT FROM AUTHOR]

This paper revisits the clay mineralogy of the "smectite" alteration zone in the Krafla geo-thermal field via the study of an exploratory well in which temperatures range from 40 °C to 215 °C. The clay alteration consists of several mineral assemblages superimposed in time and space, resulting from different stages of water-rock interaction. Trioctahedral clay minerals (chlorite, corrensite and smectite) are observed throughout the studied section. These minerals can form in nearly closed systems as replacements of groundmass minerals/glass after interactions with resident and nearly stagnant fluids not far from chemical equilibrium (neutral to basic pH conditions) or from direct precipitation from geothermal fluids. They are locally superimposed by Al clay phases (smectite, illite/smectite and kaolinite), which result from intense leaching of the host rocks due to their interaction with low pH fluids under strong W/R ratios. The absence of mineralogical zoning is explained by the fact that hydrothermal alteration is strongly dependent on very recent hydrodynamics. The current fluid circulation generates trioctahedral clays at depth that cannot be distinguished from pervasive earlier alteration. The only easily detectable signature of current activity and the most relevant signature for geothermal exploration is the presence of Al dioctahedral phases since it indicates leaching and intense hydrothermal activity. [ABSTRACT FROM AUTHOR]

Potassium feldspars (KAlSi 3 O 8) are ubiquitous minerals in the Earth's upper crust. This family of minerals has been the subject of numerous experimental and theoretical investigations concerning their dissolution kinetics and the mechanisms controlling chemical alteration at acid and neutral pH, and at temperatures ranging from ambient to hydrothermal conditions. On the other hand, considerably less research on the dissolution behavior of K-feldspars has been carried out at alkaline conditions, in particular at pH > 9 and elevated temperatures. Filling in this gap in knowledge is the major motivation for this study. More specifically, we wanted to document and understand how the K-feldspar interface structurally and chemically evolves during alteration in order to determine the mechanism of dissolution. In this study we examined interfaces of orthoclase samples that were altered in separate experiments in a Ca(OH) 2 -H 2 O solution (pH 25°C 12.4) at 190 °C for 24 h. We used a combination of focused ion beam (FIB) milling and advanced analytical transmission electron microscopy (TEM) techniques to investigate the structure and chemistry of the near surface region of post-reaction grains, with particular attention being given to the fluid-solid interface. Even though each grain diminishes in volume due to dissolution, high-resolution TEM imaging indicates that the feldspar structure itself remains completely intact and crystalline, as evidenced by lattice fringes that abruptly terminate at the grain edge. Nanometer-scale chemical composition measurements and mapping by TEM-EDXS (energy dispersive X-ray spectroscopy) and EFTEM (energy filtered TEM) show that the chemistry of the parent feldspar also remains unchanged at the interface. In particular, there is no evidence for the incursion of Ca from the fluid solvent into the structure, either by interdiffusion or by a replacement process. Taken together, the TEM observations point to a sharp chemical reaction front characterized by the congruent (i.e. stoichiometric) release of all elements from the feldspar structure. Nanometer-scale measurements by high resolution analytical TEM also reveal that a surface alteration layer (SAL) of amorphous material forms in situ at the expense of the feldspar structure. The interface demarcates a spatially coincident and nm-sharp chemical and structural discontinuity between the parent feldspar and the amorphous phase. The amorphous SAL has a variable thickness, from under 10 nm up to ~200 nm. This is likely one of the first observed occurrences of a significant surface amorphous layer on feldspar due to alteration in an alkaline solvent. The lack of a gap between the two phases points to an interfacial dissolution-reprecipitation process that continuously operates during hydrothermal alteration, and mostly likely right from the onset of contact with the fluid. After the initial formation of the amorphous layer, a 1–2 μm-thick porous amalgam of secondary crystalline phases comprised of calcite, tobermorite, and hydrogrossular, as well as other minor phases, precipitated over the SAL. These authigenic crystalline minerals formed during the experiment (hydrothermal alteration, followed by fluid loss due to evaporation) by a classical thermodynamically-controlled precipitation process as the reactor bulk fluid became increasingly concentrated. We propose that a coupled interfacial dissolution-reprecipitation (CIDR) mechanism best explains the chemical and structural properties of the interface and the formation of an amorphous surface layer. In fact, many recent studies postulate that a CIDR process controls feldspar dissolution and the formation of SALs at acid and circumneutral pH over a wide range of temperatures. Combining these previous results with our new observations supports the idea that a unique and unifying mechanism likely controls chemical alteration of feldspars in all aqueous fluids. EFTEM calcium map of reacted orthoclase grain with authigenic crystalline silicate rind. [Display omitted] [ABSTRACT FROM AUTHOR]

