Experimental data on the Tschermak substitution in the Fe-chlorite.
Résumé
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 fO2 conditions deÞ ned 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 Þ t of these data yields V0 Fe-ames = 213.06 cm3 and V0 daph = 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 Þ tted for reasonable values of V0 Daph V0 Fe-ames and VFeMg-1, with linear (ideal) or non-ideal volume models involving a positive excess volume. With these models we obtain V0 Daph between 216 cm3 and 217.49 cm3, and V0 Fe-ames between 209 and 211.35 cm3. 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 fO2 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 deÞ nitive results, obtained for the assemblage chlorite-almandine-fayalite, were used to estimate Hf 0 Fe-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 Hf 0 Fe-amesite = 7616 ± 3 kJ and WG AlFe ~ 10 kJ.