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The real topological configuration of the extra-framework content in alkali-poor beryl: A multi-methodological study

¹ Dipartimento di Scienze della Terra, Universita’ degli Studi di Milano, Via Botticelli 23, I-20133 Milano, Italy
² Bayerisches Geoinstitut, Universitaet Bayreuth, Universitaet Str. 30, D-95447 Bayreuth, Germany
³ Department of Earth Sciences, Cambridge University, Downing Street, Cambridge CB2 3EQ, U.K.
⁴ Forschungszentrum Jülich, D-52425 Jülich, Germany

Correspondence: ^* E-mail: diego.gatta{at}unimi.it

The crystal structure of alkali/water-poor beryl (H₂O + 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) Å and the H-O-H bond angle is 106.9(2.2)°. The H···H vector is oriented at ~4° from [001]. This configuration is completely different from that found in alkali-rich beryl, where the H···H vector is perpendicular to [001]. 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 (R₁) of the structural refinement was 0.037 for 34 refined parameters and 160 unique reflections with F_o > 4(F_o). 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 H···H vector parallel (or quasi-parallel) to [001].

Key Words: Chemical mineral analysis • L-A-ICP-MAS • crystal structure • single-crystal neutron diffraction • water-poor beryl • polarized IR-spectra • trace elements • REE

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