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BACKGROUND AND AIMS: Phosphorus (P) is an essential nutrient for plants but its low availability often necessitates amendments for agronomical issues. Objectives were to determine P spatial distribution and speciation that remain poorly understood in cultivated soils. METHODS: Aquic Argiudoll soil samples developed on a calcareous loam glacial till were collected from experimental plots submitted to contrasting crop rotations and amendments. Micro-X-ray fluorescence (μ-XRF) maps were collected on undisturbed samples. X-ray absorption near edge structure (XANES) spectra were collected on bulk samples and on fractions thereof, and on points of interests selected from μ-XRF maps. Results were compared with chemical analyses and extraction techniques results. RESULTS: Chemical analyses show variations in total and exchangeable P contents depending on the samples but no significant difference is observed in terms of P distribution and speciation. P distribution is dominated by a low-concentration diffuse background with a minor contribution from minute hot spots. P speciation is dominated by phosphate groups bound to clay-humic complexes. No modification of P distribution and speciation is observed close to roots. CONCLUSIONS: This study evidenced minor effect of cropping and fertilizing practices on P speciation in cultivated soils. Despite analytical challenges, the combined use of μ-XRF and XANES provides relevant information on P speciation in heterogeneous soil media.

Detachment faults in mid‐ocean ridge settings accommodate tectonic extension. These faults are potentially weakened by alteration through phyllosilicate‐ and amphibole‐mineral formation. Seven core samples taken from the Krafla and Reykjanes geothermal fields in Iceland allow for the study of the effect of mineral alteration on the frictional behavior of basic rocks in the context of high‐enthalpy geothermal fields and slow‐spreading mid‐ocean ridges. Gouge samples were prepared from the core samples and tested in a biaxial apparatus. In the core samples from Krafla, the presence of phyllosilicate minerals, which are generally stable at low (<400°$400{}^{\\circ}$ C) temperatures, enables water to cause frictional weakening, increased stability, and reduced frictional healing. The core samples from Reykjanes, however, offer a unique opportunity to study the effect of high‐temperature alteration, as they are from a spreading center at 4,600 m depth and >600°$600{}^{\\circ}$ C and have not been overprinted by low‐temperature alteration. Although these samples appear frictionally strong and more velocity‐weakening than the Krafla samples, they also exhibit negative frictional healing. This might be associated with a developing crystallographic‐preferred orientation of the amphibole minerals during inter‐seismic periods, potentially implying a new weakening mechanism that may operate along oceanic detachment faults at intermediate temperatures (approximately 400–600°$600{}^{\\circ}$ C) in conditions too hot for serpentinization and too cold for the crystal plastic deformation of peridotite. This progressive frictional strength loss along detachment faults may influence detachment‐fault morphology, but further investigation is required to both conclusively link crystallographic‐preferred orientation to negative healing and extrapolate its relevance to crustal conditions.

Background Vernadite is a nanocrystalline and turbostratic phyllomanganate which is ubiquitous in the environment. Its layers are built of (MnO 6 ) 8− octahedra connected through their edges and frequently contain vacancies and  (or) isomorphic substitutions. Both create a layer charge deficit that can exceed 1 valence unit per layer octahedron and thus induces a strong chemical reactivity. In addition, vernadite has a high affinity for many trace elements (e.g., Co, Ni, and Zn) and possesses a redox potential that allows for the oxidation of redox-sensitive elements (e.g., As, Cr, Tl). As a result, vernadite acts as a sink for many trace metal elements. In the environment, vernadite is often found associated with tectomanganates (e.g., todorokite and cryptomelane) of which it is thought to be the precursor. The transformation mechanism is not yet fully understood however and the fate of metals initially contained in vernadite structure during this transformation is still debated. In the present work, the transformation of synthetic vernadite (δ-MnO 2 ) to synthetic cryptomelane under conditions analogous to those prevailing in soils (dry state, room temperature and ambient pressure, in the dark) and over a time scale of ~10 years was monitored using high-energy X-ray scattering (with both Bragg-rod and pair distribution function formalisms) and transmission electron microscopy. Results Migration of Mn 3+ from layer to interlayer to release strains and their subsequent sorption above newly formed vacancy in a triple-corner sharing configuration initiate the reaction. Reaction proceeds with preferential growth to form needle-like crystals that subsequently aggregate. Finally, the resulting lath-shaped crystals stack, with n  × 120° ( n  = 1 or 2) rotations between crystals. Resulting cryptomelane crystal sizes are ~50–150 nm in the ab plane and ~10–50 nm along c* , that is a tenfold increase compared to fresh samples. Conclusion The presently observed transformation mechanism is analogous to that observed in other studies that used higher temperatures and (or) pressure, and resulting tectomanganate crystals have a number of morphological characteristics similar to natural ones. This pleads for the relevance of the proposed mechanism to environmental conditions.

