The structure of bornemanite, a Group III Ti-disilicate mineral from Lovozero alkaline massif, Kola Peninsula, Russia

The crystal structure of bornemanite, ideally Na6BaTi2Nb(Si2O7)2(PO4)O2(OH)F, Ti+Nb = 3 apfu (atom per formula unit), 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 942.4(2) Å3, sp. gr. P⎯1, Z = 2, Dcalc. 3.342 g.cm-3, from Lovozero alkaline massif, Kola Peninsula, Russia, has been solved and refined to R1 6.36% on the basis of 4414 unique reflections (Fo > 4σF). The crystal structure of bornemanite is as predicted by Sokolova [1]. Sokolova [1] 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. Group III includes lamprophyllite, barytolamprophyllite, nabalamprophyllite, innelite, vuonnemite and epistolite. In bornemanite, the TS block exhibits 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. The O sheet cations give Na3Ti (4 apfu) in accord with Group III. The TS block has two different H sheets, H1 and H2, where (Si2O7) groups link to [5]Ti and [6]Nb polyhedra and there are two peripheral sites, which are occupied by Ba and Na, respectively. The crystal structure of bornemanite is a combination of a TS block and an I (intermediate) block. There are two I blocks: the I1 block is a layer of Ba atoms; the I2 block consists of Na polyhedra and (PO4) tetrahedra. The I1 and I2 blocks are topologically and chemically similar to the intermediate blocks in barytolamprophyllite and vuonnemite, respectively. [1] Sokolova (2006)Can. Min. 44, 1273-1330.