Clinobarylite, BaBe2Si2O7: structure refinement, and revision of symmetry and physical properties
Krivovichev, Sergey V.; Yakovenchuk, Viktor N.; Armbruster, Thomas; Mikhailova, Yulia; Pakhomovsky, Yakov A.
published: Aug 6, 2004
The structure of clinobarylite, BaBe2Si2O7, a rare mineral from the Khibina alkaline massif, Kola peninsula, has been refined in the orthorhombic space group Pmn 21, R 1=0.030 (wR 2 = 0.082, S = 1.167) for 469 unique observed reflections with | Fo | ≥ 4σF. The structure is based upon tetrahedral framework consisting of BeO4 and SiO4 tetrahedra. BeO4 tetrahedra share corners to form chains parallel to the c axis. The chains are interlinked by the Si2O7 groups oriented parallel to the a axis. The Ba2+ cations are in the framework channels and are coordinated by eleven O atoms. The obtained orthorhombic symmetry of clinobarylite is in contradiction with the monoclinic symmetry recently reported for this mineral by Chukanov et al. (2003). Optical properties and powder X-ray diffraction pattern of clinobarylite have been re-studied and no contradictions with orthorhombic symmetry have been observed. Comparison of clinobarylite and its dimorph, barylite, show that tetrahedral frameworks in the structures of these minerals are based upon tetrahedral sheets of the same type. According to this description, barylite and clinobarylite can be considered as 2O- and 1O-polytypes of BaBe2Si2O7, respectively.