THE CRYSTAL STRUCTURE OF SEIDITE-(CE), NA4(CE, SR)2{TI(OH)2(SI8O18)

Abstract

Seidite-(Ce), Na4(Ce,Sr)2{Ti(OH 2(Si8O18)}(O,OH,F 4,•5H2O, 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, KCa5(Si2O7 (Si6O15)(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 Å, 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 TiO6, 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 TiO6 octahedra and SiO4 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 H2O 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.