Ž . Chemical Geology 175 2001 485–494 www.elsevier.comrlocaterchemgeo An experimental study of forsterite dissolution rates as a function of temperature and aqueous Mg and Si concentrations Eric H. Oelkers ) Laboratoire de Geochimie, CNRS r URM 5563, UniÕersite Paul Sabatier, 38 rue des Trente-six Ponts, 31400 Toulouse, France ´ ´ Received 23 September 1999; accepted 27 June 2000 Abstract Steady state forsterite dissolution rates, at far from equilibrium conditions and pH s2 "0.04, were measured as a function of aqueous magnesium and silica concentrations, and temperatures from 258C to 658C. All rates were measured in mixed flow reactors and exhibited stoichiometric dissolution. Measured rates are found to be independent of both aqueous magnesium and silica concentrations. The temperature dependence of the pH 2 forsterite dissolution rates obtained in this study is consistent with an Arrhenius equation of the form r sA exp yE rRT Ž . A A where r signifies the overall forsterite steady state dissolution rate, A refers to a pre-exponential factor equal to 0.190 mol A cm y2 s y1 , E designates an activation energy equal to 63.8 kJrmol, R represents the gas constant, and T denotes absolute A temperature. The observed variation of forsterite dissolution rates with aqueous composition is interpreted to originate from its dissolution mechanism. The forsterite structure consists of isolated silica tetrahedra that are branched together by magnesium ` ` octahedra chains. Mg O bonds apparently break more rapidly than Si O bonds in this structure. The breaking of octahedra ` chain linking Mg O bonds, which is apparently catalyzed by hydrogen ion adsorption at acidic conditions, leads directly to the destruction of the mineral. As the rate-controlling precursor complex for this mineral is formed by a hydrogen adsorption reaction, forsterite dissolution rates are unaffected by aqueous Mg and Si activities. q 2001 Elsevier Science B.V. All rights reserved. Keywords: Forsterite dissolution rates; Arrhenius equation; Activation energy, dissolution mechanisms 1. Introduction The motivation for this study is the improved understanding of the dissolution rates and mecha- ) Tel.: q 33-5-61-55-87-85; fax: q 33-5-61-55-81-38. Ž . E-mail address: oelkers@lucid.ups-tlse.fr E.H. Oelkers . nism of the silicate minerals. Towards this goal, a significant number of studies have been focused over the past several years on characterizing aluminosili- cate dissolution rates as a function of chemical affin- Ž ity andror aqueous Al and Si activities see for example, Nagy et al., 1991; Burch et al., 1993; Oelkers et al., 1994; Gautier et al., 1994; Oelkers and Schott, 1995a, 1999; Murphy et al., 1996; Devi- 0009-2541r01r$ - see front matter q 2001 Elsevier Science B.V. All rights reserved. Ž . PII: S0009-2541 00 00352-1