Crystal chemistry of dark blue aquamarine from the true blue showing, Yukon Territory, Canada.

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 Mossbauer spectra show that the Fe is present as both Fe(2+) and Fe(3+). Optical absorption and Mossbauer 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 angstrom) 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 Mossbauer 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.