5 years ago

Hidden Oceans? Unraveling the Structure of Hydrous Defects in the Earth’s Deep Interior

Hidden Oceans? Unraveling the Structure of Hydrous Defects in the Earth’s Deep Interior
Dominik Greim, Jürgen Senker, Daniel J. Frost, Helen Grüninger, Tiziana Boffa-Ballaran, Katherine Armstrong
High-pressure silicates making up the main proportion of the earth’s interior can incorporate a significant amount of water in the form of OH defects. Generally, they are charge balanced by removing low-valent cations such as Mg2+. By combining high-resolution multidimensional single- and double-quantum 1H solid-state NMR spectroscopy with density functional theory calculations, we show that, for ringwoodite (γ-Mg2SiO4), additionally, Si4+ vacancies are formed, even at a water content as low as 0.1 wt %. They are charge balanced by either four protons or one Mg2+ and two protons. Surprisingly, also a significant proportion of coupled Mg and Si vacancies are present. Furthermore, all defect types feature a pronounced orientational disorder of the OH groups, which results in a significant range of OH···O bond distributions. As such, we are able to present unique insight into the defect chemistry of ringwoodite’s spinel structure, which not only accounts for a potentially large fraction of the earth’s entire water budget, but will also control transport properties in the mantle. We expect that our results will even impact other hydrous spinel-type materials, helping to understand properties such as ion conduction and heterogeneous catalysis.

Publisher URL: http://dx.doi.org/10.1021/jacs.7b05432

DOI: 10.1021/jacs.7b05432

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