Synthesis of high-pressure hydrous magnesium silicates: Observations and 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 and electron microprobe analysis were used to study both major and minor phases occurring in the experimental products. We observed the formation of phase A at 16 GPa, 1172°C; phase B at 15 GPa, 1158°C; chondrodite at 12 GPa, 1158°C; and clinohumite at 12 GPa, 1170°C. These pressures and temperatures are higher than those reported in previous studies. We also observed the formation of phases E and superhydrous B at conditions at which they have been previously observed. However, we propose that there is a high-low transition in superhydrous B. We have not identified phase C in the pressure-temperature-bulk compositional space where it has been observed in previous studies. We conclude that this phase may be identical with superhydrous B, as previously suggested, or that in some studies a humite may have been misidentined as phase C.