A thermodynamic model for FE-MG aluminous chlorite using data from phase equilibrium experiments and natural pelitic assemblages in the 100 to 600 C, 1 to 25 kb range

The purpose of this study is to derive a solid solution model for aluminous (Si 1) which is not possible when using only one chlorite endmember (classically clinochlore). In particu- lar, reliable pressure estimates can be made for the common chlorite-quartz-carpholite or chloritoid or garnet bearing rocks devoid of aluminosilicates, whereas such esti- mates are impossible when using only one chlorite endmember. In the most favorable cases, temperature conditions can be estimated from the location of the temperature- dependent equilibrium 2 clinochlore 1 3 Mg-sudoite 5 4 Mg-amesite 1 7 quartz 1 4 H2O, that is from the composition of chlorite associated with quartz. Our chlorite solution model predicts that at fixed pressure and (XMg)chlorite, the location of this equilibrium is shifted toward higher temperature when decreasing the Si, Al VI , and vacancy contents and increasing the Al IV content. This result is compatible with the classical empirical thermometers based on the Al IV and vacancy contents in chlorite. However, the calculated effect of pressure is an increase of the Al IV ,A l VI , and vacancy contents. This explains why the empirical chlorite thermometers (based on the Al IV contents in chlorite) derived from low-T samples cannot be used at high pressure conditions.