Hydration of Dense Polymorphs of Silica in Subducting Slabs

二氧化硅（SiO2）是地壳和地幔的主要化学成分之一，形成了地球表面常见的矿物石英、钇石和钴矿。在地幔特有的高压下，这些矿物转变为密度更大的矿物，最终转变为矿物辉石（也在陨石坑等撞击地点发现）。Stishovite之所以引起人们的极大兴趣，是因为它的高密度和硅原子的高（6倍）配位，以及它在最初形成于地表，但通过俯冲作用被带到地球深处的岩石中的预期存在。该小组最近的发现，使用多砧高压装置，证明在高压和适度温度下，普通的水，H2O，被辉石吸收了相对大量的（高达几个重量百分比），氢以OH-离子分子的形式成为矿物晶体结构的一部分。矿物的结构体积由于水的加入而膨胀。实现此行为不需要额外的组件吗？只有纯的H2O和SiO2。这一惊人的发现提出了新的问题，关于针云石的结构化学，以及地球上针云石和其他矿物的行为。在拟议的研究中，研究人员将更详细地研究在不同压力和温度下，包括与地球内部有关的压力和温度下，辉石吸收水分的速率和数量。此外，他们将研究含水量对岩石物理性质的影响，如压缩性和声速。这项研究有望有助于了解高压下水与硅酸盐接触时的基本行为，这对于描绘地球整体水文循环过程非常重要。结果将阐明辉石水化的晶体化学含义。此外，这项研究对于了解矿物中的氢具有重要的矿物学意义，并且可能对了解气态巨行星的矿物学组成具有重要意义。

Silica, or SiO2, is one of the primary chemical components of the Earth's crust and mantle, and forms the common minerals quartz, tridymite and coesite on the Earth's surface. At higher pressures characteristic of the mantle, these minerals transform into denser minerals, ultimately transforming into the mineral stishovite (also found in impact sites such as Meteor Crater). Stishovite is of great interest because if its high density and high (6-fold) coordination of the silicon atom, as well as its expected presence in rocks initially formed at the surface, but brought deep into the Earth by subduction. Recent findings by the team, using a multi-anvil high pressure apparatus, demonstrate that under high pressure and modest temperature, ordinary water, H2O, is absorbed by stishovite in relatively significant amounts (up to several weight percent), and the hydrogen becomes part of the crystal structure of the mineral in the form of an OH- ionic molecule. The structural volume of the mineral expands as a result of the addition of H2O. No additional components are needed to achieve this behavior ? only pure H2O and SiO2. This surprising discovery raises new questions about the structural chemistry of stishovite, and the behavior of stishovite and other minerals in the Earth. In the proposed research, the investigators will pursue a more detailed study of the rates and quantities of water absorbed in stishovite at different pressures and temperatures, including those relevant to the Earth's interior. Furthermore, they will study the effect of water contents on the physical properties of the stishovite such as compressibility and sound velocity. This research is expected to contribute to fundamental understanding of the behavior of H2O in contact with silicates at high pressures, which is important for delineating processes involved in the Earth's overall hydrologic cycle. Results will clarify the crystal chemical implications of stishovite hydration. Also this research is of significant mineralogical interest in the understanding of hydrogen in minerals, and may be important in understanding the mineralogical make-up of gas giant planets.