THE CRYSTAL STRUCTURE OF SEIDITE-(Ce), Na4(Ce,Sr)2 (O,OH,F)4{middle dot}5H2O, A MODULAR MICROPOROUS TITANOSILICATE OF THE RHODESITE GROUP

Seidite-(Ce), Na 4 (Ce,Sr) 2 {Ti(OH) 2 (Si 8 O 1 8 )}(O,OH,F) 4 5H 2 O, is a titanosilicate recently discovered in the Yubileynaya pegmatite at Mount Karnasurt, Lovozero alkaline massif, Kola Peninsula, Russia. Grains consist of disordered [010] fibers; no single crystals suitable for X-ray diffraction have found. On the basis of chemical composition, electron and X-ray powder-diffraction data, and by comparison with the structure of miserite, KCa 5 (Si 2 O 7 )(Si 6 O 1 5 )(OH)F, a structure model for seidite-(Ce) has been developed in space group C2/c [a 24.61(5), b 7.23(1), c 14.53(3) A, β 94.6(3)°, Z=4]. In the crystal structure of seidite-(Ce), the isolated eight-membered silicate channels of miserite are condensed into (100) layers that are connected by isolated TiO 6 octahedra to form a framework. In the chemical formula, the composition of the framework is shown within braces. There are two types of channels involving TiO 6 octahedra and SiO 4 tetrahedra: one is eight-membered and is parallel to the silicate channel along [010]; the other is ten-membered and extends along [001]. Cations other than Si and Ti occur within the channels, together with H 2 O groups and OH,F anions. The Na cations are easily exchanged for Tl, K, Rb, Cs and Ba, indicating the microporous behavior of the seidite-(Ce) structure. We describe the mero-plesiotype modular aspects of seidite-(Ce) and related compounds belonging to the rhodesite group. comportement microporeux par la structure de la seidite-(Ce). Nous decrivons les aspects modulaires de type mero-plesiotype de la seidite-(Ce) et des composes semblables faisant partie du groupe de la rhodesite.