POLYPHITE AND SOBOLEVITE: REVISION OF THEIR CRYSTAL STRUCTURES

Abstract

The crystal structures of polyphite, Na5 (Na4Ca2) Ti2 [Si2O7] (PO4)3 O2 F2, triclinic, space group P 1, Z = 2, a 5.3933(2), b 7.0553(3), c 26.451(1) A, α 95.216(1), β 93.490(1), γ 90.101(1)°, V 1000.44(8) A3, and sobolevite, Na12 Ca (NaCaMn) Ti2 (Ti Mn) [Si2O7]2 (PO4)4 O3 F3, monoclinic, space group Pc , Z = 2, a 7.0755(5), b 5.4106(4), c 40.623(3) A, β 94.42(1)°, V 1552.8(3) A3, 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 (| F o| > 4σ F ) reflections collected with a single-crystal diffractometer fitted with a CCD detector and Mo K α 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, Na14 Ca2 Ti4 [Si2O7]2 (PO4)4 O4 F2. 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 × 7 A with γ ≈ 90°.