Crystal Chemistry of Dark Blue Aquamarine From the True Blue Showing, Yukon Territory, Canada

Abstract

Dark blue aquamarine and beryl were discovered at the True Blue showing in the southern Yukon Territory in 2003. Electronmicroprobe-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 singlecrystal 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.

Iron+magnesium-Bearing Beryl From Granitic Pegmatites: An Empa, La-Icp-Ms, Mössbauer Spectroscopy, and Powder XRD Study

“…2) in order to avoid the strong asymmetry of the main Fe 2+ doublet present at room temperature (these data not shown; see, e.g., Viana et al 2002, da Costa et al 2006, Groat et al 2010, Novák & Filip 2010, Novak et al 2011a). In all measured samples, Fe was present in a high spin Trace elements averages (ppm) are based on six data points.…”

“…Almost identical values were also found for Al. Slightly higher Si (≤ ~0.5% of the site) relative to the ideal occupancy and resulting from methods of calculation (2), (3), and (4) may have several causes, including undetermined Be, incorporation of some elements (Fe 2+ , Li) at the CH site (Sherriff et al 1991, Groat et al 2010, the presence of OHin the beryl structure (Della Ventura et al 2007), analytical error, and the formula calculation approach. Low totals of oxides for beryl samples from most localities probably reflect the presence of significant amounts of H 2 O typically found in beryl (e.g., Brown & Mills 1986, Sherriff et al 1991, Giuliani et al 1997; however, the Giuliani et al (1997) in the form of H 2 O wt.% = (Na 2 O in wt.% + 1.4829)/1.1771, yielded a sum of oxides exceeding 100 wt.%, indicating that this approach is not valid, at least for pegmatitic beryl (Table 5).…”

“…The general formula of minerals of the beryl group written as: CH (Na,Cs,Rb,K,H 2 O,He,Ar) T(2) (Be,Li) 3 O (Al,Fe 3+ ,Sc,Cr,V,Fe 2+ ,Mg,Mn) 2 T(1) [Si 6 O 18 ] (see Franz & Morteani 2002) shows that Fe 2+ and Fe 3+ mainly occur at the octahedral position (O-site) in the crystal structure of beryl; however, minor Fe 2+ may also be incorporated at a channel site (CH-site) and mainly at one of the tetrahedral positions [T(2)-site] (e.g., Viana et al 2002, da Costa et al 2006, Groat et al 2010, Novák & Filip 2010, Novák et al 2011a. Except for stoppaniite Be 3 Fe 3+ 2 Si 6 O 18 (e.g., Ferraris et al 1998, Della Ventura et al 2007, Fe 2+ always significantly predominates over Fe 3+ in the beryl-group minerals (e.g., Aurisicchio et al 1988, Viana et al 2002, Braga et al 2002, da Costa et al 2006, Adamo et al 2008, Groat et al 2010, Novák & Filip 2010.…”

“…Except for stoppaniite Be 3 Fe 3+ 2 Si 6 O 18 (e.g., Ferraris et al 1998, Della Ventura et al 2007, Fe 2+ always significantly predominates over Fe 3+ in the beryl-group minerals (e.g., Aurisicchio et al 1988, Viana et al 2002, Braga et al 2002, da Costa et al 2006, Adamo et al 2008, Groat et al 2010, Novák & Filip 2010.…”

Luminescence behaviour of beryl (aquamarine variety) from Turkey

“…To the best of our knowledge, beryl has been well characterised by spectroscopy techniques [8][9][10][11], ESR [12,13], Mössbauer spectroscopy [14,15] but no previous work on beryl CL has been performed. Only Ref.…”