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Thermodynamic properties of the Tschermak solid solution in Fe-chlorite: Application to natural examples and possible role of oxidation

¹ LGCA, UMR 5025, 1381 rue de la Piscine, BP 53, F-38041 Grenoble Cedex 09, France
² Laboratoire de Géologie, UMR 8538, Ecole Normale Supérieure, 24 rue Lhomond, F-75231 Paris Cedex 05, France
³ HYDR. ASA, UMR 6532, Université de Poitiers, 40, avenue du Recteur Pineau, 86022 Poitiers Cedex, France

Correspondence: ^* E-mail: ovidal{at}ujf-grenoble.fr

The standard-state thermodynamic properties of 14 Å Fe-amesite [Fe₄Al₄Si₂O₁₀(OH)₈] were calculated from the reequilibration experiments of Parra et al. (2005) and natural data bearing on the Fe-Mg partitioning between chlorite and chloritoid over a wide range of pressure (P), temperature (T), and rock composition. The combination of these data with the three-site mixing model and thermodynamic properties of daphnite proposed by Vidal et al. (2001) led us to reappraise the evolution of the Margules parameter W^G_AlFe with pressure and temperature. The new data [H⁰f_Fe-Am = –7 607 460 J/mol, S⁰_Fe-Am = 514.8 J/(mol·K), V⁰_Fe-Am = 20.90 J/bar, W^G_AlFe = 1200 – 31T + 0.7(P – 1)] are compatible with all the natural data and reequilibration experiments, and in fair agreement with the results of synthesis. They are used to calculate the compositional evolution of chlorite with T, P, or a_O2 at different bulk-system compositions. The calculated phase relations and chlorite compositions are compared with previous experiments conducted in the FeO-Al₂O₃-SiO₂-H₂O system. The addition of the Fe-amesite end-member increases the number of equilibria that can be calculated for any natural assemblage involving chlorite. Two independent equilibria are obtained for natural (Fe-Mg)-chlorite-quartz or diaspore assemblages. It is therefore possible to estimate metamorphic conditions for high-variance assemblages from the composition of chlorite. We discuss the reliability of such P-T estimates using natural chlorites occurring in low-pressure and low-temperature or medium- to high-pressure and temperature samples. Assuming that the proposed model and thermodynamic parameters are correct, these various natural data suggest that X_Fe³⁺ [= Fe³⁺/(Fe²⁺ + Fe³⁺)] in metamorphic chlorites is low (<0.1), whereas it can be as high as 0.3 in Si- and Mg-rich chlorites formed at low-P-T conditions.

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