New experimental data on the upper thermal stability of chlorite in the system H2O-MgO-Al2O3-SiO2 are reported here and are combined with other experimental data, molar volumes of the solid phases, heat capacities, H2O fugacities, and activity expressions for cordierite, chlorite, and orthopyroxene to obtain an array of univariant curves about an invariant point at 720 ± 10°C and 2.75 ± 0.3 kbar where chlorite, cordierite, forsterite, orthopyroxene, spinel, and water are in equilibrium. A crystallochemical study of chlorite synthesized from three different bulk compositions at 3 and 14 kbar and over the temperature range of 650-850°C suggests that Mg-chlorite attains a composition of (Mg4.8Al1.2) (Si2.8Al1.2)O10(OH)8 at its upper thermal stability at all pressures. Thermochemical data for clinochlore were derived from the experimental data on the dehydration of chlorite above 3.5 kbar, yielding values of −8220.452 ± 27 kJ/mol and −2186.2 ± 33 J/(K · mol) for ΔGf0 and ΔSf0, respectively, at 1 bar and 298 K.

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