Clinobarylite, BaBe2Si2O7: structure refinement, and revision of symmetry and physical properties

Abstract

The structure of clinobarylite, BaBe2Si2O7, a rare mineral from the Khi- bina alkaline massif, Kola peninsula, has been refined in the orthorhombic space group Pmn21, R1 = 0.030 (wR2 = 0.082, S = 1.167) for 469 unique observed reflec- tions 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 Ba 2 + cations are in the framework channels and are coordinated by ele- ven O atoms. The obtained orthorhombic symmetry of clinobarylite is in contradic- tion with the monoclinic symmetry recently reported for this mineral by Chuka- nov et al. (2003). Optical properties and powder X-ray diffraction pattern of clino- barylite 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 tetra- hedral sheets of the same type. According to this description, barylite and clinoba- rylite can be considered as 2O- and 1O-polytypes of BaBe2Si2O7, respectively.