Some calc-silicate equilibrium relations

P-T curves determined experimentally for univariant equilibria are: (1) zoisite+quartz=anorthite+grossularite+vapor; (2) grossularite+quartz=anorthite+wollastonite; (3) zoisite+sillimanite=quartz+anorthite+vapor. Thermodynamic constants calculated from the results and published data give P-T curves for other reactions in the system CaO-SiO ~2~ , -Al ~2~ O ~3~ -H ~2~ O, entropy of grossularite and zoisite, and delta G for sillimanite. Reactions 1 and 2 agree with natural assemblages. The calculated anorthite breakdown curve lies at a lower pressure but close to the curve of albite (Birch and Le Conte, 1960) indicating that intermediate plagioclase may break down as a unit during a subcrustal phase change.