The crystal structure of delindeite, Ba-2{(Na,K,square)(3)(Ti,Fe)[Ti-2(O,OH)(4)Si4O14](H2O,OH)(2)), a member of the mero-plesiotype bafertisite series

By using a crystal from a new locality, the structure of delindeite has bee n refined to R = 0.054 for 703 unique reflections with I > 2 sigma (l). Thi s titanosilicate [ideally Ba-2{(Na,K,rectangle)(3)(Ti,Fe)[Ti-2(O,OH)(4)Si4O 14](H2O,OH)(2)), space group A2/m, a 5.327(2), b 6.856(2). c 21.51(3) Angst rom,beta 93.80(4)degrees, Z = 2, D-calc = 3.815 g/cm(3), belongs to a large group of bafertisite-like structures characterized by an ideally octahedra l (O) sheet sandwiched between two heteropolyhedral (H) sheets these sheets are formed by Ti-octahedra and disilicate groups [Si2O7]. The O sheet in d elindeite comprises a large (Na,K) polyhedron together with octahedral Na a nd Ti sites; the alkali polyhedra are partially vacant. The large Ba cation is localized in the interlayer. Evidence of the leaching of alkalis, relat ed atomic disorder. and the position of delindeite within the bafertisite p olysomatic series are discussed. On the whole, this series is considered of mero-plesiotype. In fact, the heterophyllosilicate HOH layer, to a first a pproximation, represents the common module in this series, whereas the inte rlayer content is variable (merotypism), and may modify its structure in di fferent members (plesiotypism).