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Crystal chemistry of dark blue aquamarine from the True Blue showing, Yukon Territory, Canada

Groat, Lee A. and Rossman, George R. and Dyar, M. Darby and Turner, David and Piccoli, Paula M. B. and Schultz, Arthur J. and Ottolini, Luisa (2010) Crystal chemistry of dark blue aquamarine from the True Blue showing, Yukon Territory, Canada. Canadian Mineralogist, 48 (3). pp. 597-613. ISSN 0008-4476.

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Dark blue aquamarine and beryl were discovered at the True Blue showing in the southern Yukon Territory in 2003. Electron-microprobe-derived compositions show up to 5.39 wt.% FeO in the darkest material, which is among the highest Fe concentration known for true beryl. The Al site totals average 2.05, with a maximum of 2.10 apfu, which implies that there is more Fe present in the sample than can be accommodated at the Al position. Charge-balance considerations and Mössbauer spectra show that the Fe is present as both Fe^(2+) and Fe^(3+). Optical absorption and Mössbauer spectra and the results of the X-ray and neutron single-crystal refinements suggest that there is very little Fe at the tetrahedral or channel sites. Previous investigators have proposed that the color of blue beryl is due to intervalence charge-transfer (IVCT) between Fe^(2+) and Fe^(3+) cations. The anisotropy of the optical absorption spectra suggest that if the mechanism responsible for the color in our samples is IVCT, the vector between the ions involved must be oriented approximately parallel to c. The only vectors that fulfill this condition and have a realistic length (2.300 Å) are 4d–Al and 6g–Be. Given the close proximity of the Si positions (closer than any anion sites), it is difficult to conceive of the substitution taking place at the interstitial 4d site. However, Fe could substitute at the interstitial 6g position, but likely only in very small amounts, because of the need to maintain local charge-balance. Unfortunately, there is no evidence of this in the Mössbauer spectra or in difference-Fourier maps of the X-ray- and neutron-diffraction data. For the former technique, it is likely that any doublet arising from Fe in the 6gO_6 polyhedron is too similar to the Fe in the AlO_6 octahedra to be resolved for either Fe^(2+) or Fe^(3+). Calculations suggest that the concentration of Fe involved in the IVCT process is 0.08 apfu Fe, of which half (0.04 apfu, 0.17 e^–) would potentially be at the interstitial site. This amount of electrons and this nuclear density are likely too small to be seen on the difference-Fourier maps.

Item Type:Article
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Rossman, George R.0000-0002-4571-6884
Dyar, M. Darby0000-0003-4272-793X
Additional Information:© 2010 Mineralogical Association of Canada. Received September 7, 2007 ,revised June 6, 2010. We thank E.C. Sklute for her assistance with Mössbauer data processing, and P.C. Piilonen for useful discussions. The authors thank M. Parker for editing the manuscript. The manuscript was improved by comments from P.F. Zanazzi, K. Schmetzer, C. Aurisicchio, and R.F. Martin. Financial support was provided by the Natural Sciences and Engineering Research Council of Canada in the form of Research Grants to LAG, the National Science Foundation (USA) grants EAR–0337816 to GRR and EAR–0439161 to MDD, and by a White Rose Foundation grant to GRR.
Funding AgencyGrant Number
Natural Sciences and Engineering Research Council of CanadaUNSPECIFIED
White Rose FoundationUNSPECIFIED
Subject Keywords:aquamarine; beryl; color; electron microprobe; optical absorption; Mössbauer spectroscopy; X-ray diffraction; neutron diffraction; Yukon Territory, Canada
Issue or Number:3
Record Number:CaltechAUTHORS:20100818-103910137
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Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:19496
Deposited By: Jason Perez
Deposited On:18 Aug 2010 21:44
Last Modified:09 Mar 2020 13:18

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