Polyphite and sobolevite: Revision of their crystal structures

The crystal structures of polyphite, Na-5 (Na4Ca2) Ti-2 [Si2O7] (PO4)(3) O-2 F-2, triclinic, space group (P) over bar1, Z = 2, a 5.3933(2), b 7.0553(3), c 26.451(1) angstrom, alpha 95.216(1), beta 93.490(1), gamma = 90.101(1)degrees, V 1000.44(8) angstrom(3), and sobolevite, Na-12 Ca (NaCaMn) Ti-2 (Ti Mn) [Si2O7](2) (PO4)(4) O-3 F-3, monoclinic, space group Pc, Z = 2, a 7.0755(5), b 5.4106(4), c 40.623(3) angstrom, beta 94.42(1)degrees, V 1552.8(3) angstrom(3), from the Khibina-Lovozero alkaline complex, Kola Peninsula, Russia, have been refined (for the holotype single crystal of polyphite) to residual R values of 3.7 and 4.3%, respectively, using 5849 and 9030 observed (vertical bar F(o)vertical bar > 4 sigma F) reflections collected with a single-crystal diffractometer fitted with a CCD detector and MoK alpha X-radiation. The single crystals were subsequently analyzed with an electron microprobe, resulting in significant revision of both chemical formulae. There is a close relation among the structures of polyphite, sobolevite and quadruphite, Na14Ca2Ti4N2O7](2)(PO4)(4)O4F2. This type of structure consists of two structural blocks, Ti-silicate (TS) and alkali-cation (AC), stacked along [001]. There is also a close relation among the silicophosphate minerals polyphite, sobolevite, quadruphite and nacaphite, Na (Na,Ca)(2) (PO4) F. The TS block has a three-layered structure consisting of a central sheet of octahedra and two adjacent heteropolyhedral sheets. The AC block consists of nacaphite-like trimeric clusters that link along [100] to form an infinite chain. The TS and AC blocks link together through an intermediate (I) layer of Na polyhedra and P tetrahedra. In polyphite, sobolevite and quadruphite, all cations are arranged as close-packed layers parallel to (001) and characterized by a plane unit-cell approximately 5.4 X 7 angstrom with gamma approximate to 90 degrees.