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A compilation of literature data on the Fe-Mg composition of coexisting chlorite and chloritoid from metapelites metamorphosed at various P-T conditions shows that the logarithm of the Fe-Mg partitioning (lnK D ) varies linearly with the inverse of temperature, from about 2.4 at 300°C to about 1.3 at 600°C. In contrast, no trend was observed with pressure, and the molar volumes of Mg-and Fe-chlorite endmembers suggest that the pressure dependence of lnK D is not significant. Therefore, the chloritoid-chlorite Mg-Fe exchange reaction is a potential thermometer and has been empirically calibrated using the analyses of 112 chloritoid-chlorite pairs from 28 different localities. Temperatures estimated using the Chl-Cld thermometer were checked against independent estimates for 20 samples not involved in the calibration (Beni Mzala window, Morocco), and the results are in fair agreement with independent temperature estimates. However, the analytical uncertainties and errors are too large to obtain reliable temperature estimates for extremely Mg-rich or Fe-rich compositions. The Chl-Cld thermometer is unreliable at XMg-CLD <0.2 and XMg-CLD >0.8 at 700°C, and XMg-CLD <0.1 and XMg-CLD >0.9 at 300°C. Using the results of the empirical calibration, we calculated new thermodynamic data for daphnite. Implementing these data, it becomes possible to estimate T and P conditions of metamorphism for the invariant chlorite-chloritoid-quartz-aluminosilicate assemblage that is widespread in low-grade metapelites. These estimates appear to be relevant only in the stability field of kyanite, whereas the uncertainties on the calculated pressure conditions are very large in the stability field of kaolinite and pyrophyllite.

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