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

Abstract

The crystal structure of betalomonosovite, ideally Na 6 □ 4 Ti 4 (Si 2 O 7 ) 2 [PO 3 (OH)][PO 2 (OH) 2 ]O 2 (OF), a 5.3331(7), b 14.172(2), c 14.509(2) A, α 103.174(2), β 96.320(2), γ 90.278(2)°, V 1060.7(4) A 3 , from the Lovozero alkaline massif, Kola peninsula, Russia, has been refined in the space group P 1 ¯ to R = 6.64% using 3379 observed ( F o > 4σ F ) reflections collected with a single-crystal APEX II ULTRA three-circle diffractometer with a rotating-anode generator (Mo K α), multilayer optics, and an APEX-II 4K CCD detector. Electron-microprobe analysis gave the empirical formula (Na 5.39 Ca 0.36 Mn 0.04 Mg 0.01 ) Σ5.80 (Ti 2.77 Nb 0.48 Mg 0.29 Fe 3+ 0.23 Mn 0.20 Zr 0.02 Ta 0.01 ) Σ4 (Si 2.06 O 7 ) 2 [P 1.98 O 5 (OH) 3 ]O 2 [O 0.82 F 0.65 (OH) 0.53 ] Σ2 , D calc. = 2.969 g cm −3 , Z = 2, calculated on the basis of 26 (O + F) apfu , with H 2 O 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 {Na 2 □ 4 [PO 3 (OH)][PO 2 (OH) 2 ]} pfu . Betalomonosovite is an Na-poor OH-bearing analogue of lomonosovite, Na 10 Ti 4 (Si 2 O 7 ) 2 (PO 4 ) 2 O 4 . 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 H 2 O in the infrared spectrum of anhydrous lomonosovite. More extensive lamellae in betalomonosovite suggest a topotactic reaction from lomonosovite to betalomonosovite.