20 research outputs found
Lutetium incorporation in magmas at depth:changes in melt local environment and the influence on partitioning behaviour
International audienceThe structure of two Lu doped (4000 ppm) model end member silicate liquids, a highly polymerised haplogranite (Si-Al-Na-K-O) and a less polymerised anorthite-diopside (Si-Al-Mg-Ca-O), have been studied up to 8 GPa using in situ x-ray diffraction techniques. The results are the first to identify trace rare Earth element incorporation in silicate melts at high pressure. At pressures below 5 GPa, the bonding environment of Lu-O was found to be dependent on composition with coordination number CN LuâO = 8 and bond distance r LuâO = 2.36Ă
36Ă
in the haplogranite melt, decreasing to CN LuâO = 6 and r LuâO = 2.29Ă
29Ă
in the anorthite-diopside melt. This compositional variance in coordination number at low pressure is consistent with observations made for Y-O in glasses at ambient conditions and is coincident with a dramatic increase in the partition coefficients previously observed for rare Earth elements with increasing melt polymerisation. With increasing pressure we find that CN LuâO and r LuâO remain constant in the haplo-granite melt. However, an abrupt change in both Lu-O coordination and bond distance is observed at 5 GPa in the anorthite-diopside melt, with CN LuâO increasing from 6 to 8-fold and r LuâO from 2.29 to 2.39Ă
39Ă
. This occurs over a similar pressure range where a change in the P-dependence in the reported rare Earth element partition coefficients is observed for garnet-, clinopyroxene-, and olivine-melt systems. This work shows that standard models for predicting trace elements at depth must incorporate the effect of pressure-induced structural transformations in the melt in order to realistically predict partitioning behaviour
CinĂ©tique et mĂ©canismes d'oxydorĂ©duction dans les silicates fondus Ătudes expĂ©rimentales de verres nuclĂ©aires simplifiĂ©s et d'Ă©chantillons naturels
This work contributes to better understand iron redox reactions and mechanisms in silicate melts. It was conducted on compositions in both Na2O-B2O3-SiO2-FeO et Na2OAl2O3- SiO2-FeO systems. The influence of boron-sodium and aluminum-sodium substitutions and iron content on properties and structure of glasses and on the iron redox kinetics has been studied by Raman, Mössbauer and XANES spectroscopies at the B and Fe K-edges. In borosilicate glasses, an increase in iron content or in the Fe3+/ÎŁFe redox state implies a structural rearrangement of the BO4 species in the glass network whereas the BO3 and BO4 relative proportions remain nearly constant. In all studied glasses and melts, Fe3+ is a network former in tetrahedral coordination, unless for aluminosilicates of ratio Al/Na>1 where Fe3+ is a network modifier in five-fold coordination. Near Tg, diffusion of network modifying cations controls the iron redox kinetics along with a flux of electron holes. At liquidus temperatures, oxygen diffusion is considered to be the mechanism that governs redox reactions. This study shows the role played by the silicate network polymerization on the redox kinetics. In borosilicate melts, iron redox kinetics depends on the boron speciation between BO3 and BO4 that depends itself on the sodium content. Furthermore, an increase in the network-former/network-modifier ratio implies a decrease in oxygen diffusion that results in a slowing down of the redox kinetics. The obtained results allow a description of the iron redox kinetics for more complex compositions as natural lavas or nuclear waste model glasses.Ce travail contribue Ă la comprĂ©hension des rĂ©actions et