Crystal structure of synthetic Mg3Cr2Si3O12, the high-pressure Cr end-member of the knorringite-pyrope garnet series

Knorringite, the Cr-end-member of the pyrope garnet series (Nixon et al. 1968), often occur in high proportions in kimberlite garnets and is thus used for tracing high-pressure deep-earth conditions favorable to the formation of diamonds, in which knorringite-rich garnet can occur as inclusions. However, although the synthesis of knorringite is reported in the literature (Ringwood 1977; Irifune et al. 1982; Taran et al. 2004), the structure of the pure end-member has not been yet determined from experimental data. In this study, the crystal structure of knorringite, Mg3Cr2(SiO4)3, has been refined from high resolution synchrotron X-ray powder diffraction data recorded under ambient conditions on a polycrystalline sample synthesized at 12 GPa in a multi-anvil apparatus. The structure is cubic, space group Ia-3d, a = 11.5935(1), V = 1558.27(4) {\AA}3, dcalc = 3.97 g.cm-3. The Cr-O distance of 1.957(2) {\AA} is consistent with EXAFS results on the same sample. This short distance indicates a substantial compression of the CrO6 octahedron, compared to ambient pressure Cr3+-minerals such as uvarovite ( = 1.99 {\AA}, Andrut and Wildner 2002). Our experimental results thus confirm early empirical predictions based on series of high-pressure Cr-garnet end-members (Fursenko 1981), showing that the values of the Cr-O distance and the Cr-O-Si angle decrease with the augmentation of pressure and with the diminution of the size of the divalent cation

LES NANOPARTICULES: QUELS RISQUES EN SEINE ?

32 pInternational audienceL'obtention de nanoparticules (NPs) manufacturées et les propriétés physico-chimiques spécifiques qu'elles présentent autorisent aujourd'hui des applications de plus en plus nombreuses et innovantes. La quantité de nanoparticules manufacturées mises sur le marché est en constante augmentation et elles sont aujourd'hui présentes dans de nombreux produits de consommation courante. Cela alimente un débat croissant sur les coûts environnementaux et sociétaux qui pourraient dépasser les bénéfices escomptés par l'utilisation des nanotechnologies en général et des nanomatériaux ou nanoparticules (i.e. taille < 100 nm) en particulier. Elles sont devenues un enjeu majeur de santé publique du point de vue de la toxicité potentielle qu'elles pourraient engendrer dans les écosystèmes. Les nanoparticules peuvent en effet présenter un risque écotoxicologique (dispersion et dégradation dans l'environnement) et un risque en termes de santé humaine (exposition au poste de travail par exemple). Il est donc nécessaire d'évaluer la persistance, le devenir et l'impact de ces nouveaux polluants sur les écosystèmes et sur la qualité des ressources naturelles (eaux, cultures, etc...). Dans ce contexte, l'objectif de cette étude était de mieux cerner l'état physique et chimique des NPs dans des concentrations représentatives des niveaux de toxicité observés et dans des conditions proches de celles des milieux naturels. Elle s'est focalisée sur l'eau de Seine, représentative des eaux naturelles de surface qui sont un des vecteurs principaux de la dispersion de ces NPs manufacturées. Les NPs étudiées ici sont produites en grande quantité et largement utilisées dans différents domaines industriels : il s'agit de NPs d'oxydes de zinc (ZnO) et de dioxyde de titane (TiO2). L'analyse des NPs de ZnO par XPS a mis en évidence l'existence d'un cœur de ZnO et d'un coquille de Zn(OH)2 en surface. Parallèlement, des NPs enrobées ont été étudiées et caractérisées, afin de mettre en évidence le rôle de l'enrobage organique sur la solubilité des NPs. Les expériences de mesures de solubilité des nanoparticules manufacturées en milieu naturel ont été réalisées par utilisation combinée des techniques de DMT et d'UF, associée à des calculs thermodynamiques. Il s'avère que la forme nanoparticulaire du TiO2 n'est pas davantage soluble que ses homologues microparticulaire ou macroparticulaire. A l'inverse, une fraction non négligeable des nanoparticules d'oxydes de zinc est rapidement dissoute dans l'eau de Seine. Puis les NPs sont "piégées" dans des phases secondaires carbonatées, ce qui peut signifier leur isolement par rapport au milieu et donc l'arrêt des réactions impliquant les NPs, soit une forme de passivation des nanoparticules. Le comportement des NPs dans le milieu est donc en grande partie contrôlé par la couche directement à leur surface (Gélabert et al., 2014; Sivry et al., 2014) : la couche d'hydroxydes de zinc contrôle le Ks apparent, l'enrobage organique augmente la vitesse et le taux de dissolution des NPs et, enfin, la formation d'une gangue carbonatée emprisonne les NPs et provoque potentiellement leur passivation

