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

The crystal structure of alkali/water-poor beryl (1120 + Na(2)O + Cs(2)O <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 H(...)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 H(...)H vector is perpendicular to [00 1]. Na is probably located, with the H(2)O 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 4 sigma(F(0)). The topological configuration of the H(2)O molecule into the channel is confirmed by the spectroscopic investigation. Polarized single-crystal IR spectra show that the H(2)O molecule is oriented with the molecular symmetry axis perpendicular to the hexagonal axis and H(...)H vector parallel (or quasi-parallel) to [001].