Equilibrium data for the reaction 5KMg3AlSi3O10(OH)2 + 6CaCO3 + 24SiO2 → 3Ca2Mg5Si8O22(OH)2 + 5 KAlSi3O8 + 6CO2 + 2H2O have been determined reversibly over the pressure range of 2-8 kbar. For XCO2 = 0.75 in the fluid the equilibrium curve lies at 664 ± 18°C at 8 kbar, 600 ± 25°C at 6 kbar, 540 ± l5°C at 4 kbar, and 475 ± l5°C at 2 kbar. The enthalpy of reaction at 600°C and 6 kbar is 91 ± 12 kcal. The T-Xco2 curve at 6 kbar, calculated assuming ideal fluid mixing from a log fco26fH2O2 - 1/T plot of the data, place the equilibrium at Xco2 = 0.58 and 0.81 at 600°C, Xco2 = 0.41 and 0.95 at 575°C, Xco2 = 0.28 and 0.98 at 550°C, Xco2 = 0.20 and 0.99 + at 525°C and xco2 = 0.13 and 0.99 + at 500°C

The intersection of this reaction with the reaction anorthite + calcite + H2O ⇌ zoisite + CO2 is indifferent in the end member system at 6 kbar. The common occurrence in the natural system of the assemblage present at the intersection of these reactions indicates that additional components raise the equilibrium temperature of the ph-cc-qtz reaction relative to that of the reaction muscovite + calcite + quartz → anorthite + K-feldspar + CO2 + H2O. Although the assemblage ph-cc-qtz is more stable at high Xco2, and less stable at low Xco2, than the mu-cc-qtz assemblage in the pure system, the addition of other components favors the relatlive stability of ph-cc-qtz over the whole range of Xco2.

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