Redetermination of clinobarylite, BaBe₂Si₂O ₇

Clinobarylite, ideally BaBe₂Si₂O₇ (chemical name barium diberyllium disilicate), is a sorosilicate mineral and dimorphic with barylite. It belongs to a group of compounds characterized by the general formula BaM²⁺₂Si₂O₇, with M²⁺ = Be, Mg, Fe, Mn, Zn, Co, or Cu, among which the Be-, Fe-, and Cu-members have been found in nature. The crystal structure of clinobarylite has been re-examined in this study based on single-crystal X-ray diffraction data collected from a natural sample from the type locality (Khibiny Massif, Kola Peninsula, Russia). The structure of clinobarylite can be considered as a framework of BeO₄ and SiO₄ tetrahedra, with one of the O atoms coordinated to two Be and one Si, one coordinated to two Si, and two O atoms coordinated to one Si and one Be atom. The BeO₄ tetrahedra share corners, forming chains parallel to the c axis, which are interlinked by the Si₂O₇ units oriented parallel to the a axis. The Ba²⁺ cations (site symmetry m..) are in the framework channels and are coordinated by eleven O atoms in form of an irregular polyhedron. The Si - O_br (bridging O atom, at site symmetry m..) bond length, the Si - O_nbr (non-bridging O atoms) bond lengths, and the Si - O - Si angle within the Si₂O₇ unit are in marked contrast to the corresponding values determined in the previous study [Krivovichev et al. (2004). N. Jb. Miner. Mh. pp. 373-384].