Fayalite (Fe2SiO4) dissolution kinetics determined by X-ray absorption spectroscopy | INSTITUT DE PHYSIQUE DU GLOBE DE PARIS

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  Fayalite (Fe2SiO4) dissolution kinetics determined by X-ray absorption spectroscopy

Type de publication:

Journal Article

Source:

Chemical Geology, Volume 275, Ticket 3-4, p.161-175 (2010)

ISBN:

0009-2541

Numéro d'accès:

ISI:000280614900005

URL:

http://www.sciencedirect.com/science/article/pii/S0009254110001695

Mots-clés:

UMR 7154 ; Minéralogie ; Physico-chimie des Fluides Géologiques ; N° Contribution :3003 ; Fayalite; Dissolution rate; In situ X-ray absorption spectroscopy; Hydrothermal fluids; Iron speciation; Acetate ligands

Résumé:

Dissolution of fayalite has been monitored in a small size batch reactor at a constant pressure of 30 MPa and temperatures of 50, 65, 80 and 100 degrees C, either in dilute HCl or in acetate buffered solutions. The technique, based on in situ X-ray absorption spectroscopy measurements, allowed simultaneous measurements of aqueous iron concentration and speciation as a function of time under high pressure and high temperature conditions. Far from equilibrium, dissolution rate normalised to geometric surface area (R-net) can be described in dilute HCl solutions by: R-net = k(0) exp (-E-a/RT)a(H)(n)+ with log(k(0)) = 0.17 +/- 0.63 (k(0) being expressed in mol cm(-2) s(-1)); E-a = 45,100 +/- 4200 J mol(-1): n = 0.74 +/- 0.15, so that R-net is expressed in mol cm(-2) s(-1). These results are found to be consistent with some previous fayalite dissolution studies. In acetate buffered solutions, the dissolution rate was found to be enhanced and positively correlated with the activity of acetate. Whereas speciation of Fe2+ in HCl solutions is dominated by hexahydrated octahedral species, the combination of ab initio XANES calculations, measurement of X-ray absorption spectra, and thermodynamic modelling allowed us to determine that aqueous Fe2+ was complexed octahedrally with two acetate ligands, each bonded to iron through one oxygen atom, the corresponding formula being Fe(CH3COO)(2)center dot 4H(2)O. Supposing that surface complexes resemble the major complex species in solution, this solvation by acetate ions would explain the observed enhancement of the dissolution rate, because of the nucleophilic attack of acetate ligands at the silicate surface. Crown Copyright (C) 2010 Published by Elsevier B.V. All rights reserved.

Notes:

Daval, Damien Testemale, Denis Recham, Nadir Tarascon, Jean-Marie Siebert, Julien Martinez, Isabelle Guyot, François