Following our previous works on dioctahedral clays, we extend the classical Polarizable Ion Model (PIM) to trioctahedral clays, by considering dry Na-, Cs-, Ca- and Sr-hectorites as well as hydrated Na-hectorite. The parameters of the force field are determined by optimizing the atomic forces and dipoles on density functional theory calculations. The simulation results are validated by comparison with experimental X-ray diffraction (XRD) data. The XRD patterns calculated from classical molecular dynamics simulations performed with the PIM force field are in very good agreement with experimental results. In the bihydrated state, the less structured electronic density profile obtained with PIM compared to the one from the state-of-the-art non-polarizable force field clayFF explains the slightly better agreement between the PIM results and experiments.

Flow-like landslides in clayey soils represent serious threats for populations and infrastructures and have been the subject of numerous studies in the past decade. However, despite the rising need for landslide mitigation with growing urbanization, the transient mechanisms involved in the solid-fluid transition are still poorly understood. One way of characterizing the solid-fluid transition is to carry out rheometrical tests on clayey soil samples to assess the evolution of viscosity with the shear stress. In this study, we carried out geotechnical and rheometrical tests on clayey samples collected from six flow-like landslides in order to assess if these clayey soils exhibit similar characteristics when they fluidize (solid-fluid transition). The results show that (1) all tested soils except one exhibit a yield-stress fluid behavior that can be associated with a bifurcation in viscosity (described by the critical shear rate γċ) and in shear modulus G; (2) the larger the amplitude of the viscosity bifurcation, the larger the associated drop in G; and (3) the water content (w) deviation from the Atterberg liquid limit (LL) seem a key parameter controlling a common mechanical behavior of these soils at the solid-fluid transition. We propose exponential laws describing the evolution of the critical shear stress τc, the critical shear rate γċ, and the shear modulus G as a function of the deviation w-LL.

Smectite interlayer water plays a key role in the mobility of elements and molecules and affects a variety of geological processes. In trioctahedral smectites, in contrast to saponite and hectorite, the layer charge of which originates from isomorphic substitutions, the stevensite layer charge originates from the presence of octahedral vacancies. Despite its common occurrence in lacustrine environments, stevensite hydration has received little attention compared to saponite and hectorite. Early reports mention a specific hydration behaviour, however, with the systematic presence of a low-angle reflection attributed to the regular interstratification of various hydration states. The present study aims to revisit this specific hydration behaviour in more depth. Within this scope, the hydration behaviour of the three smectite varieties above are compared using synthetic trioctahedral smectites of similar layer charge and various compositions of their octahedral sheets. The chemical composition of the octahedral sheet does not appear to influence significantly smectite hydration for saponite and hectorite. Compared to its saponite and hectorite equivalents, H 2 O content in stevensite is lower by ~2.0 mmol H 2 O per g of dry clay. Consistent with this lower H 2 O content, Zn-stevensite lacks a stable monohydrated state, with dehydrated layers prevailing from 60% to 0% relative humidity. The presence of the regular interstratification of 0W and 1W layers is responsible for the low-angle reflection commonly observed for stevensite under air-dried conditions. Finally, the stevensite identification method based on X-ray diffraction of heated and ethylene glycol-solvated samples is challenged by the possible influence of the octahedral sheet chemical composition (Zn or Mg in the present study) on hectorite swelling behaviour in synthetic Zn-smectites. The origin of this effect remains undetermined and further work is needed to propose a more general identification method.

