期刊名称：Proceedings of the National Academy of Sciences
印刷版ISSN：0027-8424
电子版ISSN：1091-6490
出版年度：2016
卷号：113
期号：49
页码：13971-13976
DOI：10.1073/pnas.1611571113
语种：English
出版社：The National Academy of Sciences of the United States of America
摘要：SignificanceHydrous minerals help transport water deep into Earths mantle, and form part of a cycle that regulates the sustained presence of surface water on Earth. To understand the deep-water cycle, it is crucial to study the properties of hydrous minerals under the conditions present in Earths mantle. Brucite is one of the simplest hydrous minerals and stores significant amounts of water as hydroxyl groups. It is assumed to decompose in the mantle transition zone, but we show here that a more compact high-pressure phase is stabilized instead that pushes the stability region of brucite into the lower mantle. Brucite might be present in much larger quantities, and play a larger role in water transport and storage, than previously thought. We investigate the high-pressure phase diagram of the hydrous mineral brucite, Mg(OH)2, using structure search algorithms and ab initio simulations. We predict a high-pressure phase stable at pressure and temperature conditions found in cold subducting slabs in Earths mantle transition zone and lower mantle. This prediction implies that brucite can play a much more important role in water transport and storage in Earths interior than hitherto thought. The predicted high-pressure phase, stable in calculations between 20 and 35 GPa and up to 800 K, features MgO6 octahedral units arranged in the anatase-TiO2 structure. Our findings suggest that brucite will transform from a layered to a compact 3D network structure before eventual decomposition into periclase and ice. We show that the high-pressure phase has unique spectroscopic fingerprints that should allow for straightforward detection in experiments. The phase also has distinct elastic properties that might make its direct detection in the deep Earth possible with geophysical methods.
