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POLYPHITE AND SOBOLEVITE: REVISION OF THEIR CRYSTAL STRUCTURES

Elena Sokolova^1,, Frank C. Hawthorne¹ and Alexander P. Khomyakov²

¹ Department of Geological Sciences, University of Manitoba, Winnipeg, Manitoba R3T 2N2, Canada
² Institute of Mineralogy, Geochemistry and Crystal Chemistry of Rare Elements, Russian Academy of Sciences, Veresaev Street, 15, Moscow 121357, Russia

E-mail address: elena_sokolova{at}umanitoba.ca

The crystal structures of polyphite, Na₅ (Na₄Ca₂) Ti₂ [Si₂O₇] (PO₄)₃ O₂ F₂, triclinic, space group P, Z = 2, a 5.3933(2), b 7.0553(3), c 26.451(1) Å, 95.216(1), ß 93.490(1), 90.101(1)°, V 1000.44(8) ų, 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) Å, ß 94.42(1)°, V 1552.8(3) ų, 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| > 4F) reflections collected with a single-crystal diffractometer fitted with a CCD detector and MoK 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 x 7 Å with 90°.

Keywords: polyphite, sobolevite, crystal structure, Khibina-Lovozero, Kola Peninsula, Russia.

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