A portion of the join anorthite-water in the quaternary system CaO–Al2O3–SiO2–H2O has been reinvestigated in the range 550°–990°C, from 7 to 16 kbar. The reaction 4 anorthite + H2O ⇄ 2 zoisite + kyanite + quartz can be described by the equation P = −4590 + 20.4T, where P is in bars, T is in °C. The reaction is terminated at an invariant point at 725°C and 10.2 kbar, where melt first appears. The two reactions at higher temperatures and pressures are zoisite + kyanite + quartz + vapor ⇄ liquid, and anorthite + vapor ⇄ liquid, the latter delimiting the fields of anorthite + vapor and zoisite + kyanite + liquid + vapor. The incongruent melting of anorthite + water, beginning at 725°C, is significantly lower than earlier reported, and is unusual in that the solid assemblage anorthite + vapor lies on the high-temperature side of the boundary.

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