From structure topology to chemical composition. IX. Titanium silicates : revision of the crystal chemistry of lomonosovite and murmanite, Group-IV minerals

The crystal structures of lomonosovite, ideally Na10Ti4(Si2O7)2(PO4)2 O4, a = 5.4170(7) Å, b = 7.1190(9) Å, c = 14.487(2) Å, α = 99.957(3)°, β = 96.711(3)°, γ = 90.360(3)°, V = 546.28(4) Å3, Dcalc. = 3.175 g cm-3, and murmanite, ideally Na4Ti4(Si2O7)2O4(H2O)4, a = 5.3875(6) Å, b = 7.0579(7) Å, c = 12.176(1) Å, α = 93.511(2)°, β = 107.943(4)°, γ = 90.093(2)°, V = 439.55(2) Å3, Dcalc. = 2.956 g.cm-3, from the Lovozero alkaline massif, Kola Peninsula, Russia, have been refined in the space group P1 (Z = 1) to R values of 2.64 and 4.47%, respectively, using 4572 and 2222 observed |Fo ≥ 4σF| reflections collected with a single-crystal Bruker AXS SMART APEX diffractometer with a CCD detector and Mo-Kα radiation. Electron microprobe analysis gave empirical formulae for lomonosovite (Na9.50Mn0.16Ca0.11)σ9.77(Ti4+2.83Nb0.51Mn2+0.27Zr0.11Mg0.11Fe2+0.10Fe3+0.06Ta0.01)σ4.00(Si2.02O7)2 (P0.98O4)2(O3.50F0.50) σ4, Z = 1, calculated on the basis of 26(O+F) a.p.f.u., and murmanite (Na3.32Mn0.15Ca0.21K0.05)σ3.73(Ti4+3.08Nb0.51Mn2+0.18Fe3+0.15Mg0.07Zr0.01)σ4.00(Si1.98O7)2(O3.76F0.24)σ4 (H2O)4, Z = 1, calculated on the basis of 22(O+F) a.p.f.u., with H2O determined from structure refinement and Fe3+/(Fe2++Fe3+) ratios obtained by Mössbauer spectroscopy. The crystal structures of lomonosovite and murmanite are a combination of a titanium silicate (TS) block and an intermediate (I) block. The TS block consists of HOH sheets (H-heteropolyhedral, O-octahedral), and is characterized by a planar cell based on translation vectors, t1 and t2, with t1 ~5.5 and t2 ~7 Å and t1 ^ t2 close to 90°. The TS block exhibits linkage and stereochemistry typical for Group IV (Ti = 4 a.p.f.u.) of the Ti disilicate minerals: two H sheets connect to the O sheet such that two (Si2O7) groups link to Ti polyhedra of the O sheet adjacent along t1. In murmanite and lomonosovite, the invariant part of the TS block is of composition Na4Ti4(Si2O7)2O4. There is no evidence of vacancy-dominant cation sites or (OH) groups in the O sheet of lomonosovite or murmanite. In lomonosovite, the I block is a framework of Na polyhedra and P tetrahedra which gives 2[Na3 (PO)4] p.f.u. In murmanite, there are four (H2O) groups in the intermediate space between TS blocks. In lomonosovite, TS and I blocks alternate along c. In murmanite, TS blocks are connected via hydrogen bonding. The H atoms were located and details of the hydrogen bonding are discussed.