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

Ohtani和Mizobata（1998）使用多砧装置在1680 ℃和22 GPa条件下合成的35 × 35 × 24 μ m含水环木石Mg_ SiH_ O_4单晶的X射线衍射强度高达7.9 GPa<1.97><0.06>，使用同步辐射在光束线BL-10A（Photon Factory，High Energy Accelerator Research Organization，Tukuba，Japan）测量。使用改进的Merrill-Bassett型金刚石压砧压力室。压力介质为甲醇和乙醇的4：1混合流体。单胞的压缩系数与MO_6八面体的压缩系数接近。平均Si-O距离在7.9GPa以下几乎保持不变。晶体结构的压缩是由MO_6八面体的压缩控制的。这与体积模量受Mg被2 H取代的显著影响的事实一致。在<0.84><0.07><0.09>1000 ℃和21 GPa条件下，用多砧装置合成了δ-（Al_4 Mg_ Si_）OOH晶体，研究了晶体的空间群和氢位置。单晶X射线衍射确定的空间群为Pnn 2，晶胞参数为a=4.6975（8）&lt;$，B=4.2060（6）&lt;$，c=2.8327（4）&lt;$，V=55.97（1）&lt;$^3。Mg和Si对Al位的部分占据表明δ-AlOOH、中硅镁石和密度比水镁石高16%的CaCl_2型Mg（OH）_2之间可能存在有限固溶体。差分傅立叶图揭示了两个小的，但重要的傅立叶峰归因于氢原子。第一个峰周围的原子距离和角度表明氢键与O[三键]O距离为2.511 π，而第二个峰周围的原子距离和角度表明氢键分叉，O[三键]O距离为2.743 π和2.743 π。

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.