SYNTHESIS OF HIGH-PRESSURE HYDROUS MAGNESIUM SILICATES - OBSERVATIONSAND ANALYSIS

Specimens of dense high-pressure hydrous magnesium silicates, notably phase A, phase B, and superhydrous phase B, have been synthesized for use in calorimetric and spectroscopic experiments. X-ray diffraction a nd electron microprobe analysis were used to study both major and mino r phases occurring in the experimental products. We observed the forma tion of phase A at 16 GPa, 1172 degrees C; phase B at 15 GPa, 1158 deg rees C; chondrodite at 12 GPa, 1158 degrees C; and clinohumite at 12 G Pa, 1170 degrees C. These pressures and temperatures are higher than t hose reported in previous studies. We also observed the formation of p hases E and superhydrous B at conditions at which they have been previ ously observed. However, we propose that there is a high-low transitio n in superhydrous B. We have not identified phase C in the pressure-te mperature-bulk compositional space where it has been observed in previ ous studies. We conclude that this phase may be identical with superhy drous B, as previously suggested, or that in some studies a humite may have been misidentified as phase C.