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-Mg1.97SiH0.03O4) 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 KT'=4 was KT=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. Copyright © 2007 by The Japanese Association of Mineralogists, Petrologists and Economic Geologists/The Mineralogical Society of Japan.