High Pressure Collaborative Access Team, Geophysical Laboratory, Carnegie Institution of Washington, Argonne, IL 60439;
Frontier Research Institute for Interdisciplinary Sciences, Tohoku University, Aoba-ku, 980-8578 Sendai, Japan.
Proc Natl Acad Sci U S A. 2018 Feb 20;115(8):1742-1747. doi: 10.1073/pnas.1716748115. Epub 2018 Feb 5.
Knowledge of the structure and properties of silicate magma under extreme pressure plays an important role in understanding the nature and evolution of Earth's deep interior. Here we report the structure of MgSiO glass, considered an analog of silicate melts, up to 111 GPa. The first (r1) and second (r2) neighbor distances in the pair distribution function change rapidly, with r1 increasing and r2 decreasing with pressure. At 53-62 GPa, the observed r1 and r2 distances are similar to the Si-O and Si-Si distances, respectively, of crystalline MgSiO akimotoite with edge-sharing SiO structural motifs. Above 62 GPa, r1 decreases, and r2 remains constant, with increasing pressure until 88 GPa. Above this pressure, r1 remains more or less constant, and r2 begins decreasing again. These observations suggest an ultrahigh-pressure structural change around 88 GPa. The structure above 88 GPa is interpreted as having the closest edge-shared SiO structural motifs similar to those of the crystalline postperovskite, with densely packed oxygen atoms. The pressure of the structural change is broadly consistent with or slightly lower than that of the bridgmanite-to-postperovskite transition in crystalline MgSiO These results suggest that a structural change may occur in MgSiO melt under pressure conditions corresponding to the deep lower mantle.
了解硅酸盐熔体在极端压力下的结构和性质对于理解地球深部内部的性质和演化起着重要作用。在这里，我们报告了 MgSiO 玻璃的结构，该玻璃被认为是硅酸盐熔体的类似物，压力高达 111 GPa。配分函数的第一（r1）和第二（r2）近邻距离迅速变化，r1 随压力增加而增加，r2 随压力减小而减小。在 53-62 GPa 时，观察到的 r1 和 r2 距离分别与具有边缘共享 SiO 结构基元的晶态 MgSiO akimotoite 的 Si-O 和 Si-Si 距离相似。在 62 GPa 以上，随着压力的增加，r1 减小，r2 保持不变，直到 88 GPa。在此压力以上，r1 或多或少保持恒定，r2 再次开始减小。这些观察结果表明，在 88 GPa 左右存在超高压结构变化。在 88 GPa 以上的结构被解释为具有最接近的边缘共享 SiO 结构基元，类似于晶态后钙钛矿的结构基元，具有密集堆积的氧原子。结构变化的压力与晶态 MgSiO 中 bridgmanite 到后钙钛矿转变的压力大致一致或略低。这些结果表明，在对应于深部下地幔的压力条件下，MgSiO 熔体可能发生结构变化。
