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X ray diffraction intensities were measured for single crystal, spinel structure ringwoodite (γ Mg 1.97 SiH 0.03 O 4 ) 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 H 2 O 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 MO 6 octahedron.