Global food security concerns have spurred increasing demand for locally sourced and produced K-fertilizers. Various processes have been explored for more than a century; one promising solution is based on the alkaline aqueous alteration of feldspar-rich rocks at elevated temperatures. However, knowledge of the overall physico-chemical reactions comprising dissolution of feldspar and precipitation of secondary phases is still rudimentary, in particular how the feldspar structure evolves at the nm-scale during hydrolysis at alkaline conditions. Here we report on the results of a study aimed at converting potassium feldspars to K-rich fertilizer based on the alteration of sanidine and microcline samples at 190 °C in pH 12 Ca(OH) 2 solutions for 24 h. Based on X-ray diffraction and Rietveld refinement, the secondary authigenic minerals that precipitated are primarily composed of Ca-carbonate (calcite, vaterite), and Ca-(Al)-silicates, such as tobermorite and hydrogrossular. Short-term bench top leaching experiments in water prove that the hydrothermal product releases up to two orders of magnitude more K than the unaltered K-feldspar starting material, pointing to its application as a ready-to-use fertilizer for K-deficient soils. Detailed chemical mapping and energy dispersive X-ray spectroscopy (FESEM- and TEM-EDXS) analyses of the precipitates at the μm to nm-scale show that the distribution of K associated with the secondary phases is very heterogeneous, both spatially and in terms of concentrations. Using various analytical transmission electron microscopy (TEM) techniques, e.g., HRTEM, TEM-EDXS, EFTEM, to investigate the structure and chemistry of the feldspar interface, we find no evidence for a change in chemistry or structure at the nm-scale, even though dissolution continuously decreases the volume of each grain. Our observations also show the existence of an amorphous surface altered layer (SAL) of variable thickness (10–~100 nm) forming at the feldspar interface. Nanometer-scale chemical measurements show that this amorphous SAL is rich in K, and therefore may also be an important reservoir of easily leachable K. We hypothesize that it forms continuously and in situ at the expense of the feldspar by a coupled interfacial dissolution-reprecipitation process (CIDR). • Alkaline hydrothermal alteration of K-feldspars yields K-fertilizer for soils. • SEM and TEM techniques used to measure [K] in secondary phases and interface. • Labile K in hydrothermal product associated with soluble secondary phases. • The K-feldspar structure dissolves stoichiometrically, with no change in chemistry. • Coupled interfacial dissolution-reprecipitation (CIDR) controls dissolution process. [ABSTRACT FROM AUTHOR]

Rare earth elements (REE) speciation in laterites are commonly estimated from sequential extraction procedure (SEP). In this study, the selectivity of a 5-steps SEP protocol was evaluated on individual synthetic Ce-material and applied to a well-characterized lateritic profile developing over a granite bedrock (Madagascar). The synthetic Ce-materials are representative of laterite minerals: Ce-doped oxyhydroxides and kaolinite, cerianite, Ce-rhabdophane and Ce-bastnäsite. The morphology, specific surface area, Ce sorption concentrations were thoroughly characterized prior to SEP. X-Ray absorption spectroscopy on synthetic material indicates that Ce(III) dominates apart for cerianite and Mn-oxide. The evaluation of the 5-steps SEP scheme shows that reagent selectivity is generally good, but the use of hydroxylamine hydrochloride is strongly discouraged. First, distinction between Ce sorbed at the surface of Mn- and amorphous or crystalline Fe-oxyhydroxides remains ambiguous. Second, Ce-rhabdophane (phosphate) and Ce-bastnäsite (carbonate) dissolve partially with this reagent. These experimental results combined with previous mineralogical and geochemical characterization were fundamental to interpret SEP results in the laterite profile from Madagascar. In the three laterite samples from the A-, B- and C-horizons, it is proposed that REE mainly distribute in authigenic phosphates and Fe- and (Mn-)oxyhydroxides. In the oxidized B-horizon, there is a significant LREE/HREE and Ce(IV)/Ce(III) fractionation between laterite minerals. While LREE-Ce is mainly concentrated in phosphates (alunite-jarosite supergroup), it is proposed that HREE and Ce(IV) uptake is dominated by oxyhydroxides. In the A- and C-horizons, the LREE and HREE speciations are similar, indicating no significant LREE/HREE fractionation in laterite minerals and organic matter. In all horizons, the REE ion exchangeable is negligible, although kaolinite is a main mineral of the A- and B-horizons. [ABSTRACT FROM AUTHOR]

