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Experimental data on the Tschermak substitution in Fe-chlorite

¹ Laboratoire de Géologie, UMR 8538, Ecole Normale Supérieure, 24 rue Lhomond, F-75231 Paris Cedex 05, France
² LGCA, UMR 5025, 1381, rue de la Piscine, BP 53, F-38041 Grenoble Cedex 09, France
³ Institut für Mineralogie und Kristallchemie, Universität Stuttgart, Azenbergstrasse 18, D-70174 Stuttgart, Germany

Correspondence: ^* E-mail: parra{at}geologie.ens.fr

Iron chlorite with compositions intermediate between the two end-members daphnite [Fe₅Al₂Si₃O₁₀(OH)₈] and Fe-amesite [Fe₄Al₄Si₂O₁₀(OH)₈] were synthesized from gels, under f_O2 conditions defined by the solid oxygen buffer Fe-FeO. The unit-cell parameters and volume of chlorite with Si-content ranging from 2.3 to 2.7 were calculated. A least-squares fit of these data yields V_Fe-ames⁰ = 213.06 cm³ and V_daph⁰ = 216.50 cm³. The molar volume of daphnite is similar to that estimated by Vidal et al. (2001), but the volume difference between Fe-amesite and daphnite is too low. The experimental data were also fitted for reasonable values of V_Daph⁰ – V_Fe-ames⁰ and V_FeMg-1, with linear (ideal) or non-ideal volume models involving a positive excess volume. With these models we obtain V_Daph⁰ between 216 cm³ and 217.49 cm³, and V_Fe-ames⁰ between 209 and 211.35 cm³.

Equilibration experiments involving chlorite with almandine-hercynite/fayalite or chloritoid-hercynite/fayalite provide data on the chlorite composition as a function of T and P at temperatures between 420 and 520 °C and pressures between 3 and 20 kbar, at f _O2 buffered by the assemblage Fe-FeO. Initial Si-rich and Si-poor chlorite compositions converged in most cases toward an equilibrium composition during the experiments. The results show that the Si-content of chlorite is sensitive to temperature for the various divariant assemblages. The most definitive results, obtained for the assemblage chlorite-almandine-fayalite, were used to estimate H_fFe-amesite⁰ and the Al-Fe Margules parameter for the various sets of daphnite and Fe-amesite molar volumes constrained from the synthetic chlorites. The results indicate that H_fFe-amesite⁰ = –7616 ± 3 kJ and W_AlFe^G ~ –10 kJ.

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				O. Vidal, T. Parra, and P. Vieillard Thermodynamic properties of the Tschermak solid solution in Fe-chlorite: Application to natural examples and possible role of oxidation American Mineralogist, February 1, 2005; 90(2-3): 347 - 358. [Abstract] [Full Text] [PDF]