High temperature high pressure crystal structures and phase changes of subduction zone minerals.

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Sets of X-ray diffraction intensities up to 7.9 GPa of a single crystal of 35x35x24 micron hydrous ringwoodite Mg_<1.97>SiH_<0.06>O_4, synthesized by Ohtani and Mizobata (1998) using a multi-anvil apparatus at conditions of 1680 ℃ and 22 GPa were measured using synchrotron radiation at the beam line BL-10A, Photon Factory, High Energy Accelerator Research Organization, Tukuba, Japan. The modified Merrill-Bassett type diamond anvil pressure cell was used. The 4:1 fluid mixture of methanol and ethanol was used for pressure medium. The compressibility of the unit cell is close to the compressibility of the MO_6 octahedron. The mean Si-O distance stay almost constant up to 7.9 GPa. The compression of the crystal structure is governed by the compression of MO_6 octahedron. This is consistent to the fact that the bulk modulus is affected significantly by the substitution of Mg by 2H. The octahedral shared edges are compressed more than the unshared edges, keeping the edge lengths of SiO_4 tetrahedron almost constant.The space group and hydrogen positions of δ-(Al_<0.84>4Mg_<0.07>Si_<0.09>)OOH are investigated using a single crystal synthesized using a multi-anvil apparatus under conditions of 1000 ℃ and 21 GPa. The space group determined by single-crystal x-ray diffraction is to Pnn2, with unit cell parameters of a=4.6975(8)Å, b=4.2060(6)Å, c=2.8327(4)Å, and V=55.97(1)Å^3. Partial occupancy of the Al site by Mg and Si suggests the possibility of a limited solid solution between δ-AlOOH, stishovite, and a hypothetical CaCl_2-type Mg(OH)_2 that is 16% denser than brucite. Difference Fourier maps reveal two small but significant Fourier peaks attributable to hydrogen atoms. Atomic distances and angles around the first peak indicate a hydrogen bond with O【triple bond】O distances of 2.511Å, while those around the second peak are suggestive of a bifurcated hydrogen bond with O【triple bond】O distances of 2.743Å and 2.743Å.