STRUCTURE REFINEMENTS OF BERYL BY SINGLE-CRYSTAL NEUTRON AND X-RAY-DIFFRACTION

The crystal structure of beryl has been refined from single-crystal ne utron and X-ray data obtained on two samples with different alkali and H2O contents [morganite: Al2Be2.65Li0.32Si6O18.(Na0.22Cs0.080.54H2O); aquamarine: Al1.94Fe0.06Be3Si6O18. (Na0.020.28H2O)] in order to eluci date the ambiguities affecting the assignment of atomic species to the channel sites. Neutron data for morganite were obtained at 295 and 30 K; neutron data for aquamarine and X-ray data for both samples were o btained at room temperature. Final agreement factors for the three neu tron and the two X-ray refinements are in the order [R(unweighted)] 0. 034, 0.052, 0.033, 0.023, 0.022. The expected Li substitutions in the Be tetrahedral site and of Fe in the Al octahedral site were clearly c onfirmed by the neutron data refinements. Accurate site occupancy refi nements in the 2a and 2b channel sites at 0,0, 1/4 and 0,0,0, obtained also through location of the H atoms, yielded two distinct configurat ions with Cs and H2O at 0,0,1/4 and Na at 0,0,0 in morganite, and Na a nd H2O at 0,0,1/4 in aquamarine (with 2b empty). The H2O molecules att ain two distinct orientations in the two samples with H-H vectors on t he ab plane in morganite and parallel to the c axis in aquamarine. The H2O orientations are in agreement with type II H2O being present in a lkali-containing beryl, and type I H2O being present in alkali-free sa mples, as found by spectroscopic investigations.