Modélisation ab initio périodique de quelques défauts azotés du diamant (contribution au code CRYSTAL)

PARIS7-Bibliothèque centrale (751132105) / SudocSudocFranceF

Etude des rapports Fe3+/Fe2+ et OH-/F-, des éléments-traces dans les amphiboles des roches volcaniques des Petites Antilles ; estimation de fHF/fH2O

Study of separated amphiboles and biotites from calc-alkaline series of the Lesser Antilles permit to precise their crystal-chemistry : oxidation ratio of iron, trace element contents and to estimate the composition of fluid in equilibrium with them, fH2O/f HF ratio is about 106 in dacites.L'étude d'amphiboles et de micas séparés provenant des séries calco-alcalines de l'arc des Petites Antilles permet de préciser leur cristallochimie : degré d'oxydation du fer, éléments en trace notamment et d'estimer la composition de la phase fluide en équilibre avec ces minéraux : le rapport fH2O/f HF est voisin de 106 dans les dacites.D'Arco Philippe, Cotten Joseph. Etude des rapports Fe3+/Fe2+ et OH-/F-, des éléments-traces dans les amphiboles des roches volcaniques des Petites Antilles ; estimation de fHF/fH2O. In: Bulletin de Minéralogie, volume 108, 2, 1985. pp. 153-159

Le cœur alcalin du complexe du Messum, Namibie (description pétrologique, interprétation de l évolution minéralogique et relations roches sous- et sur-saturées en silice)

Le complexe anorogénique annulaire du Messum, associé à la province basaltique de l Etendeka (Namibie), montre la coexistence de roches sous- et sur-saturées en silice. Une étude pétrologique détaillée est nécessaire pour comprendre l évolution minéralogique de ces deux ensembles de roches. Dans le premier, l ensemble des observations a permis de proposer une route de cristallisation mettant en jeu des lignes (ou surfaces) péritectiques et expliquant le développement de la néphéline tardivement aux dépens des phases précoces (olivine, plagioclase, clinopyroxène alumineux) et la signification intermédiaire de la pargasite. Des minéraux significatifs non encore signalés sont décrits : spinelles dans les roches basiques, wohlérite et hiortdahlite dans les syénites les plus évoluées, ainsi que des pyrochlores considérés comme des indices potentiels de la présence de matériaux à affinités carbonatitique. Les syénites à quartz apparaissent comme un ensemble composite avec plusieurs venues différemment évoluées. Les paragenèses sont plus ou moins peralcalines, avec fayalite, hédenbergite acmitique, et développement tardif de riebéckite. La chevkinite, titano-silicate rare, y est décrit en quantité relativement abondante comme un minéral magmatique. Aucun lien minéralogique entre ces deux ensembles ne peut être établi ce qui rend peut crédible l hypothèse du passage de l un à l autre par contamination crustale.PARIS-BIUSJ-Thèses (751052125) / SudocPARIS-BIUSJ-Sci.Terre recherche (751052114) / SudocSudocFranceF

Phase transitions and superexchange mechanism in transition metal compounds. The case of KMnF 3 perovskite

International audienceMany space groups are proposed in the literature for the KMnF3 perovskite (see, for example, Knight et al., J. Alloys Compd., 2020, 842, 155935), ranging from cubic (C) (Pm[3 with combining macron]m) to tetragonal (T) (Image ID:d1cp03816h-t1.gif or I4/m) down to orthorhombic (O) (Pbnm). The relative stability ΔE of these phases, both ferromagnetic (FM) and antiferromagnetic (AFM), has been investigated quantum mechanically by using both the B3LYP hybrid functional and the Hartree–Fock Hamiltonian, an all-electron Gaussian type basis set and the CRYSTAL code. The O phase is slightly more stable than the T phase which in turn is more stable than the C phase, in agreement with experimental evidence. The C to T to O transition is accompanied by a volume reduction. The mechanism of stabilization of the AFM solution with respect to the FM one is discussed. Spin density maps and profiles, Mulliken charges, magnetic moments and bond population data are used for supporting the proposed mechanism. The IR and Raman spectra of the FM and AFM C, T and O cells are discussed; the only noticeable difference between the C, T and O spectra appears at wavenumbers lower than 150 cm−1. The effect of pressure is also explored in the 0–20 GPa interval. The stability order (O > T > C) at 0 GPa persists also at high pressure, and the differences between the phases increase

