The crystal structure of vicanite-(Ce), a borosilicate showing an unusual (Si3B3O18)15- polyanion

The crystal structure of holotype vicanite-(Ce) has been solved and refined to R = 1.8% for 1398 observed reflections with the aid of a new crystal from the same locality (Tre Croci, Vetralla, Italy), found more than 10 years after the first. The new unit formula is (Ca,REE,Th)15Fe3+(SiO4)3 (Si3B3O18)(BO3)(As5+O4)(As3+O3)x(NaF3)1–xF7·0.2H2O with x = 0.4. The structure is trigonal, R3m, Z = 3, a = 10.8112(2), c = 27.3296(12) Å, and layered along [001] with three distinct layers. Layer A at z ca. 0 (1/3, 2/3) contains an Fe(SiO4)6 group and a threefold B3O9 borate ring. Each 15– tetrahedron of the ring shares one oxygen atom with one Si tetrahedron, forming an unusual Si3B3O18 polyanion. Layer B at z ca. 1/9 (4/9, 7/9) contains an AsO4 tetrahedron and a BO3 triangle. Layer C at z ca. 2/9 (5/9, 8/9) represents the disordered part of the structure, containing two very close (0.85 Å) As3+O3 2– polyhedra, the occurrence of which is mutually exclusive and 3– and NaF3 statistically disordered. A 3-dimensional network of M-(O,F)n polyhedra (M = Ca, REE, Th; 8 < n < 10) provide connections among neighboring layers.