Host: The Mineralogical Society of Japan
Phase A (Mg7Si2O8(OH)6) is a dense hydrous magnesium silicate phase in which two fully occupied hydrogen sites have been found. Previously (Kagi et al., 2000) carried out neutron diffraction measurements to 3.2 GPa in fluorinert (3M Company) pressure-transmitting medium using the Paris-Edinburgh opposed-anvil cell. Reliable refinements were not possible using data collected beyond this pressure because of the relative complexity of the diffraction pattern and the substantial peak broadening due to deviatoric stress introduced above the freezing point of Fluorinert, . Recently however an encapsulated TiZr gasket has been developed by Marshal and Francis (2002) and this allows the use of deuterated methanol-ethanol mixtures as a pressure transmitting medium. We applied this new gasketting technique to the deuterated Phase A sample and obtained neutron diffraction patterns at pressures up to ca. 8 GPa on the POLARIS beam line at the UK pulsed neutron facility, ISIS at the Rutherford Appleton Laboratory. The quasi-hydrostatic condition minimized peak broadening and allowed us to take advantage of the higher resolution at low d-spacing at POLARIS to refine high quality structural models for Phase A with the Rietveld technique, without resorting to constraints involved in the previous study. Furthermore, the bulk modulus of Phase A was determined to be 106.2 (1.6) GPa by the single crystal X-ray diffraction method at high pressure. X ray diffraction intensities were collected by a four-circle diffractometer with monochromatized MoKa X-ray (50kV, 40mA) or synchrotron light source at Photon Factory, KEK. This bulk modulus gave consistent pressure values from the unit cell volumes of Phase A and coexisting brucite. The Raman spectra of Phase A measured at high pressures using a diamond anvil cell and a relationship between OH stretch frequency and atomic positions of hydrogen atoms will be presented.
