Hydration of Dense Polymorphs of Silica in Subducting Slabs

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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.