The adsorption of two pharmaceuticals, carbamazepine and paracetamol, onto the expandable clay mineral saponite has been studied through the combination of kinetic experiments, X-ray diffraction, and theoretical modeling. Kinetic experiments indicate low adsorption for carbamazepine and paracetamol on expandable smectite clay. Accordingly, X-ray diffraction experiments show that neither compound enters smectite interlayer space. Molecular dynamics simulations were carried out to understand the interactions between the two pharmaceuticals and the saponite basal surface in the presence of Na+ cations. Calculations reveal that paracetamol almost does not coordinate solution cations, whereas a rather low coordination to cation is observed for carbamazepine. As a result, the adsorption onto the clay surface results mainly from van der Waals interactions for both pharmaceuticals. Carbamazepine does adsorb the surface via two configurations, one involving cation coordination, which corresponds to a rather stable adsorption compared to paracetamol. This is confirmed by structural analyses completed with desorption free energy profile.

The formation of iron- and/or manganese-rich dark patinas on sandstones is a common natural phenomenon that occurs also on building stones. Lunéville château, in eastern France, presents such patinas that developed either under natural conditions (rain and time) or after an accidental fire and exposure to significant amounts of water as part of attempts to extinguish the fire. The present study aimed at characterizing both types of patinas in an effort to determine their formation mechanisms and Mn sources. In both cases, Mn required for patina formation likely derives from the reductive dissolution of Mn-rich minerals present in pristine sandstones, as suggested by the contrasting mineralogy and chemistry of Mn-rich phases present in the bulk and in the patina of a given building block. Reduced Mn species then migrate to the exposed surface of building blocks where they are re-oxidized via undetermined processes. Patinas developing \"naturally\" over time result from the alternation of wetting-reducing and drying-oxidizing cycles and appear to be composed of birnessite. Patinas formed after the 2003 fire result from this single accidental event and form a much thinner, heterogeneous, and discontinuous layer of poorly crystalline lithiophorite at the sandstone surface (â¼ 0-150 µm compared to â¼ 300-600 µm for \"natural\" patinas). The lack of Mn-rich patinas on areas of Lunéville château is likely related to the lower Mn content of pristine sandstone blocks.

The formation of iron- and/or manganese-rich dark patinas on sandstones is a common natural phenomenon that occurs also on building stones. Lunéville château, in eastern France, presents such patinas that developed either under natural conditions (rain and time) or after an accidental fire and exposure to significant amounts of water as part of attempts to extinguish the fire. The present study aimed at characterizing both types of patinas in an effort to determine their formation mechanisms and Mn sources. In both cases, Mn required for patina formation likely derives from the reductive dissolution of Mn-rich minerals present in pristine sandstones, as suggested by the contrasting mineralogy and chemistry of Mn-rich phases present in the bulk and in the patina of a given building block. Reduced Mn species then migrate to the exposed surface of building blocks where they are re-oxidized via undetermined processes. Patinas developing “naturally” over time result from the alternation of wetting-reducing and drying-oxidizing cycles and appear to be composed of birnessite. Patinas formed after the 2003 fire result from this single accidental event and form a much thinner, heterogeneous, and discontinuous layer of poorly crystalline lithiophorite at the sandstone surface (∼ 0–150 µm compared to ∼ 300–600 µm for “natural” patinas). The lack of Mn-rich patinas on areas of Lunéville château is likely related to the lower Mn content of pristine sandstone blocks.