Pressure dependence of u parameter in ringwoodite up to 7.9 GPa

Abstract

X-ray diffraction intensities were measured for single-crystal, spinel structure ringwoodite (γ-Mg_1.97SiH_0.03O₄) at pressures up to 7.9 GPa in the BL-10A synchrotron beam line of the Photon Factory. The isothermal bulk modulus calculated using the Birch-Murnaghan equation of state assuming K_T’ = 4 was K_T = 184(6) GPa. The difference between this result and the value for anhydrous ringwoodite is not significant within the limit of experimental error due to a minimal H₂O content. The oxygen positional parameter u was revealed to increase significantly with pressure, approaching the ideal value of 1/4 for cubic closest packing. The mean Si-O distance remained almost constant up to 7.9 GPa, with compression of the unit cell occurring in proportion to that of the MO₆ octahedron.