Vitrification, cristallisation et structure des verres d'aluminosilicate de zinc

International audienceThe glass formation, crystallization behavior and structure were investigated in the ZnO-Al2O3-SiO2 system. The boundaries of the glass forming region are defined, indicating the absence of glasses along the ZnO-SiO2 binary and the role of Al to improve glass formation. The crystallization behavior has been clarified for representative glass compositions showing first crystallization of willemite near the ZnO-SiO2 binary and a metastable zinc aluminosilicate solid solution for higher Al2O3/SiO2 content. Gahnite appears as a second phase and is mainly present near the center of the tectosilicate join. Glass structure was studied using 27 Al NMR spectroscopy, neutron and X-ray diffraction and molecular dynamics simulation. Both Al and Zn are shown to be distributed between four-fold and five-fold sites. The proportion of [4] Al is more important near the ZnAl2O4-SiO2 join. A remarkable similar behavior is revealed for the glass forming ability and structural properties between MgO and ZnO aluminosilicate systems.La formation du verre, le comportement en cristallisation et la structure ont été étudiés dans le système ZnO-Al2O3-SiO2. Les limites de la région de formation du verre sont définies, indiquant l’absence de verres le long du binaire ZnO-SiO2 et le rôle d’Al pour améliorer la formation du verre. Le comportement en cristallisation a été clarifié pour des compositions de verre représentatives montrant la première cristallisation de willemite près du binaire ZnO-SiO2 et une solution solide d’aluminosilicate de zinc métastable pour une teneur plus élevée en Al2O3/SiO2. La gahnite apparaît comme une deuxième phase et est principalement présente près du centre du joint tectosilicate. La structure du verre a été étudiée en utilisant la spectroscopie 27Al RMN, la diffraction des neutrons et des rayons X et la simulation par dynamique moléculaire. Il est démontré que Al et Zn sont répartis entre des sites de coordinence quatre et cinq La proportion de [4]Al est plus importante près du joint ZnAl2O4-SiO2. Un comportement similaire est révélé pour la capacité de formation du verre et les propriétés structurelles entre les systèmes d'aluminosilicate de MgO et de ZnO

Calcium speciation and coordination environment in intracellular amorphous calcium carbonate (ACC) formed by cyanobacteria

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Phyllomanganate vein-infillings in faulted and Al-poor regoliths of the New Caledonian ophiolite: periodic and sequential crystallization of Ni-asbolane, Alk-birnessite and H-birnessite

Mineralogy, chemistry and spatial distribution of phyllomanganates, found in abundance at the bottom of thick Ni-laterite deposits, were established on dislocated vein-infillings showing banded and fibrous patterns (i.e. colloforms with rows of tiny boxworks) to decipher their conditions of formation in intensively faulted and Al-poor regoliths of the New Caledonian ophiolite (Koniambo klippe). Phyllomanganates of the vein-infillings belong to a Ni-rich, 9.5 angstrom type (Ni-asbolane) and two 7 angstrom types of contrasted alkali (K and Na) contents (Alk-birnessite and H-birnessite). Periodic development of radial cracks and differentiation of sequences of phyllomanganates enabled distinguishing up to five episodes of infillings The first outer infilling of a botryoidal shape is devoid of Ni asbolane and made of flakes of Alk-birnessite of different lengths and thicknesses, mixed and then replaced toward the inner section of the infilling by fine-grained H-birnessite. For the other infillings, the sequence of phyllomanganates is invariably the same from their outer to inner parts: (1) laths of Ni-asbolane in sealed cracks, (2) flakes of Alk birnessite at the margin and extremity of the cracks or isolated in (3) a continuous groundmass of fine-grained H-birnessite. The relative proportions of these three phyllomanganate species can change drastically from one infilling to another. That of Ni-asbolane is closely related to the development of cracks, which are both important in the last two infillings. Thermal effects (e.g. cooling of a hot fluid on an older surface deposit) likely contributed to the development of radial cracks and, by self-organized diffusion and precipitation processes, to the sequential production of phyllomanganates (Ni-asbolane, Alk-birnessite and then H-birnessite) of decreasing particle sizes. As for the Mg/Ni phyllosilicate ("garnierite") ore found at greater depth in the vein-infillings of the saprolite, the Ni/Co phyllomanganate ("chocolate") ore observed abundantly as relics (dislocated veins) at the transition with the overlying laterite could also result from the early hydrothermal alteration of Al-poor peridotites following the emergence, dislocation and cooling of the ophiolite nappe onto the New Caledonian basement

