Experimental determination of the solubility product of magnesite at 50 to 200 °C Pascale Bénézeth ⁎, Giuseppe D. Saldi 1 , Jean-Louis Dandurand, Jacques Schott Géosciences Environnement Toulouse (GET)—CNRS—IRD—OMP—Université de Toulouse, 14, Avenue Edouard Belin, 31400 Toulouse, France abstract article info Article history: Received 17 December 2010 Received in revised form 5 April 2011 Accepted 11 April 2011 Available online 30 April 2011 Editor: Dr. J. Fein Keywords: Magnesite solubility product Thermodynamic properties Stability Accurate knowledge of magnesite thermodynamic properties over a wide range of temperatures is crucial for characterizing mineral stabilities in the system MgO–CO 2 –H 2 O and for modeling carbon dioxide fate in important natural and industrial processes. However, available databases, especially for its solubility product, are sparse and contradictory, leading to considerable uncertainties in the calculation of chemical equilibria and phase transformations among carbonates. In this study, the solubility of synthetic magnesite was investigated from 50 to 200 °C in 0.1 mol kg -1 NaCl solutions and in some cases under constant CO 2 partial pressure (4–30 bars) both by means of a hydrogen electrode concentration cell (HECC) and a traditional batch Ti-reactor. The obtained apparent solubility products (Q sp-mgs ) were extrapolated to infinite dilution to generate the solubility products (K sp°-mgs ), allowing calculation of the thermodynamic properties of magnesite. Of all the temperature functions tested, the equation giving a reliable fit of our data in the investigated temperature range (50–200 °C) has the following form: log 10 K sp°-mgs =a +b / T (K) +cT (K) with: a = 7.267, b =-1476.604 and c =-0.033918. Based on this equation and its first and second derivatives with respect to T, we were able to derive values at 25 °C, 1 bar for magnesite thermodynamic functions: Δ f G 298.15 o =(-1026.48 ± 2) kJ mol -1 (log 10 K sp°-mgs =-7.80 ± 0.3), Δ f H 298.15 o =(-1111.75 ± 2) kJ mol -1 , S 298.15 o = (60.00 ± 2) J mol -1 K -1 , and C p 298.15 o = (75.91 ± 2) J mol -1 K -1 (uncertainties are 3σ). © 2011 Elsevier B.V. All rights reserved. 1. Introduction 1.1. Stability of carbonates in the system MgO–CO 2 –H 2 O The stability of the phases forming in the system MgO–CO 2 –H 2 O (see Table 1 for a list of names and chemical formulas) has been the object of numerous studies. Despite the abundance of data concerning the relative stability of the carbonate phases belonging to this system, surprising discrepancies exist between the results of different studies, reflecting the experimental difficulties encountered in studying Mg-carbonate phases. For instance, some confusion exists in the literature with respect to the exact formula of hydromagnesite, that according to some authors (Baron and Favre, 1958; Kazakov et al., 1959; Sayles and Fyfe, 1973; Dandurand and Schott, 1977; Könisberger et al., 1999), corresponds to 5MgO·4CO 2 ·5H 2 O, whereas Palache et al. (1951), Carpenter (1963) and Langmuir (1965) proposed the chemical formula 4MgO·3CO 2 ·4H 2 O and Takahashi (1927) used 5MgO·4CO 2 ·7H 2 O to indicate the same mineral phase. As a consequence, the different chemical formulae generate contradic- tions between corresponding solubility product values. Although rare, magnesite (MgCO 3 ) is present in a wide array of geologic settings (Pohl and Siegl, 1986) such as (1) sediment-hosted deposits, which tend to occur in Precambrian strata, (2) deposits that form at or near ultramafic complexes of all ages, and (3) extremely localized deposits forming in modern sabkha and alkaline lake environments. However, major discrepancies exist for the solubility product and free energy of formation of magnesite (MgCO 3 ). Reviews of existing data (e.g., Langmuir, 1965; Christ and Hostetler, 1970; Kittrick and Peryea, 1986; Könisberger et al., 1999; Marion, 2001) showed that reported values of the solubility product (see Reaction (1) below) at room temperature ranged from 10 -10.3 to 10 -5.1 . This large variation of magnesite solubility product has deep consequences on the relative stability of the various mineral phases of the system MgO–CO 2 –H 2 O and results in different phase relations as a function of pCO 2 and temperature. Langmuir (1965) explained the general scarcity of Mg-carbonates in natural environments by their high solubilities and assigned to magnesite a solubility product of 10 -5.1 , higher than that of nesquehonite (MgCO 3 ⋅ 3H 2 O) and hydromagnesite (4MgCO 3 ⋅ Mg (OH) 2 ⋅ 4H 2 O) (K sp =10 -4.9 ). However, according to more recent determinations of solubility and Gibbs free energy of formation (Kittrick and Peryea, 1986; Könisberger et al., 1999), magnesite appears to be the most stable carbonate phase forming in the system MgO–CO 2 –H 2 O, under all conditions of temperature and CO 2 partial pressure, as also reported by Marion (2001) and Hänchen et al. (2008). All other hydrated Mg-carbonates are unstable with respect to magnesite but their formation is favored by the slow kinetics of crystallization of magnesite, commonly attributed to the strongly Chemical Geology 286 (2011) 21–31 ⁎ Corresponding author. Tel.: +33 5 61 33 26 17; fax: +33 5 61 33 25 60. E-mail address: benezeth@get.obs-mip.fr (P. Bénézeth). 1 Present address: Earth Science Division, Lawrence Berkeley National Laboratory, 1 Cyclotron Road, Berkeley, CA 94720, USA. 0009-2541/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.chemgeo.2011.04.016 Contents lists available at ScienceDirect Chemical Geology journal homepage: www.elsevier.com/locate/chemgeo