Phase equilibria in the system MgO-Al2O3-SiO2-H2O) were studied in order to obtain further information on the stability of metamorphic minerals and mineral assemblages. The equilibria are described in terms of composition triangles, of phases in equilibrium with H2O. Seventeen sets of triangles are given to describe changing equilibria with increasing temperature, at a constant water pressure of 10,000 psi. Syntheses of phases and equilibrium data are discussed, along with factors influencing the attainment of equilibrium.

At low temperatures montmorillonite “solid solutions” cover a large field in the high-silica portion of the diagram, whereas mixtures high in magnesia yield aluminian serpentine, in combination with other phases. Alumina hydrates and kaolinite dominate the high-alumina portion of the diagram in this temperature range. Aluminian serpentine at higher temperatures gives way to clinochlore. Cordierite, the only anhydrous ternary phase encountered, is stable in equilibrium with H20 above ca. 500° C., and the stable joins, quartz-cordierite-H2O and cordierite-spinel-H2O, divide the quaternary system into two areas: that part dominated by the alumino-silicates, and the area involving magnesium silicates. In the alumina-silica side of the system pyrophyllite is the hydrate stable to the highest temperature, 575° C.; but in the magnesia-silica side, talc is stable to a much higher temperature, 780° C. Applications of the data obtained to geologic problems are discussed.

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