New insight into the crystal chemistry of alkali-poor beryl: the real topological configuration of the extra-framework content

The crystal chemistry of natural alkali/water-poor beryl from Gilgit, Pakistan (H2O+Na2O+Cs2O<1.2wt%) is reinvestigated by means of laser ablation inductively coupled plasma mass spectroscopy, thermogravimetric analysis, neutron diffraction and polarised infrared spectroscopy in order to determine the real topological configuration of the extra-framework content in the 6-membered ring channels. Fine chemical analysis was performed by means of laser ablation inductively coupled plasma mass spectroscopy with a laser ablation system made by Wave UP213 (Nd:YAG laser source) coupled with ICP-MS Thermo Electron X7. Calibration was performed using NIST SRM 612 as external calibration sample in conjunction with internal standardization using 29Si, previously measured by EMPA-WDS. The following masses (isotopes) have been analysed: 7Li, 9Be, 23Na, 24Mg, 27Al, 39K, 44Ca, 55Mn, 56Fe, 85Rb and 133Cs. Thermal analysis were performed with a NETZSCH STA 449C instrument using 7.442 mg of powdered sample, previously dried at 130ºC for 4h. The thermal curve (weight loss vs T, 1ºC/min), showed a monotonic trend that became flat at about 1100 ºC. The experiment was conducted up to 1200 ºC. The calculated amount of H2O, on the basis of the weight loss, is 0.86 wt%. The chemical composition of sample is: Al1.93Fe0.03Be2.94Si5.93O18(Na 0.03 0.26H2O). Single-crystal polarised IR-spectra were obtained with a Bruker IFS-120-HR spectrometer coupled with a Bruker IR-microscope. In order to avoid absorption bands due to external contaminants (H2O vapour or CO2), the optics of the spectrometer were kept under vacuum and the optics of the microscope were continuously purged with purified air (free of H2O and CO2) during the measurements. The resolution of the spectra was 1 cm-1. IR-spectra were obtained using radiation polarised parallel to the a and c axes of the beryl samples. To do this, two small single-crystals were oriented using a Huber four-circle X-ray diffractometer, and then cut and polished in thin platelets (50 μm thick) with the polished surfaces parallel to (001) and (100) respectively. The single-crystal neutron diffraction experiment was performed at room T with a Huber four-circle diffractometer (SV28 beam-line) installed at the DIDO reactor - Forschungszentrum Juelich, Germany. The incident radiation (CW, λ=1.24126Å) was obtained using a Cu(200) monochromator. The unit-cell parameters of the neutron measurement are: a=b=9.2099(35)Å, c=9.1894(18)Å (c/a=0.9977). A total of 1534 reflections were recorded (2θmax = 100.78˚), of which 191 were unique. The systematic extinction rules agreed with the space group P6/mcc. Diffraction data were then corrected for Lorentz effect. No absorption correction was applied. Analysis of the nuclear density Fourier map suggests that the (water) oxygen is located along the 6-fold axis at the 2a-position site (0,0,1/4), whereas the (water) protons are at -0.028(7), -0.071(3), 0.332(1). The hydrogens 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 also located, with the H2O-oxygen, at the 2a-position site. The final agreement index (R1) of the structural refinement was 0.037 for 34 refined parameters and 160 unique reflections with Fo > 4σ(Fo). The topological configuration of the H2O-molecule into the channel is confirmed by the spectroscopic investigation. Polarised 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].