Structure refinement of natural beryls by single crystal neutron and X-ray diffraction

The crystal. structure of beryl has been refined from single-crystal .neutronand X-ray data obtained on two samples with different alkali and H20 contents [rnorganite: A12BC2.6SLiO',3S,i26 018 '(NaO,ì2 CSO,08 O.54fI2 O); aquamarine: AlL04FeoDo Be3 Si6 o., . (Nao.o20.28H20)) in order 10 elucidate the ambiguities affecting the assignmenr 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 wereobtained at roorn temperature. Final agreement factors for the three neutron and the two Xsray refinements are in the order LR\unwc;gh'Cd)] 0.034. Q.052, O.O:}J,0.023, 0.022. The expected Li substitutions in the Be tetrahedral site and of Fe in the Al octahedral site were clearly confirmed by the neutron data refìnements. Accurate site occupancy refìnements in the 2a and 2b charrnel sites at 0,0,1/4and Q,O,O,obtained also through location ofthe H atoms, yielded two distinct confìgurations with Cs and H10 at 0,0,% and Na at 0,0,0 in morganite, and Na and H,O at 0;0,1/4in aquamarine (with 2b empty). The H20 molecules attain two distinct orientations in the two sarnples with H-H vectors on the ab pIane in morganite and parallel to the c axis in aquamarine. The fIlO orientations are in agreement with type II H20 being present in alkali-containing beryl, and type I H20 being present in alkali-free samples, as found by spectroscopic investigations