Smectite hydration impacts dynamical properties of interlayer cations and thus the transfer and fate of H 2 O, contaminants, and nutriments in surficial environments where this ubiquitous clay mineral is often one of the main mineral components. The influence of key crystal-chemical parameters, such as the amount of charge or the presence of fluorine, rather than hydroxyl groups, in smectite anionic framework, on hydration, organization of interlayer species, and related properties has been described for tetrahedrally substituted trioctahedral smectites (saponites). Despite the ubiquitous character of octahedrally substituted smectites, that make most of the world bentonite deposits, the influence of charge location on smectite hydration properties has not received similar attention. A set of octahedrally substituted trioctahedral smectites (hectorites) with a common structural formula Na x Mg 6-x Li x Si 8.0 O 20 (OH) 4 and a layer charge (x) varying from 0.8 to 1.6 was thus synthesized hydrothermally. The distribution of charge-compensating Na+ cations and of associated H 2 O molecules was determined experimentally from the modeling of X-ray diffraction data obtained along water vapor desorption isotherms. Consistent distributions of charge-compensating cations and of associated H 2 O molecules were also computed from GCMC simulations as a function of layer charge. Interlayer H 2 O contents [2.5–5.5 and 8.0–10.0 H 2 O molecules per O 20 (OH) 4 for 1W and 2W hydrates, respectively] are similar in all Na-saturated smectite samples, independent of the location and amount of their layer charge. In contrast to synthetic saponite, for which stability of most hydrated layers was increased by increasing layer charge, the stability of synthetic hectorite hydrates is only marginally affected by layer charge. Consistently, the layer-to-layer distance of Na-saturated hectorite 2W (and 1W) layers is independent of layer charge (15.10–15.65 Å and 12.0–12.6 Å, respectively). The contrasting hydration behavior of synthetic Na-saturated saponite and hectorite is likely due to different electrostatic attraction between the 2:1 layer and interlayer cation, the charge undersaturation of O atoms at the surface of hectorite 2:1 layer being more diffuse compared to saponite. Combined with previous results on saponites, the present data and sample set provides key constraints to assess the validity of force fields simulating clay-water interactions for an unmatched variety of smectite with contrasting locations and amounts of layer charge deficits. Unlabelled Image • Hectorites with a layer charge varying from 0.8 to 1.6 were synthesized hydrothermally. • H 2 O contents of 1W and 2W layers are independent of layer charge amount and location. • Hectorite hydration is marginally affected by the amount of layer charge. • Saponite & hectorite different hydration behaviors due to undersaturation of O atoms. [ABSTRACT FROM AUTHOR]

The microstructural evolution of CaSiO3 wollastonite subjected to carbonation reactions at T = 90 °C and pCO2 = 25 MPa was studied at three different starting conditions: (1) pure water; (2) aqueous alkaline solution (0.44 M NaOH); and (3) supercritical CO2. Scanning and transmission electron microscopy on reacted grains prepared in cross-section always revealed unaltered wollastonite cores surrounded by micrometer-thick pseudomorphic silica rims that were amorphous, highly porous, and fractured. The fractures were occasionally filled with nanometer-sized crystals of calcite and Ca-phyllosilicates. Nanoscale chemical profiles measured across the wollastonite-silica interfacial region always revealed sharp, step-like decreases in Ca concentration. Comparison of the Ca profiles with diffusion modeling suggests that the silica rims were not formed by preferential cation leaching (leached layer), but rather by interfacial dissolution-precipitation. Extents of carbonation as a function of time were determined by quantitative Rietveld refinement of X-ray diffractograms performed on the reacted powders. Comparing the measured extents of carbonation in water (condition 1) with kinetic modeling suggests that carbonation was rate-controlled by chemical reactions at the wollastonite interface, and not by transport limitations within the silica layers. However, at conditions 2 and 3, calcite crystals occurred as a uniform surface coating covering the silica layers, and also within pores and cracks, thereby blocking the connectivity of the originally open nanoscale porosity. These crystals ultimately may have been responsible for controlling transport of solutes through the silica layers. Therefore, this study suggests that pure silica layers were intrinsically non-passivating, whereas silica layers became partially passivating due to the presence of calcite crystallites. [ABSTRACT FROM AUTHOR]