Experimental reversals of chlorite compositions in divariant MgO + Al2O3 + SiO2 + H2O assemblages

The compositions of chlorite in divariant assemblages involving orthopyroxene + forsterite and spinel + corundum were reversed at a range of temperatures for P = 14 kbar and a range of pressures for T = 800°C. These compositions, determined using a new calibration of the variation in the c unit-cell parameter with Xchl, were then compared with those calculated using several simple activity-composition models for charge-balanced Tschermak substitutions in MASH chlorite. The discrepancies between the observations and calculations suggest that entropies of mixing are very low in chlorite solid solutions.