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

在日本土波高能加速器研究机构光子工厂的BL-10A束流线上，利用同步辐射，在1680℃和22 GPa条件下，对大谷和Mizobata（1998）用多顶仪合成的35x35x24微米含水菱木石Mg_<1.97>SiH_<0.06>O_4单晶进行了高达7.9 GPa的x射线衍射测量。采用改良的Merrill-Bassett型金刚石顶砧压力传感器。压力介质为甲醇与乙醇的4:1混合液。单元胞的可压缩性接近MO_6八面体的可压缩性。平均Si-O距离基本保持不变，达到7.9 GPa。晶体结构的压缩是由MO_6八面体的压缩控制的。这与体积模量受到2H取代Mg的显著影响这一事实是一致的。八面体共享边比非共享边受到更大的压缩，使得SiO_4四面体的边缘长度几乎保持不变。在1000℃、21 GPa条件下，用多砧装置合成单晶，研究了δ-(Al_<0.84>4Mg_<0.07>Si_<0.09>)OOH的空间基团和氢位置。单晶x射线衍射确定的空间群为Pnn2，单位胞参数为a=4.6975(8)Å， b=4.2060(6)Å， c=2.8327(4)Å， V=55.97(1)Å^3。Mg和Si占据了Al的部分位置，表明δ-AlOOH与水辉石和假想的比水辉石密度高16%的cacl_2型Mg(OH)_2之间可能存在有限的固溶体。差分傅里叶图显示了两个小而重要的可归因于氢原子的傅里叶峰。第一个峰附近的原子距离和角度表明氢键的O（三键）O距离为2.511Å，而第二个峰附近的原子距离和角度表明氢键的O（三键）O距离为2.743Å和2.743Å。

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.