mĂ©canismes d'oxydorĂ©duction du fer dans les silicates fondus. Il a Ă©tĂ© rĂ©alisĂ© Ă partir de compositions appartenant aux systĂšmes Na2O-B2O3-SiO2-FeO et Na2O-Al2O3-SiO2-FeO. L'influence des substitutions bore-sodium et aluminium-sodium et du taux de fer sur les propriĂ©tĂ©s et la structure des verres ainsi que sur les cinĂ©tiques d'oxydorĂ©duction a Ă©tĂ© Ă©tudiĂ©e par spectroscopie Raman, Mössbauer et XANES aux seuils K du bore et du fer. Dans les borosilicates, une augmentation du taux de fer ou de l'Ă©tat rĂ©dox Fe3+/ÎŁFe entraĂźne un rĂ©arrangement structural des espĂšces BO4 au sein du verre alors que les proportions relatives de BO3 et BO4 sont trĂšs peu affectĂ©es. Dans tous les verres et silicates fondus Ă©tudiĂ©s, Fe3+ est un formateur de rĂ©seau en coordinence 4, sauf pour les aluminosilicates de rapport Al/Na>1 oĂč il joue un rĂŽle de modificateur de rĂ©seau en coordinence 5. Les cinĂ©tiques d'oxydorĂ©duction sont contrĂŽlĂ©es prĂšs de Tg par la diffusion des cations modificateurs de rĂ©seau, accompagnĂ©e d'un contreflux de trous Ă©lectroniques. Aux tempĂ©ratures proches du liquidus, la diffusion de l'oxygĂšne est considĂ©rĂ©e comme le mĂ©canisme gĂ©rant les processus d'oxydorĂ©duction. Cette Ă©tude met en Ă©vidence le rĂŽle jouĂ© par la polymĂ©risation du rĂ©seau sur les cinĂ©tiques rĂ©dox. Dans les borosilicates fondus, les cinĂ©tiques rĂ©dox sont dĂ©pendantes de la spĂ©ciation du bore entre BO3 et BO4 qui elle-mĂȘme dĂ©pend du taux de sodium. Par ailleurs, plus le rapport formateurs/modificateurs de rĂ©seau est important, plus la diffusivitĂ© de l'oxygĂšne diminue et plus les vitesses de cinĂ©tiques rĂ©dox sont rĂ©duites. Les rĂ©sultats obtenus ont permis de dĂ©crire les cinĂ©tiques rĂ©dox pour des compositions plus complexes de laves naturelles ou s'approchant de celles de verre de confinement de dĂ©chets nuclĂ©aires
Diffusion of sodium ions driven by charge compensation as the rate-limiting step of internal redox reactions
International audienc
Cinétique et mécanismes d oxydoréduction dans les silicates fondus (études expérimentales de verres nucléaires simplifiés et d échantillons naturels)
Ce travail contribue Ă la comprĂ©hension des rĂ©actions et mĂ©canismes d oxydorĂ©duction du fer dans les silicates fondus. Il a Ă©tĂ© rĂ©alisĂ© Ă partir de compositions appartenant aux systĂšmes Na2O-B2O3-SiO2-FeO et Na2O-Al2O3-SiO2-FeO. L influence des substitutions bore-sodium et aluminium-sodium et du taux de fer sur les propriĂ©tĂ©s et la structure des verres ainsi que sur les cinĂ©tiques d oxydorĂ©duction a Ă©tĂ© Ă©tudiĂ©e par spectroscopie Raman, Mössbauer et XANES aux seuils K du bore et du fer. Dans les borosilicates, une augmentation du taux de fer ou de l Ă©tat rĂ©dox Fe3+/ Fe entraĂźne un rĂ©arrangement structural des espĂšces BO4 au sein du verre alors que les proportions relatives de BO3 et BO4 sont trĂšs peu affectĂ©es. Dans tous les verres et silicates fondus Ă©tudiĂ©s, Fe3+ est un formateur de rĂ©seau en coordinence 4, sauf pour les aluminosilicates de rapport Al/Na>1 oĂč il joue un rĂŽle de modificateur de rĂ©seau en coordinence 5. Les cinĂ©tiques d oxydorĂ©duction sont contrĂŽlĂ©es prĂšs de Tg par la diffusion des cations modificateurs de rĂ©seau, accompagnĂ©e d un contreflux de trous Ă©lectroniques. Aux tempĂ©ratures proches du liquidus, la diffusion de l oxygĂšne est considĂ©rĂ©e comme le mĂ©canisme gĂ©rant les processus d oxydorĂ©duction. Cette Ă©tude met en Ă©vidence le rĂŽle jouĂ© par la polymĂ©risation du rĂ©seau sur les cinĂ©tiques rĂ©dox. Dans les borosilicates fondus, les cinĂ©tiques rĂ©dox sont dĂ©pendantes de la spĂ©ciation du bore entre BO3 et BO4 qui elle-mĂȘme dĂ©pend du