From structure topology to chemical composition. XVIII. Titanium silicates: Revision of the crystal structure and chemical formula of betalomonosovite, a group-IV TS-block mineral from the Lovozero alkaline massif, Kola peninsula, Russia

The crystal structure of betalomonosovite, ideally Na6□4Ti4 (Si2O7)2 [PO3 (OH)][PO2 (OH) 2]O2 (OF), a 5.3331(7), b 14.172 (2), c 14.509(2) Å, α 103.174(2), β 96.320(2), γ 90.278(2)°, V 1060.7(4) Å3, from the Lovozero alkaline massif, Kola peninsula, Russia, has been refined in the space group P1 to R = 6.64% using 3379 observed (Fo > 4σF) reflections collected with a single-crystal APEX II ULTRA three-circle diffractometer with a rotating-anode generator (MoKα), multilayer optics, and an APEX-II 4K CCD detector. Electron-microprobe analysis gave the empirical formula (Na5.39Ca0.36Mn0.04Mg0.01)Σ5.80 (Ti2.77Nb0.48Mg0.29Fe3+ 0.23Mn0.20Zr0.02Ta0.01)Σ4 (Si2.06O7)2 [P1.98O5 (OH) 3]O2 [O0.82F0.65 (OH) 0.53]Σ2, Dcalc. = 2.969 g cm-3, Z = 2, calculated on the basis of 26 (O + F) apfu, with H2O determined from structure refinement. The crystal structure of betalomonosovite is characterized by extensive cation and anion disorder: more than 50% of cation sites are partly occupied. The crystal structure of betalomonosovite is 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 exhibits linkage and stereochemistry typical for Group IV (Ti + Mg + Mn = 4 apfu) of the TS-block minerals. The I block is a framework of Na polyhedra and P tetrahedra which ideally gives {Na2□ 4 [PO3 (OH)][PO2 (OH) 2]} pfu. Betalomonosovite is an Na-poor OH-bearing analogue of lomonosovite, Na10Ti4 (Si2O7)2 (PO4)2O4. In the betalomonosovite structure, there is less Na in the I block and in the TS block when compared to the lomonosovite structure. The OH groups occur mainly in the I block where they coordinate P and Na atoms and in the O sheet of the TS block (minor). The presence of OH groups in the I block and in the TS block is supported by IR spectroscopy and bond-valence calculations on anions. High-resolution TEM of lomonosovite shows the presence of pervasive microstructural intergrowths, accounting for the presence of signals from H2O in the infrared spectrum of anhydrous lomonosovite. More extensive lamellae in betalomonosovite suggest a topotactic reaction from lomonosovite to betalomonosovite.