Some Calc-Silicate Equilibrium Relations

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Abstract
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.

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