SINGLE-CRYSTAL PULSED-NEUTRON DIFFRACTION OF A HIGHLY HYDROUS BERYL

The crystal structure of a hydrous beryl with the highest water content on record and coupled with a very high sodium content (Be-3.0(Al1.3Fe0.3Mg0.4)Si6.0O18[Na-0.45(H2O)(0.95)]) has been refined from single-crystal diffraction data obtained at the ISIS pulsed neutron source in the time-of-flight Laue geometry. The results of the structure analysis allow unambiguous discrimination between the Na cations and the water molecules located in the channel Wyckoff positions 2(b) and 2(a), respectively, and contribute to the elucidation of the crystal chemical relations among the extra framework species and the cations in the framework sites. The water hydrogens are disordered over multiple sites, with the H-H vectors inclined ca 38 degrees with respect to the sixfold symmetry axis, and one O-H vector directed along the axis. The resulting water geometry is different from the orientation of type I and type II water molecules, as reported in the literature from spectroscopic data. The results of the structure refinement based on the neutron diffraction data are compared with those from a refinement based on conventional single-crystal X-ray diffraction data collected on a fragment of the same beryl crystal, and with the results of a combined refinement carried out simultaneously on both data sets.