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Cation order/disorder behavior and crystal chemistry of pyrope-grossular garnets: An ^(17)O 3QMAS and ^(27)Al MAS NMR spectroscopic study

Kelsey, Kimberly E. and Stebbins, Jonathan F. and Du, Lin-Shu and Mosenfelder, Jed L. and Geiger, Charles A. and Asimow, Paul D. (2008) Cation order/disorder behavior and crystal chemistry of pyrope-grossular garnets: An ^(17)O 3QMAS and ^(27)Al MAS NMR spectroscopic study. American Mineralogist, 93 (1). pp. 134-143. ISSN 0003-004X. doi:10.2138/am.2008.2623.

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The thermodynamic mixing properties of the pyrope-grossular solid solution show large deviations from ideality, which could be partly related to Ca-Mg order/disorder. In this study, synthetic pyrope-grossular garnets with X_(Mg) = 1.00, 0.91, 0.75, 0.50, 0.24, 0.10, and 0.00 are observed using ^(17)O 3QMAS, ^(27)Al MAS, and ^(29)Si MAS NMR to examine Ca-Mg order/disorder behavior and crystal chemical variations. The ^(17)O 3QMAS NMR spectra show four distinct resonances, assigned to four different local oxygen coordination environments; two resemble end-member garnets (oxygen bonded to two Mg or two Ca) and two are intermediate (oxygen bonded to one Ca and one Mg), indicating that there are two distinct bond distances for the Mg-O and/or Ca-O bonds through the entire solid solution. Noticeable changes in the NMR peak position for two of the oxygen sites suggest that as X_(Mg) increases, the longer Ca-O bond shortens. The relative areas for the different oxygen sites are close to those predicted using a model of random Ca/Mg mixing. The maximum allowed reduction in configurational entropy from first neighbor Ca-Mg ordering is insignificant relative to other configurational entropy reductions and excess vibrational entropy. These conclusions are not inconsistent with published theoretical calculations suggesting some Ca-Mg ordering that involves correlations beyond the first neighbor, as suggested by published theoretical calculations. Even at 18.8 Tesla, the ^(27)Al MAS NMR spectra do not resolve different local Al sites with varying combinations of X cation neighbors. The ^(29)Si MAS NMR spectra have resonance broadening, probably caused by the addition of 0.15 wt% Fe_2O_3 in the synthetic samples, and are consistent with published results suggesting a small degree of Ca-Mg ordering that is not reflected in the ^(17)O NMR spectra.

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Asimow, Paul D.0000-0001-6025-8925
Additional Information:© 2008 American Mineralogist. Manuscript received February 26, 2007. Manuscript accepted September 12, 2007. Manuscript handled by Michael Fechtelkord. We are grateful to Tom Sisson, Ben Hankins, and the USGS, Menlo Park for the use of piston cylinder apparatus, to NSF for continued funding under grant number EAR 0408410, and to J. Puglisi and C. Liu for access to the 18.8 T NMR spectrometer at the Stanford Magnetic Resonance Laboratory. We thank Sung Keun Lee, Robtera Oberti, and an anonymous reviewer for helpful comments on an early version of this paper.
Group:UNSPECIFIED, Division of Geological and Planetary Sciences
Funding AgencyGrant Number
NSFEAR 0408410
Subject Keywords:NMR spectroscopy, pyrope, grossular, order-disorder, garnet, oxygen-17, aluminum- 27, silicon-29
Issue or Number:1
Record Number:CaltechAUTHORS:20120905-151541763
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Official Citation: Kimberly E. Kelsey, Jonathan F. Stebbins, Lin-Shu Du, Jed L. Mosenfelder, Paul D. Asimow, and Charles A. Geiger Cation order/disorder behavior and crystal chemistry of pyrope-grossular garnets: An 17O 3QMAS and 27Al MAS NMR spectroscopic study American Mineralogist, January, v. 93, p. 134-143, doi:10.2138/am.2008.2623
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:33876
Deposited By: Ruth Sustaita
Deposited On:05 Sep 2012 22:42
Last Modified:09 Nov 2021 23:05

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