Static compression of the hydrous magnesium silicate phase D to 30 GPa at room temperature

Phase D is a dense hydrous magnesium silicate (ideal formula MgSi2H2O6) whi ch contains silicon cations exclusively in octahedral coordination. Measure ments of the unit cell parameters of phase D were made to pressures of 30 G Pa using a diamond anvil cell and employing synchrotron X-ray diffraction. A neon pressure medium was used. Using a third order Birch-Murnaghan equati on of state the isothermal bulk modulus of phase D was determined as 166(+/ -3) GPa with K' equal to 4.1(+/-0.3). The compression of phase D is anisotr opic with the c-axis twice as compressible as the a-axis. Above 20 GPa, how ever, the c/a ratio becomes pressure independent.