Experimental data on the Tschermak substitution in Fe-chlorite

Abstract

Iron chlorite with compositions intermediate between the two end-members daphnite [Fe 5 Al 2 Si 3 O 10 (OH) 8 ] and Fe-amesite [Fe 4 Al 4 Si 2 O 10 (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 0 Fe-ames = 213.06 cm 3 and V 0 daph = 216.50 cm 3 . 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 0 Daph - V 0 Fe-ames and V FeMg-1 , with linear (ideal) or non-ideal volume models involving a positive excess volume. With these models we obtain V 0 Daph between 216 cm 3 and 217.49 cm 3 , and V 0 Fe-ames between 209 and 211.35 cm 3 . Equilibration experiments involving chlorite with almandine-hercynite/fayalite or chloritoidhercynite/ 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 0 fFe-amesite 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 0 fFe-amesite = -7616 ± 3 kJ and W G AlFe ~ -10 kJ.