Synchrotron-based speciation of chromium in an Oxisol from New Caledonia: Importance of secondary Fe-oxyhydroxides

abstract In New Caledonia, the weathering of ultamafic rocks under a tropical climate has led to the residual accumulation of trace elements in lateritic soils widely dominated by Fe-oxyhydroxides. The speciation of trace elements, such as Cr, Ni, and Co, in these Oxisols remains a major subject of interest regarding mining and environmental issues. We have assessed the speciation of chromium in the upper part of an Oxisol, by combining bulk and spatially resolved chemical analyses (EPMA and SEM-EDS) with synchrotron-based spectroscopic data (EXAFS and XANES). EPMA indicates that the main hosts for chromium in the bedrock sample are the silicates forsterite, enstatite, and lizardite. Hosting of chromium in these easily weatherable mineral species could lead to a significant loss of this element upon weathering. However, total analyses of major elements indicate only a slight depletion of Cr, together with an immobility of Fe and Al and drastic losses of Si and Mg, after the weathering of the bedrock. Such a low mobility of chromium is likely related to its significant incorporation in goethite and hematite formed after the weathering of Fe 2+ -bearing primary silicates. This efficiency of secondary Fe-oxyhydroxides at immobilizing chromium is demonstrated by quantitative analysis of EXAFS data that indicates that these mineral species host between 67 and 75 wt% of total Cr (compared to the 18 to 22 wt% of total Cr hosted by chromite). In addition, SEM observation and SEM-EDS analyses performed on the Oxisol samples also show some evidence for chemical weathering of chromite. Chromite could then represent a past and/or present source of chromium upon extended tropical weathering of the studied Oxisol, rather than a stable host. These results emphasize the importance of secondary Fe-oxyhydroxides, compared to Cr-spinels, on chromium hosting in Oxisols developed upon tropical weathering of ultramafic rocks. Although the trapping mechanism of chromium mainly corresponds to incorporation within the structural network of goethite and hematite, sorption reactions at the surface of these mineral species could also be involved in such a process. In addition, considering their potential oxidative reactivity that can generate Cr 6+ or enhance the chemical weathering of chromite, the occurrence of Mn oxides could significantly modify the behavior of chromium upon weathering. These considerations indicate that further studies are needed to assess the actual potential of chromium release from Oxisols developed upon weathering of ultramafic rocks under a tropical climate

Electron sources for plasma electronics and different technological application

There are the following advantages of applying electron guns with plasma cathodes in devices exciting microwave radiation: stability of their parameters, high density of current, relative insensitivity to ion bombardment and the possibility of operating over a wide range of pressure values of a plasma-generating gas [1-5]. The given work aims at constructing the guns with the parameters necessary for the excitation of microwaves of high amplitudes in the slow-wave structures: the beam energy is 20-30 kV, the current is up to 5 A, and the pulse duration is 0,11÷1 ms. The principal problem arising during construction of heavy-current electron sources with plasma emitters consists in the following: it is necessary to provide such conditions of the gas volume, under which the discharge firing would be stable and the emissive plasma generation be effective, whereas a gas breakdown in the accelerating gap must be eliminated

EXAFS Signatures of Structural Zn at Trace Levels in Layered Minerals

Many in situ XAFS studies have shown that zinc incorporated in layered minerals is a major form of zinc in Zn-contaminated soils. Quantitative information on the local structural environment(s) and ordering of Zn in these minerals is required to better understand its behavior in soils. In this study, EXAFS spectroscopy was used to assess the structural environment of zinc incorporated at trace levels (40 ppm to 4,000 ppm) within the octahedral sheets of various natural and synthetic layered minerals. Results indicate that EXAFS data analyzed using ab initio FEFF calculations (FEFF 8.10) can unambiguously distinguish between zinc incorporation within the octahedral sheet of dioctahedral versus trioctahedral layered minerals and can determine the distribution (random or ordered) of zinc cations within the octahedral sheets of these minerals

Unsupervised clustering for seabed color with Sentinel-2 imagery: a case study on Voh-Koné-Pouembout lagoon (New Caledonia)

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Unsupervised Optical Classification of the Seabed Color in Shallow Oligotrophic Waters from Sentinel-2 Images: A Case Study in the Voh-Koné-Pouembout Lagoon (New Caledonia)

International audienceMonitoring chlorophyll-a concentration or turbidity is crucial for understanding and managing oligo- to mesotrophic coastal waters quality. However, mapping bio-optical components from space in such shallow settings remains challenging because of the strong interference of the complex bathymetry and various seabed colors. Correcting the total satellite reflectance signal from the seabed reflectance in ocean color with high resolution sensors is promising. This article shows how unsupervised clustering approaches can be applied to Sentinel-2 images to classify seabed colors in shallow waters of a tropical oligotrophic lagoon in New Caledonia. Data processing included Lyzenga correction for estimating the water column reflectance, optical spectra standardization for attenuating water absorption effects and clustering using the unsupervised k-means method. This methodological approach was applied on the 497, 560, 664 and 704 nm optical bands of the selected Sentinel-2 image. When applied on non-standardized data, our unsupervised classification retrieved three seafloor clusters, whereas five seafloor clusters could be retrieved using standardized data. For each of these two trials, the computed membership values explained more than 75% of the inertia in each Sentinel-2 wavelength band used for the clustering. However, the accuracy of the method was slightly improved when applied on standardized data. Confusion index mapping of the unsupervised clustering retrieved from these data emphasized the relevance and robustness of our methodological approach. Such an approach for seabed colors classification in optically complex shallow settings will be particularly helpful to improve remote sensing of biogeochemical indicators such as chlorophyll-a concentration and turbidity in fragile coastal environments

Unsupervised clustering for seabed color with Sentinel-2 imagery: a case study on Voh-Koné-Pouembout lagoon (New Caledonia)

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Mecafloc (volet 1) : mécanismes de floculation

Workshop "Au fil de l'eau", Nouméa, NCL, 17-/09/2019 - 18/09/201