Solvothermal synthesis, temperature-dependent structural study and magnetic characterization of a multipolydentate oxamate-based 2D coordination network

International audienceA multi-polydentate N-substituted oxamate ligand bearing an additional carboxylato group has been successfully reacted in solvothermal conditions to form the 2D coordination network (TMA) 3 [CuMn(paba) 2 (OAc)]•8H 2 O (1). This coordination network displays a brick-wall type morphology with, thanks to the augmented connectivity of the ligand, a lower than usual metal ion nuclearity for the building sub-unit. Temperature-dependent structural studies indicate that the layered structure undergoes dehydration at 90°C and remain stable up to 200°C. Magnetic characterizations show that 1 behaves as a ferrimagnet, with a Curie temperature of 2.8 K

Pressure Control of Cuprophilic Interactions in a Luminescent Mechanochromic Copper Cluster

International audienceFor the development of applications based on mechanochromic luminescent materials, acomprehensive study of the mechanism responsible for the emission changes is required. We report thestudy of a mechanochromic copper iodide cluster under hydrostatic pressure, which allows control ofcrystal packing via modification of the intermolecular interactions. In situ single-crystal powder X-raydiffraction analysis and emission measurements under pressure permit one to establish a directcorrelation between the molecular structure and luminescence properties and, in particular, todemonstrate that cuprophilic interactions are responsible for the stimuli-responsive luminescenceproperties of such multinuclear coordination compounds

Characterization of untransformed ferrite in 10Cr and 12Cr ODS steels

International audienceTwo new ferrito-martensitic oxide dispersion strengthened (ODS) steels reinforced with (Y, Ti, O) nanoparticles were elaborated using a high-energy attritor. The milled powder was consolidated by hot extrusion at 1050 °C. The two types of ODS steels differ by chromium content, with 10 wt% Cr and 12 wt% Cr respectively. According to thermodynamic calculations, those grades are supposed to exhibit an austenitic transformation at high temperatures. X-ray diffraction (XRD) above austenitic temperature transformation reveals the presence of both ferrite and austenite phase. This unexpected ferrite phase is assumed to be untransformed low temperature ferrite. The → phase transformation specific enthalpy is monitored by differential scanning calorimetry (DSC). The untransformed ferrite fraction is calculated using dilatometric data and confirmed by electron backscatter diffraction (EBSD) microstructural analysis. The quenched samples from the austenitic domain give an image of the high-temperature partitioning. EBSD maps reveal two distinct elementary microstructures, one martensitic inherited from austenite and the other corresponds to the untransformed ferrite. This untransformed ferrite keeps the crystallographic-fiber conferred by hot-extrusion. The 10 Cr ODS has equiaxed untransformed ferrite areas. In contrast, the untransformed ferrite into 12 Cr ODS is distributed as elongated areas, parallel to the hot-extrusion direction. Moreover, electron probe micro analyzer (EPMA) mapping exhibits chromium content gradients, consistent with phase partitioning at high temperatures. Creep properties are evaluated at 650 °C for both grades. Small-angle X-rays scattering (SAXS) shows a similar size and distribution of the oxide particles in both grades

