Étude cristallochimique de solutions solides de minéraux argileux (Impact de la déshydratation des smectites sur les surpressions dans les bassins sédimentaires)

L'augmentation de la demande énergétique incite les compagnies pétrolières à prospecter dans des zones où les conditions de forage et d'exploitation sont difficiles, comme les réservoirs profondément enfouis qui peuvent présenter des surpressions. Elles sont classiquement expliquées par différents mécanismes parmi lesquels, la libération d'eau par réaction minérale. Nous nous sommes intéressés à la réaction de déshydratation des smectites. Les smectites sont des phyllosilicates et ont la particularité de pouvoir insérer des molécules d'eau dans leur structure, entraînant de fortes variations de volumes. Elles présentent également une grande variabilité chimique. Dans une partie expérimentale, nous avons calculé les paramètres cristallochimiques de smectites dioctaédriques alumino-magnésiennes sodiques ou calciques dans une large gamme de compositions chimiques par analyse Rietveld à partir de diffractogrammes de rayons X pour différents états d'hydratation. La structure chimique a été déterminée par microscopie électronique à balayage munie d'un spectromètre à dispersion d'énergie. Les paramètres de Margules de chaque solution solide ont été déterminés pour un modèle non idéal asymétrique. Il apparaît que l'hydratation est le phénomène principal faisant varier le volume molaire des smectites mais que la composition chimique a un impact également. Dans une partie de modélisation, nous avons combiné les résultats de thermodynamique avec des données de modélisation de bassin afin d'estimer la contribution sur les surpressions dans les bassins sédimentaires, en prenant un exemple dans le delta du Niger, ayant un fort taux de sédimentation et d'épaisses couches argileuses.Since the energetic demand is increasing, the oil companies have to prospect in more hostile drilling conditions, such as in deep buried reservoirs that can present overpressures. Among the mechanisms put in evidence to explain the generation of overpressure there is water released from mineral reaction. This PhD thesis focuses on the contribution of the reaction of smectite dehydration. Smectites are clay minerals belonging to the phyllosilicates swelling family. Their particularity is to incorporate water in their structure, leading to huge volume variations. They present a wide chemical variability: cationic substitutions impart a net negative charge to the 2:1 layer, compensated by interlayer cations, such as Na+ or Ca2+. In a first part, we calculated cell parameters of dioctahedral smectites in the systems SiO2-Al2O3-MgO-Na2O-H2O and SiO2-Al2O3-MgO-CaO-H2O, in a wide range of chemical compositions by Rietveld analysis from X-ray diffraction patterns, for different hydration states. Chemical formulae have been determined by scanning electronic microscopy with an energy dispersive probe. Margules parameters of each solid solution have been determined for a non ideal asymetric model. It appears that hydration is the main factor influencing molar volume variations but chemical composition has an effect too. In a second part, we combined results from the thermodynamic study with data extracted from basin modelling, in order to estimate the contribution of smectite dehydration on overpressure generation in sedimentary basins. The case studied is from Niger Delta, where can be found high sedimentary rates, thick shale layers and overpressures due to undercompaction.PARIS-BIUSJ-Physique recherche (751052113) / SudocSudocFranceF

Structural study of polymorphism and thermal behavior of CaZr(PO4)2

International audienceThe crystal structure of CaZr(PO4)2 has been revised by ab initio Rietveld analysis of X-ray powder diffraction data. At room temperature, CaZr(PO4)2 crystallizes in the orthorhombic space group Pna21 (Z = 4). Differential thermal analysis suggests a reversible second order transition at 1000°C confirmed by high temperature XRD analysis that brings out the existence of a high temperature form, very similar to the room temperature one, but more symmetrical (Pnma, Z = 4). Analysis of the crystal parameters evolution during heating reveals that CaZr(PO4)2 exhibits a quite low thermal expansion coefficient of 6.11 10-6K-1. This value stems from a combination of several mechanisms, including Coulombic repulsion and bridging oxygen rocking motion

Reducing ultra-low thermal expansion of β-Zr2O(PO4)2 by substitutions?

