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American Mineralogist

Journal of Earth and Planetary Materials

Ed. by Baker, Don / Xu, Hongwu / Swainson, Ian


IMPACT FACTOR 2017: 2.645

CiteScore 2018: 2.55

SCImago Journal Rank (SJR) 2018: 1.355
Source Normalized Impact per Paper (SNIP) 2018: 1.103

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1945-3027
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Volume 100, Issue 5-6

Issues

Transition metal cation site preferences in forsterite (Mg2SiO4) determined from paramagnetically shifted NMR resonances

Ryan J. McCarty
  • Corresponding author
  • Department of Geological and Environmental Sciences, Stanford University, Stanford, California 94305, U.S.A.
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/ Aaron C. Palke
  • Department of Geological and Environmental Sciences, Stanford University, Stanford, California 94305, U.S.A.
  • Present address: Gemological Institute of America, Carlsbad, CA 92008, U.S.A.
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/ Jonathan F. Stebbins
  • Department of Geological and Environmental Sciences, Stanford University, Stanford, California 94305, U.S.A.
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/ Stephen Hartman
Published Online: 2015-05-12 | DOI: https://doi.org/10.2138/am-2015-5150

Abstract

In marked contrast to the single, narrow 29Si MAS NMR resonance for pure forsterite (Mg2SiO4), the spectra for synthetic forsterite containing 0.05 to 5% of the Mg2+ replaced with Ni2+, Co2+, or Fe2+ display between 4 and 26 additional, small, paramagnetically shifted peaks that are caused by interactions of the unpaired electron spins on the transition metal cations and the nuclear spins. Analyses of these relative peak areas, their numbers, and comparison of their positions to those in spectra of synthetic monticellites (CaMgSiO4) containing similar levels of transition metals, allows at least partial assignment to the effects of cations in either the M1 octahedral site only or to both M1 and M2 sites. More detailed analyses indicate that in forsterite, Ni2+ occupies only M1, Fe2+ occupies M1 and M2 roughly equally, and Co2+ occupies both M1 and M2 in an approximately 3:1 ratio. These findings for low concentrations agree with expectations from previous studies by other methods (e.g., XRD) of olivines with much higher transition metal cation contents. However, even low concentrations of Mn2+ (e.g., 0.1%), as well as higher Fe2+ contents (e.g., in natural San Carlos olivine) can broaden NMR peaks sufficiently to greatly reduce this kind of information content in spectra.

Keywords: Forsterite; olivine; NMR; transition metal cations; paramagnetic shift; site preference

About the article

Received: 2014-08-05

Accepted: 2014-12-01

Published Online: 2015-05-12

Published in Print: 2015-05-01


Citation Information: American Mineralogist, Volume 100, Issue 5-6, Pages 1265–1276, ISSN (Online) 1945-3027, ISSN (Print) 0003-004X, DOI: https://doi.org/10.2138/am-2015-5150.

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© 2015 by Walter de Gruyter Berlin/Boston.

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