Hydrostatic compression and crystal structure of pyrope to 33 GPa

The equation of state and crystal structure of pyrope were determined by single crystal X-ray diffraction under hydrostatic conditions to 33 GPa, a pressure that corresponds to a depth of about 900 km in the lower mantle. The bulk modulus K-TO and its pressure derivative K-TO' were determined simultaneously from an unweighted fit of the volume data at different pressures to a third order Birch-Murnaghan equation of state. They are 171(2) GPa and 4.4(2), respectively. Over the whole pressure range, MgO8 polyhedra showed the largest compression of 18.10(8)%, followed by AlO6 and SiO4 polyhedra, with compression of 11.7(1)% and 4.6(1)%, respectively. The polyhedral bulk moduli for MgO8, AlO6 and SiO4 are 107(1), 211(11) and 580(24) GPa, respectively, with K-TO' fixed to 4. Significant compression of up to 1.8(1)% in the very rigid Si-O bonding in pyrope could be detected to 33 GPa. Changes in the degree of polyhedral distortion for all three types of polyhedra could also be observed. These changes could be found for the first time for AlO6 and SiO4 in pyrope. It seems that the compression of pyrope crystal structure is governed by the kinking of the Al-O-Si angle between the octahedra and tetrahedra. No phase transition could be detected to 33 GPa.