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Transmission Kikuchi Diffraction, a powerful imaging technique for nanoscale structural characterisation of cultural heritage materials

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Structure et propriétés des verres de silicates de plomb

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High-temperature behavior of dicesium molybdate Cs2MoO4: Implications for fast neutron reactors

International audienceDicesium molybdate (Cs 2 MoO 4)'s thermal expansion and crystal structure have been investigated herein by high temperature X ray diffraction in conjunction with Raman spectroscopy. This first crystal-chemical insight at high temperature is aimed at predicting the thermostructural and thermomechanical behavior of this oxide formed by the accumulation of Cs and Mo fission products at the periphery of nuclear fuel rods in sodium-cooled fast reactors. Within the temperature range of the fuel's rim, Cs 2 MoO 4 becomes hexagonal P6 3 /mmc, with disordered MoO 4 tetrahedra and 2D distribution of Cs-O bonds that makes thermal axial expansion both large (50 rα l r 70 10 À 6 1C À 1 , 500-800 1C) and highly anisotropic (α c À α a ¼ 67 Â 10 À 6 1C À 1 , hexagonal form). The difference with the fuel's expansion coefficient is of potential concern with respect to the cohesion of the Cs 2 MoO 4 surface film and the possible release of cesium radionuclides in accidental situations

Structure and properties of lead silicate glasses

International audienceGiven their specific properties, glasses containing lead oxide have a very broad field of application. This extends from crystal glasses, enamels and glazes, to optical glasses with high refractive index, passing through glasses absorbing ionizing radiation and glasses for low-temperature welding of metals, ceramics or glasses. This chapter focuses first of all on the chemistry of lead, and more specifically on the properties and stereochemistry of the Pb$^{2+}$ ion (species under which lead is present in oxide glasses), the nature of the Pb-O chemical bond and the crystalline structure of pure PbO oxide (stereochemistry and mode of connection of the polyhedrons). Then, since the lead-based oxide glasses of interest are overwhelmingly silicate glasses, the focus is on how the Pb$^{2+}$ ions get inserted in the glassy network of silica and modify its structure, first of all in the case of the simple binary SiO2-PbO system (for which the evolution of the structure of the binary crystalline phases will also be approached depending on their PbO content compared to glasses), then in the case of more complex systems such as the ternary SiO$_2$-PbO-R$_2$O (R: alkaline) and SiO$_2$-PbO-Al$_2$O$_3$ systems

Structural study and luminescence of TlSrLa(AsO4)2

This work presents the crystal structure and luminescent properties of TlSrLa(AsO4)2. In this phase Tl+ ions are located in large tunnels delimited by chains of alternating (AsO4) and (Sr,La)O8 polyhedra. Thallium atoms are eightfold coordinated with C1 symmetry. Large Tl---O distances are observed revealing a low stereochemical activity of the 6s2 lone pair..

Microstructure imaging of Florentine stuccoes through X-ray tomography: A new insight on ancient plaster-making techniques

International audienceGypsum-based plasters or stuccoes, in spite of their importance and diffusion, received little attention in cultural heritage materials studies. This work introduces a new, non-destructive methodology, using micro-tomography to measure the water/plaster ratio and the morphology of the hemihydrate powder used to make plasters on < 1 mm3 samples. This methodology give insight in both the raw material (and ultimately provenance) and the technique used to make plaster. The methodology was tested first on mock-up samples of known composition, then in a case study on 13 low-relief cast plaster sculptures from 15th century Florentine artists. Preliminary conclusions on this limited corpus show relative uniformity across most reliefs in terms of raw materials and techniques. The casts of one model (Nativity, attributed to Donatello and B. Bellano) were made with a different raw material, in line with prior geochemical analyses; these results support the previous attribution to a North Italian rather than Florentine origin. The casts of a second model (Virgin and Child, type of Saint Petersburg, attributed to Antonio Rossellino) were prepared with a different technique. This surprising result was not expected from Art History or previous studies