Experiments on the upper stability limit of pyrophyllite at 1.8 kb and 3.9 kb water pressure

The reaction pyrophyllite=andalusite + 3 quartz + water sets a lower limit for the stability of andalusite in quartz-bearing rocks. Tests show complete decomposition of pyrophyllite at 473 degrees + or -7 degrees C and 3.9 kb, 495 degrees + or -7 degrees C at 3.9 to 3.8 kb, and 447 degrees + or -7 degrees C at 1.7 to 1.8 kb. Partial decomposition occurred at 455 degrees + or -7 degrees C at 3.8 to 3.9 kb. Experiments on quartz and andalusite suggest equilibrium at 430 degrees + or -15 degrees C at 3.9 kb, and 410 degrees + or -15 degrees C at 1.8 kb. The present results, considering knowledge of Al ~2~ SiO ~5~ polymorphs, allow an appreciable stability field for andalusite and kyanite in quartz-bearing rocks at moderate temperatures, whatever the water pressure.