posted on 2017-07-07, 00:00authored byHelen Grüninger, Katherine Armstrong, Dominik Greim, Tiziana Boffa-Ballaran, Daniel J. Frost, Jürgen Senker
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.