CRYSTAL-STRUCTURE OF PERRAULTITE FROM THE COASTAL REGION OF THE SEA OF AZOV

The crystal structure of the mineral perraultite (2)(Mn,Fe)(8)[(Ti,Nb) (4)O-4(OH)(2)Si8O28](OH,F)(4) is determined by X-ray diffraction analy sis (Syntex P (1) over bar automated diffractometer, 3458 reflections, direct method, refinement in the anisotropic approximation of atomic vibrations, R = 6.2%). The unit-cell parameters are a = 10.731(2), b = 13.841(4), and c = 24.272(6) Angstrom; beta = 121.19(2)degrees; sp. g r. C2; Z = 4, and d(calc) = 3.67 g/cm(3). The basic units of the perra ultite structure are three-layer slabs parallel to (001). They consist of a central octahedral sheet sandwiched between two networks, which are formed of Ti octahedra and Si tetrahedra. In distinction to bafert isite, which is similar in composition and structure, the slabs are in terlinked through common OH vertices of Ti octahedra. The large-sized Ba, K, Na, and Ca cations are situated in the interlayer space of the structure.