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Forsterite, wadsleyite, and ringwoodite (Mg₂SiO₄): ²⁹Si NMR constraints on structural disorder and effects of paramagnetic impurity ions

¹ Department of Geological and Environmental Sciences, Stanford University, Stanford, California 94305, U.S.A.
² School of Earth Sciences, Ohio State University, Columbus, Ohio 43210, U.S.A.
³ Department of Geological Science, University of Colorado, Boulder, Colorado 80309, U.S.A.
⁴ Bayerisches Geoinstitut, Universität Bayreuth, Bayreuth, Germany

Correspondence: ^* E-mail: stebbins{at}stanford.edu

We present here high-resolution ²⁹Si MAS NMR data for synthetic samples of forsterite (-Mg₂SiO₄), wadsleyite (β), and ringwoodite (). Enrichment to >99% ²⁹Si provides greatly enhanced signal-to-noise ratios and thus great sensitivity to small features in the spectra. At a detection limit of 0.1 to 0.5%, no six-coordinated Si (^VISi) is observed in any of the polymorphs, although these results could be consistent with theoretical predications of 1 to 2% Mg-Si site disorder in ringwoodite if re-ordering occurs rapidly during cooling. Several small ^IVSi peaks in ringwoodite samples may be related to residual defects from this process. In forsterite and wadsleyite, several very small "extra" peaks are observed, many of which are at positions far outside the known range of chemical shifts for ²⁹Si in silicates. These may be caused by "pseudo-contact" shifts from dipolar interactions with unpaired electron spins on trace impurities of paramagnetic transition metal cations.

Key Words: NMR spectroscopy • forsterite • ringwoodite • wadsleyite • high-pressure studies

This article has been cited by other articles:

				J. F. Stebbins, J. R. Smyth, W. R. Panero, and D. J. Frost Forsterite, hydrous and anhydrous wadsleyite and ringwoodite (Mg2SiO4): 29Si NMR results for chemical shift anisotropy, spin-lattice relaxation, and mechanism of hydration American Mineralogist, July 1, 2009; 94(7): 905 - 915. [Abstract] [Full Text] [PDF]