Stability of untransformed ferrite in 10Cr ODS steel

International audienceThe development of generation IV nuclear reactor requires improvements in structural and cladding materials. Oxide dispersion strengthened (ODS) steels are promising candidates because of improved creep properties. Two main families of ODS steels are known, the first one is martensitic ODS steel with a full reverse ferrite to austenite phase transformation at high temperatures. The second one is ferritic ODS steels with a ferritic matrix whatever is the temperature. Previous papers mention the existence of untransformed ferrite (UF) phase at high temperature in very particular martensitic ODS steels [S. Ukai 2009, M. Yamamoto 2010, T. Yamashiro 2016, A. Durand 2021]. UF is unexpected according to experimental observation and thermodynamic calculations of the non-strengthened materials (NR) with same chemical composition. This UF is particularly interesting because UF is a way to improve the creep resistance of martensitic grades [H. Sakasegawa 2008, A. Durand 2021] and then conciliates the benefits of both ferritic and martensitic grades. The present study is focused on the thermal stability of untransformed ferrite (UF) in a martensitic ODS steel at 10 wt% Cr. Effects of the heat treatments holding times and temperatures are studied by considering the UF fraction evolution and microstructural changes. The decrease of UF fraction is correlated with the decrease of the nano-oxides density coupled to the increase of their size. The phase transformation temperatures of ODS steels are also affected by heat treatments. Those temperatures appear as a mix of the ODS and the equivalent non-strengthened materials ones

Phases supergènes de cuirasses ferrugineuses : micro-échantillonnage non destructif et minéralogie avant analyse géochronologique (U–Th) ∕ He

International audienceAbstract. Interpreting the ages of supergene mineralogical phases in laterite is complex because they consist of polycrystalline mixtures of different phases at the microscopic scale that could be crystalized at different epochs. Among the geochronometers, the (U-Th)/He method on hematite and goethite is more often used, but ages can be difficult to interpret due to phases mixing. To resolve this issue, this study proposes a methodology for performing detailed mineralogical analysis of hematite and goethite single grains prior to their dating using the (U-Th)/He method. Strictly non-destructive mineralogy of single grains is not achievable by classical tools, such as conventional powder XRD (X-ray diffraction; requiring at least some milligrams of powder) or SEM (scanning electron microscopy; that can contaminate the grain by coating or fixing). Therefore, we performed X-ray diffraction patterns of single grains using high-flux X-ray beams from both a rotating anode (XRD_rotat) laboratory diffractometer and a synchrotron beamline (XRD_synch) and compared the results in order to design a method based on XRD_rotat only. For this purpose, two samples from the pisolitic facies of a Brazilian ferruginous duricrust (Alto Paranaíba region, Minas Gerais State, Brazil) were chosen because they presented a usual heterogeneity. Rietveld refinements of the XRD patterns obtained from both XRD_rotat and XRD_synch yielded similar results for the weight percentage ratio of the main phases and mean coherent domain sizes and less similar results for Al substitution rates, thus validating the XRD_rotat approach. No beam damage was observed when increasing X-ray exposure time, neither on XRD patterns nor on (U-Th)/He ages. Hence, sub-millimeter, undisturbed grains can be used to analyze the mineralogy of ferruginous duricrusts by XRD_rotat with a short exposure, and the same grains can subsequently be dated by (U-Th)/He geochronology analysis. The (U-Th)/He dating of pisolitic core and cortex grains also provided meaningful ages: they revealed two evolution phases of the ferruginous duricrust, which occurred at or before the Oligocene for the pisolitic core and middle Miocene for the pisolitic cortex, agreeing with the previous model for the development of pisolites. The mineralogy of single grains selected for dating is helpful for discussing the crystallization ages, and the high-flux XRD approach may be applied to other supergene mineral parageneses used for absolute dating

Early entombment within silica minimizes the molecular degradation of microorganisms during advanced diagenesis

International audienceMost ancient organic microfossils delicately preserved in 3D have been found in cherts. Although entombment within silica has been shown to promote morphological preservation, the impact of early silicification on the molecular evolution of fossilized microorganisms during burial remains poorly constrained. Here, we report results of advanced fossilization experiments performed under pressure (250 bars) and temperature (250 °C) conditions typical of sub-greenschist facies metamorphism for different durations up to 100 days on microorganisms experimentally entombed (or not) within a silica gel. The experimental residues have been characterized using XRD and XANES spectroscopy. The present study demonstrates that entombment within silica limits the degradation of microorganism molecular signatures, likely through specific chemical interactions, despite the progressive conversion of silica into quartz during the experiments. Extrapolation of the present results suggests that such protection may persist during geological timescales. The present experimental study provides molecular evidence that, in addition to morphologies, cherts may support the chemical preservation of remains of ancient life. The present results thus constitute a step forward towards the reconstruction of the original chemistry of pu-tative fossilized microorganisms

The degradation of organic compounds impacts the crystallization of clay minerals and vice versa

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