The crystal structure and crystal chemistry of bornemanite, a group III Ti-disilicate mineral

ABSTRACT Here we present the results of the direct determination of the crystal structure of bornemanite, ideally Na6 BaNbTi2(Si2O7)2 (PO4)O2 (OH)F, from the Yubileinaya vein, Karnasurt Mt., Lovosero alkaline massif, Kola Peninsula, Russia, and discuss the crystal chemistry of bornemanite. INTRODUCTION Bornemanite was originally described from the Yubileinaya pegmatite, Mt. Karnasurt, Lovosero alkaline massif, Kola peninsula, Russia by Men’shikov et al. (1975) who reported the ideal formula Na7BaTi2NbP Si4O21 F, orthorhombic unit-cell, a 5.48, b 7.10, c 48.2Å, V 1875.4 Å3, possible space groups Imma and Ima2, Z = 4, Dcalc = 3.49 g cm-3, Dmeas. 3.47-3.50 g.cm-3, and emphasized the apparent chemical and structural similarity of bornemanite to the seidozerite and lomonosovite groups of minerals. Ferraris et al. (2001) proposed a model of the crystal structure of bornemanite based on seidozerite and lomonosovite moduls, reported the symmetry as monoclinic, sp. gr. I11b, a 5.498, b 7.120, c 47.95Å, γ 88.1O V 1876 Å3, and wrote the chemical composition of bornemanite as BaNa2.5Mn0.5 {Na3Ti [TiNbO2 Si4O14] (O0.5OH0.75F0.75)} PO4 . Sokolova (2006) questioned the validity of this model of the bornemanite structure. Sokolova (2006) established the relation between structure topology and chemical composition for 24 minerals with the TS (titanium-silicate) block, a central trioctahedral (O) sheet and two adjacent (H) sheets containing different polyhedra including (Si2O7) groups, and divided these minerals into four groups characterized by different topology and stereochemistry of the TS block. Each group of structures has a different linkage of H and O sheets in the TS block and a different arrangement of Ti (= Ti + Nb) polyhedra. In Groups I, II III and IV, Ti equals 1, 2, 3 and 4 apfu, respectively. The invariant core part of the TS block has the formula MH2 MO4 (Si2O7)2 XO4, where MH2 = cations of the H sheet, MO4 = cations of the O sheet, XO4 = anions of the O sheet not shared with (Si2O7) groups. Sokolova (2006) pointed out that in bornemanite, Ti equals 3 apfu and thus bornemanite must belong to Group III, and the TS block must exhibit linkage 1 and the stereochemistry of Group III: Ti occurs in the H and O sheets, two (Si2O7) groups link to trans edges of a Ti octahedron in the O sheet. EXPERIMENTAL DETAILS The crystal structure of bornemanite, ideally Na6 BaNbTi2(Si2O7)2 (PO4)O2 (OH)F, a 5.4587(3), b 7.1421(5), c 24.528(2)Å, α 96.790(1), β 96.927(1), γ 90.326(1)o, V 942.42 Å3, sp. gr. P1, Z = 2, Dcalc. 3.342 g.cm-3, has been solved by direct methods and refined to R1 6.36% on the basis of 4414 unique reflections (Fo > 4σF), collected on a Bruker AXS SMART APEX diffractometer with a CCD detector and MoKα-radiation. One fragment of the crystal of bornemanite used for the X-ray data collection was analyzed with a Cameca SX-100 electron-microprobe. The empirical formula is (Na6.07Mn2+0.23 Ca0.06 )6.36 (Ba0.73 K0.13Sr0.06)Σ0.92 (Ti2.05 Nb0.80 Zr0.02 Ta5+0.01 Fe3+0.03 Al0.02 Mn2+0.06 Mg0.01)3.00 (Si2O7)2 (P0.97O4) O2 [F1.27 (OH)0.73]Σ2.00, Z =2, calculated on the basis of 22 (O+F) apfu, and the ideal formula is Na6 Ba Ti2 Nb (Si2O7)2 (PO4) O2 (OH)F, in good agreement with structure data. STRUCTURE TOPOLOGY The structure of bornemanite is as predicted by Sokolova (2006). The O sheet cations give Na3Ti (4 apfu) in accord with Group III. There are two H sheets in bornemanite, and their topology is identical except for coordination of the MH sites. In the H1 sheet, (Si2O7) groups share common vertices with [5]-coordinated Ti-dominant polyhedral; in the H2 sheet, (Si2O7) groups share common vertices with Nb-dominant octahedra. In the H1 and H2 sheets, the peripheral AP sites are occupied by Ba and Na, respectively. In bornemanite, the TS block exhibits linkage 1 and stereochemistry typical for Group III: two H sheets connect to the O sheet such that two (Si2O7) groups link to the trans edges of a Ti octahedron of the O sheet. In bornemanite, TS blocks do not link directly through apical vertices of polyhedra of the H sheets, but alternate with (intermediate) I blocks. A layer of Ba atoms forms the first I1 block. The second I2 block has two I layers between adjacent TS blocks. Each layer consists of [4]- and [6]-coordinated Na polyhedra and (PO4) tetrahedra. Two I layers link together through common vertices of Na and P polyhedra. Alternating TS and I blocks along c build up the crystal structure of bornemanite (Fig.), with two TS blocks and two I blocks per the c cell parameter. The two TS blocks are related by an inversion centre and are identical. RELATED MINERALS There are six other minerals in Group III, lamprophyllite, nabalamprophyllite, barytolamprophyllite, innelite, epistolite and vuonnemite. For all these minerals, the invariant core part of the TS block is MH2 MO4 (Si2O7)2 XO4, where MH2 = Ti, Nb and MO4 = Na3Ti. There are two types of I block in bornemanite. The I1 block of composition (Ba0.73 K0.13Sr0.06)2 is topologically identical and chemically similar to the I block of composition (BaK) in barytolamprophyllite. The I2 block of composition Na3(PO4)2 is topologically similar and chemically identical to the I block in vuonnemite. The bond topology found for bornemanite lends further credence to the findings of Sokolova (2006) concerning these minerals. REFERENCES: Ferraris, G., Belluso, E., Gula, A., Soboleva, S.V., Ageeva, O.A. & Borutskii, B.E. (2001): A structural model of the layer titanosilicate bornemanite based on siederozite and lomonosovite modules. Can. Mineral. 39, 1665-1673. Men’shikov, Yu.P., Bussen, I.V., Goiko, E.A., Zabavnikova, N.I., Mer’kov, A.N. & Khomyakov, A.P. (1975): Bornemanite - a new silicophosphate of sodium, titanium, niobium and barium. Zap. Vses. Mineral. Obshchest. 104(3), 322-326 (in Russian). Sokolova, E. (2006): From structure topology to chemical composition. I. Structural hierarchy and stereochemistry in titanium disilicate minerals. Can. Mineral. 44, 1273-1330.