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| Home > Publications database > The real topological configuration of the extra-framework content in alkali-poor beryl: a multi methodological study |
| Journal Article | PreJuSER-57967 |
; ; ;
2006
Soc.
Washington, DC
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Please use a persistent id in citations: doi:10.2138/am.2006.1896
Abstract: The crystal structure of alkali/water-poor beryl (1120 + Na2O + Cs2O < 1.2 wt%) was reinvestigated by means of laser ablation inductively coupled plasma mass spectroscopy, thermogravimetric analysis, neutron diffraction, and polarized infrared spectroscopy to determine the real topological configuration of the extra-framework content in the six-membered ring channels. Analysis of the nuclear density Fourier map suggests that the (water) oxygen is located along the sixfold axis at the 2a site (0,0, 1/4), whereas the (water) protons are at -0.028(7), -0.071(3), 0.332(1). The hydrogen atoms are distributed in 6 x 2 equivalent positions, above and below the oxygen site. Geometrical configuration of the water molecule is well defined: the O-H bond distance is 0.949(18) angstrom and the H-O-H bond angle is 106.9(2.2)degrees. The (HH)-H-... vector is oriented at similar to 4 degrees from [001]. This configuration is completely different from that found in alkali-rich beryl, where the (HH)-H-... vector is perpendicular to [00 1]. Na is probably located, with the H2O oxygen, at the 2a site. According to the chemical analysis, which shows that the amounts of other alkali and earth-alkali cations are negligible (Rb, K, Mg, Mn <= 110 ppm, Ca <= 225 ppm, Cs <= 430 ppm), no effect of other cations on the extra-framework population was observed in the structural refinement. The final agreement index (R-1) of the structural refinement was 0.037 for 34 refined parameters and 160 unique reflections with F-0 > 4 sigma(F-0). The topological configuration of the H2O molecule into the channel is confirmed by the spectroscopic investigation. Polarized single-crystal IR spectra show that the H2O molecule is oriented with the molecular symmetry axis perpendicular to the hexagonal axis and (HH)-H-... vector parallel (or quasi-parallel) to [001].
Keyword(s): J ; chemical mineral analysis (auto) ; L-A-ICP-MAS (auto) ; crystal structure (auto) ; single-crystal neutron diffraction (auto) ; water-poor beryl (auto) ; polarized IR-spectra (auto) ; trace elements (auto) ; REE (auto)
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