Crystal Chemistry of Aquamarine from the True Blue Showing, Yukon Territory

Lee A. Groat

Department of Earth and Ocean Sciences, University of British Columbia, Vancouver, British Columbia V6T 1Z4

George R. Rossman

Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, California 91125-2500, U.S.A.

M. Darby Dyar

Department of Astronomy, Mount Holyoke College, 50 College Street, South Hadley, Massachusetts 01075, U.S.A.

David Turner

Department of Earth and Ocean Sciences, University of British Columbia, Vancouver, British Columbia V6T 1Z4

Paula M. B. Piccoli, Arthur J. Schultz

Intense Pulsed Neutron Source, Building 360, Argonne National Laboratory, Argonne, Illinois 60439-4814, U.S.A.

Luisa Ottolini

CNR-Istituto di Geoscienze e Georisorse (IGG), Sezione di Pavia, Via Ferrata 1, I-27100 Pavia, Italy


Dark blue aquamarine and beryl were discovered at the True Blue showing in the southern Yukon Territory in 2003. Electron microprobe compositions show up to 5.39 wt.% FeO in the darkest material, which is among the highest Fe concentration known for true beryl.  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 Fe2+ and Fe3+.  Optical absorption and Mössbauer spectra and the results of the X-ray and neutron single-crystal refinements suggest that there is no Fe at the tetrahedral or channel sites.  Previous studies have proposed that the color of blue beryl is due to intervalence charge-transfer (IVCT) between Fe2+ and Fe3+ cations.  The anisotropy of the optical absorption spectra suggest that 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 4dAl and 6gBe.  Given the close proximity of the Si positions (closer than any anion sites), it is difficult to conceive of 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 6gO6 polyhedron is too similar to the Fe in the AlO6 octahedra to be resolved for either Fe2+ or Fe3+.  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 electron and nuclear density is likely too small to be seen on the difference Fourier maps.

Dark blue aqumarine from the Tatu Mine, Minas Gerais Brazil, is one of the darkest blue beryls discovered, to date.

blue beryl
 True Blue beryl on matrix

Polarization anisotropy of the True Blue beryl

Additional images of True Blue beryl in matrix

1 2 3 4 5

Additional closer view of the locality


Canadian Mineralogist 48, 597-613