Etude théorique des grenats silicatés et calcul des fréquences de vibrations dans les solides

PARIS-BIUSJ-Thèses (751052125) / SudocPARIS-BIUSJ-Sci.Terre recherche (751052114) / SudocFONTAINEBLEAU-MINES ParisTech (771862302) / SudocSudocFranceF

Crystal chemistry and partitioning of halogens in hydrous silicates

International audienceUnderstanding how halogens are distributed among usual hydrous silicates in the lithosphere is important to constrain their deep geochemical cycle and fluid-rock interactions in subduction zones. This article presents firstprinciples modelling of halogen (F-, Cl-, Br-) incorporation in hydrous silicates including mica, chlorite, serpentine, amphibole, epidote and carpholite. The approach allows studying the impact of crystal chemistry on halogen partitioning by quantification of the energetic cost of halogen incorporation in minerals. Calculations are carried out in large systems where halogens are in minor to trace concentrations. Estimations show that F-bearing defects must be separated at least 9 Å from one another to reproduce trace element behaviour, this value increasing to at least 10 Å for Cl and Br. Results highlight the competition between the effects of electrostatic interactions and steric hindrance for incorporation of halogens, where steric hindrance has greater importance for heavy halogens, in particular for Br. Interaction with alkalis is a major control for F incorporation, especially in mica. Other parameters such as octahedral site occupancy, Si/Al ratio of tetrahedral sites and the nature of alkalis in amphibole and mica (K or Na) appear to play subordinate roles. Partition coefficients have been estimated in mineral assemblages in an effort to be representative of subduction zone metamorphism. Results show that pargasite, biotite and lizardite are favoured hosts for all three halogens, followed by clinochlore, tremolite and carpholite. The energetic cost of incorporating halogens into dioctahedral phyllosilicates and epidote is comparatively higher, and partitioning is predicted as unfavourable to these minerals. Fractionation between halogens in subduction zones is predicted by the evolution of mineral assemblages and partition coefficients, a consequence of the influence of crystal chemistry over halogen incorporation in hydrous silicates

Partitioning of chromium between garnet and clinopyroxene: first-principle modelling versus metamorphic assemblages

International audienceUnderstanding the geochemical behaviour of trace and minor elements in mineral assemblages is of primary importance to study small-and large-scale geological processes. Partition coefficients are frequently used to model the chemical evolution of minerals and fluids during melting and in metamorphic rocks of all grades. However, kinetic effects hampering equilibrium partitioning may invalidate the modelling. This study aims at calculating partition coefficients and testing their applicability in natural mineral assemblages, choosing Cr in garnet and clinopyroxene via exchange with Al as a case study. First-principle modelling has been combined with measurements and element mapping to estimate partition coefficients for Cr and the deviation from equilibrium. Results highlight the role of crystal chemistry over the strain field around point defects, controlling the dynamics of the Cr 3+ = Al 3+ exchange between clinopyroxene and garnet. Ab initio calculations allowed estimation of Cr partition coefficients between garnet and clinopyroxene, using a thermodynamic approach based on endmembers and mixing models simplified for trace element behaviour. The Cr 3+ = Al 3+ exchange reaction between garnet and the jadeite component of clinopyroxene depends on the grossular and pyrope content, with Cr preferentially incorporated into grossular over jadeite but preferentially incorporated into jadeite over pyrope. Comparison of predicted partition coefficients to measured concentrations in natural samples, together with element mapping, shows large disequilibrium. Cr-rich and Cr-poor sectors exhibit disequilibrium attributed to slow diffusivity of Cr during crystal growth and interface-coupled dissolution-precipitation, even for garnet-clinopyroxene assemblages crystallized around 850 • C

Strategies for the optimization of the structure of crystalline compounds

International audienc

On the use of the symmetry-adapted Monte Carlo for an effective sampling of large configuration spaces. The test cases of calcite structured carbonates and melilites

