Some compositions in the system CaO-Al 2 O 3 -SiO 2 -H 2 O have been studied in the temperature range 400-900 degrees C. and H 2 O pressures ranging from 5000 to 42,000 bars. The phase assemblage boundary of the reaction anorthite+corundum+water = zoisite+sillimanite has a slight negative slope. The assemblage anorthite+quartz+water is stable at H 2 O pressure below 12,000 bars at 800 degrees C. and 14,000 bars at 400 degrees C. Clinozoisite and zoisite are the stable phases at high pressures, but at H 2 O pressures above 23,000 bars at 400 degrees -600 degrees C. and 20,000 bars at 850 degrees C., lawsonite is the stable phase.