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Observation of "hidden" magnesium: first-principles calculations and Mg-25 solid-state NMR of enstatite

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<mark>Journal publication date</mark>09/2011
<mark>Journal</mark>Solid State Nuclear Magnetic Resonance
Issue number2
Volume40
Number of pages9
Pages (from-to)91-99
StatePublished
Early online date11/08/11
Original languageEnglish

Abstract

Mg-25 NMR parameters have been determined for two polymorphs of enstatite (MgSiO3), an important magnesium silicate phase present as a major component of the Earth's upper mantle. The crystal structures of both polymorphs contain two crystallographically distinct magnesium sites; however, only a single resonance is observed in Mg-25 MAS NMR spectra recorded at 14.1 and 20.0 T. First-principles calculations performed on geometry-optimised crystal structures reveal that the quadrupolar interaction for the second site is expected to be very large, resulting in extensive broadening of the spectral resonance, explaining its apparent absence in the NMR spectrum. Mg-25 QCPMG NMR experiments employing variable offset cumulative spectroscopy (VOCS) are used to observe the broadened site and enable measurement of NMR parameters. The large difference in quadrupolar interaction between the two crystallographic magnesium sites is rationalised qualitatively in terms of the distortion of the local coordination environment as well as longer-range effects using a simple point charge model. (C) 2011 Elsevier Inc. All rights reserved.