taux de sodium. Par ailleurs, plus le rapport formateurs/modificateurs de rĂ©seau est important, plus la diffusivitĂ© de l oxygĂšne diminue et plus les vitesses de cinĂ©tiques rĂ©dox sont rĂ©duites. Les rĂ©sultats obtenus ont permis de dĂ©crire les cinĂ©tiques rĂ©dox pour des compositions plus complexes de laves naturelles ou s approchant de celles de verre de confinement de dĂ©chets nuclĂ©airesThis work contributes to better understand iron redox reactions and mechanisms in silicate melts. It was conducted on compositions in both Na2O-B2O3-SiO2-FeO et Na2OAl2O3- SiO2-FeO systems. The influence of boron-sodium and aluminum-sodium substitutions and iron content on properties and structure of glasses and on the iron redox kinetics has been studied by Raman, Mössbauer and XANES spectroscopies at the B and Fe K-edges. In borosilicate glasses, an increase in iron content or in the Fe3+/ Fe redox state implies a structural rearrangement of the BO4 species in the glass network whereas the BO3 and BO4 relative proportions remain nearly constant. In all studied glasses and melts, Fe3+ is a network former in tetrahedral coordination, unless for aluminosilicates of ratio Al/Na>1 where Fe3+ is a network modifier in five-fold coordination. Near Tg, diffusion of network modifying cations controls the iron redox kinetics along with a flux of electron holes. At liquidus temperatures, oxygen diffusion is considered to be the mechanism that governs redox reactions. This study shows the role played by the silicate network polymerization on the redox kinetics. In borosilicate melts, iron redox kinetics depends on the boron speciation between BO3 and BO4 that depends itself on the sodium content. Furthermore, an increase in the network-former/network-modifier ratio implies a decrease in oxygen diffusion that results in a slowing down of the redox kinetics. The obtained results allow a description of the iron redox kinetics for more complex compositions as natural lavas or nuclear waste model glassesPARIS-BIUSJ-Physique recherche (751052113) / SudocSudocFranceF
The structure of GeO2âSiO2 glasses and melts: A Raman spectroscopy study
International audienceArticle history: Received 11 July 2008 Received in revised form 14 January 2009 Available online 11 March 2009 PACS: 61.20
In situ local environment and partitioning of Ni2+ ions during crystallization of an oxyfluoride glass
International audienceThe local structure of Ni 2+ ions during crystallization of an oxyfluoride aluminosilicate glass was examined by high temperature in situ spectroscopic experiments coupled with in situ X-ray diffraction to characterize the different crystallization steps. We show that Ni 2+ ions in the glass are located in the silicate glass network. After heat treatment Ni 2+ ions do not partition into the fluorine crystallites as observed for rare-earth metals. Instead, we observed the crystallization of a NiAl 2 O 4 crystalline phase, a largely inverse spinel. The in situ spectroscopic results (XRD, UV-Vis-NIR and Ni K-edge XANES) give new insights on the nickel partitioning between the supercooled liquid and the new crystals and show that the inversion degree of NiAl 2 O 4 spinel during its crystallization depends not only on the temperature but also on the annealing timescale. We also show that the addition of fluorine into aluminosilicate systems favors the formation of spinel crystals at lower temperature than usually observed, thereby playing a role to promote nucleation
Dynamics of iron-bearing borosilicate melts: Effects of melt structure and composition on viscosity, electrical conductivity and kinetics of redox reactions
International audienc