Experimental data on the Tschermak substitution in Fe-chlorite
-
Teddy Parra
,
Olivier Vidal
Abstract
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 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 V0Fe-ames = 213.06 cm3 and V0daph = 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 V0Daph - V0Fe-ames and VFeMg-1, with linear (ideal) or non-ideal volume models involving a positive excess volume. With these models we obtain V0Daph between 216 cm3 and 217.49 cm3, and V0Fe-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 definitive results, obtained for the assemblage chlorite-almandine-fayalite, were used to estimate H0fFe-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 H0fFe-amesite = -7616 ± 3 kJ and WGAlFe ~ -10 kJ.
© 2015 by Walter de Gruyter Berlin/Boston
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- The origin of orthopyroxene/biotite + plagioclase coronas from the Bolangir anorthosite complex (India), and implications for reconstructing P-T paths
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