THE CRYSTAL STRUCTURE OF DELINDEITE, Ba2 )3(Ti,Fe)[Ti2(O,OH)4Si4O14](H2O,OH)2}, A MEMBER OF THE MERO-PLESIOTYPE BAFERTISITE SERIES

Abstract

By using a crystal from a new locality, the structure of delindeite has been refined to R = 0.054 for 703 unique reflections with I > 2σ ( I ). This titanosilicate [ideally Ba 2 {(Na,K,□) 3 (Ti,Fe)[Ti 2 (O,OH) 4 Si 4 O 14 ](H 2 O,OH) 2 }, space group A 2/ m , a 5.327(2), b 6.856(2), c 21.51(3) A, β 93.80(4)°, Z = 2, D calc. = 3.815 g/cm 3 , belongs to a large group of bafertisite-like structures characterized by an ideally octahedral ( O ) sheet sandwiched between two heteropolyhedral ( H ) sheets; these sheets are formed by Ti-octahedra and disilicate groups [Si 2 O 7 ]. The O sheet in delindeite comprises a large (Na,K) polyhedron together with octahedral Na and Ti sites; the alkali polyhedra are partially vacant. The large Ba cation is localized in the interlayer. Evidence of the leaching of alkalis, related atomic disorder, and the position of delindeite within the bafertisite polysomatic series are discussed. On the whole, this series is considered of mero-plesiotype. In fact, the heterophyllosilicate HOH layer, to a first approximation, represents the common module in this series, whereas the interlayer content is variable (merotypism), and may modify its structure in different members (plesiotypism).