The crystal structure of bornemanite: a Group III Ti-disilicate mineral

The crystal structure of bornemanite, ideally Na6Ba Ti2Nb (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. There are six other minerals in Group III, lamprophyllite, nabalamprophyllite, barytolamprophyllite, innelite, epistolite and vuonnemite. For all these minerals, the invariant part of the TS block is MH2 MO4 (Si2O7)2 XO4, where MH2 = Ti, Nb and MO4 = Na3Ti. 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. There are two H sheets in bornemanite, and their topology is identical except for coordination of the MH sites. In the H1 and H2 sheets, (Si2O7) groups link to [5]Ti and [6]Nb polyhedra. There are two peripheral sites 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 types of I block in bornemanite: the I1 block is a layer of Ba atoms; the I2 block consists of Na polyhedra and (PO4) tetrahedra. 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. [1] Sokolova (2006) Can. Min. 44, 1273-1330.