Raman Spectra of Bredigite at High Temperature and High Pressure.

Bredigite was synthesized by using the Piston-Cylinder in 1.2 GPa and 1 473 K. With external heating device and diamond anvil cell, high temperature and high pressure Raman spectra of bredigite were collected at temperatures 298, 353, 463, 543, 663, 773 and 873 K and with pressure from 1 atm up to 14.36 GPa (room temperature). The SEM image showed that the sample consisted of one crystalline phase with grain size ranging from 10～20 μm. The EPMA data suggest a chemical formula of Ca7.03(2)Mg0.98(2)Si3.94(2)O16 which was identical to the theoretical component of bredigite. The Raman spectroscopic results indicate there were 29 vibration bands of bredigite at high temperature. Some bands were merging, weakening and disappearing increasingly with the temperature, which was obvious in the range of 800~1 200 cm-1. The vibration bands of 909, 927 and 950 cm-1 disappeared at 873, 773 and 873 K, respectively. The results primarily indicated that the structure of bredigite was stable under experimental condition. In addition, isobaric mode-Grüneisen parameters and isothermal mode-Grüneisen parameters were calculated, yielding 1.47(2) and 0.45(3) as their mean values, respectively. Anharmonic coefficients were estimated based on the high temperature and high pressure Raman experiments, showing that the contributions to anharmonic-effect induced with the Si—O vibration modes were smaller than other modes.