International audienceThe symmetry-adapted Monte Carlo sampling scheme is applied for the ab initio study of two mineralsystems, namely the calcite structured compound Ca0.75Mg0.25CO3 and soda-melilite (Na,Ca)AlSi2O7. Itis shown how an extensive use of symmetry, from the sampling of atomic configurations up to thequantum-mechanical calculation, makes feasible the investigation of large configuration spaces. As forthe sampling, we describe an effective procedure to specifically target low-energy configurations onthe potential energy surface of supercells of virtually any size. It is based on the suggestion that a correlationbetween symmetry and energy of the configurations exists according to which atomic distributionsof minimum and maximum energy are likely to have some spatial symmetry. This hypothesis is verifiedempirically and leads to a significant alleviation of the original problem by virtue of the possibility of tailoringthe symmetry-adapted Monte Carlo to select only symmetric configurations. The latter are alsofound to display a probability distribution similar to that of the entire set of configurations, thus providing,eventually, a suitable ab initio reference for the parameterization of model Hamiltonians. The moststable configuration so identified is used as pivot for the selection of new configurations having the sameatomic distribution but for the exchange of a couple of atoms. These are called ‘‘neighbors” to highlightboth their structural and energetic proximity to the pivot. We illustrate how, by collecting neighbors ofconfigurations of increasing energy, the description of the system can be progressively and deterministicallyimproved up to convergence of the calculated average properties, whatever the temperature.The same scheme works when moving to a supercell larger than the initial one (but of equivalent symmetry)since it is shown that stable structures remain so at any volume

Signification du grenat et de la cordiérite dans les laves du Sud-Ouest martiniquais

Mineralogical data are presented for three garnet or cordierite-bearing lavas from southwestern Martinique (Lesser Antilles island arc). Garnets and cordierite occur as megacrysts, and garnets show compositional variations reflecting the andesitic or dacitic nature of their host rocks. These megacrysts contain rhyolitic glassy inclusions and mineral inclusions (orthopyroxene, plagioclase, iron-titanium oxides) whose compositions are similar to those of the same mineral phases present as phenocrysts in the lavas. Reaction rims developed around the megacrysts and the minerals generated in them are also a function of the composition of the host rocks. The application of several geothermometers and geobarometers to the garnet, the cordierite and their inclusions show that they crystallized at moderate temperatures (850-680°C) and under relatively low pressures (c.a. 3 kbar only for garnet). These P-T conditions are close to those estimated for the crystallization of the phenocrysts from the host lavas, and are compatible with Green's (1977) experiments on the stability of garnet in Mn-bearing liquids. Garnets and cordierite from these Martinique occurrences are thus considered as phenocrysts which have undergone partial resorption during magma ascent ; their thermal and chemical disequilibrium with respect to the groundmass of the host lavas may also indicate the occurrence of magma mixing processes.Une andésite à cordiérite, une andésite à grenat et une dacite à grenat provenant du Sud-Ouest de la Martinique (arc insulaire des Petites Antilles) ont fait l'objet d'études minéralogiques visant à déterminer les conditions de cristallisation de ces minéraux. Grenats et cordiérite se présentent en mégacristaux millimétriques à centimétriques, dont la composition varie en fonction de celle des roches-hôtes ; ils contiennent des inclusions vitreuses rhyolitiques, ainsi que des minéraux (orthopyroxène, plagioclase, oxydes de fer-titane) dont les compositions sont très voisines de celles des phénocristaux de ces laves ; ils sont entourés d'auréoles réactionnelles dont la nature reflète également la minéralogie des roches encaissantes. L'application de différents géothermomètres et géobaromètres aux paragenèses étudiées montre que les grenats, la cordiérite et leurs minéraux inclus ont cristallisé dans des conditions comparables à celles des phénocristaux de leurs laves-hôtes, à des températures modérées (850 à 680°C) et sous des pressions de l'ordre de 3 kbar seulement pour les grenats. Ces résultats sont compatibles avec les travaux expérimentaux de Green (1977) sur la stabilité des grenats en présence de manganèse. Les grenats et la cordiérite de Martinique peuvent donc être considérés comme des phénocristaux partiellement déstabilisés lors de leur remontée ; leur déséquilibre thermique et chimique vis-à-vis de la mésostase des laves-hôtes témoigne, par ailleurs, de l'intervention de mélanges magmatiques.Maury René Charles, Clocchiatti Roberto, Coulon Christian, D'Arco Philippe, Westercamp Denis. Signification du grenat et de la cordiérite dans les laves du Sud-Ouest martiniquais. In: Bulletin de Minéralogie, volume 108, 1, 1985. pp. 63-79