FROM STRUCTURE TOPOLOGY TO CHEMICAL COMPOSITION. II. TITANIUM SILICATES: REVISION OF THE CRYSTAL STRUCTURE AND CHEMICAL FORMULA OF DELINDEITE

The crystal structure of delindeite, ideally Ba 2 Na 2 Ti 3 (Si 2 O 7 ) 2 O 2 (OH) 2 (H 2 O) 2 , a 10.6452(9), b 13.713(1), c 21.600(2) A, β 93.804(3)°, V 3146.15(5) A 3 , space group C 2/ c , Z = 8, D calc. 3.74 g.cm −3 from Mt. Yuksporr, Khibina alkaline massif, Kola Peninsula, Russia, has been solved by direct methods and refined to an R 1 value of 6.4% on the basis of 3559 unique reflections ( F o ≥ 4σ F ) collected on a Bruker AXS SMART APEX diffractometer with a CCD detector and Mo K α radiation. Electron-microprobe analyses gave (wt.%): SiO 2 27.08, Al 2 O 3 0.29, Nb 2 O 5 1.98, TiO 2 24.33, Fe 2 O 3 1.07, FeO 0.40, MnO 0.19, BaO 33.20, Na 2 O 4.63, K 2 O 1.37, F 1.41, H 2 O 5.29 (calculated from structure refinement), O=F −0.59, total 100.65. The empirical formula is (Ba 1.92 K 0.08 ) ∑2.00 (Na 1.33 K 0.18 Fe 2+ 0.05 Mn 2+ 0.02 □ 1.42 ) ∑3.00 (Ti 2.70 Fe 3+ 0.12 Nb 0.13 Al 0.05 ) ∑3.00 (Si 2 O 7 ) 2 [O 1.53 (OH) 0.47 ] ∑2.00 [(OH) 1.34 F 0.66 ] ∑2.00 (H 2 O) 1.70 , Z = 8, calculated on the basis of 4 Si ( apfu ). The crystal structure of delindeite can be described as combination of two blocks. The first block consists of HCH sheets (H: heteropolyhedral, C: central), and it is related to the TS (titanium silicate) block in Ti disilicate minerals. In the TS block, there are seven fully occupied cation sites, four [4]-coordinated Si sites with 1.622 A and three [6]-coordinated Ti-dominant sites. Two sites, M (1) and M (2), are coordinated by five O atoms and one OH group, 1.974 A; they give Ti 1.88 Nb 0.07 Al 0.05 ; the M (3) site, of composition Ti 0.82 Fe 3+ 0.12 Nb 0.06 , is coordinated by four O atoms and two OH groups, 1.964 A. There are four partially occupied sites: the Na (1) and Na (2) sites are occupied by Na with minor Fe 2+ and Mn 2+ at 70%, 2 O> 2.31 A; the [8]-coordinated Na (3) site of composition (Na 0.70 □ 0.30 , 2 O> 2.52 A), and the [7]-coordinated K site of composition (K 0.18 □ 0.82 , 2.90 A), are at a short distance, 1.25 A. The M(1,2) octahedra and (Si 2 O 7 ) groups constitute the H sheet that is completely ordered. In the C sheet, M(3) octahedra and H 2 O groups [O(19)] are long-range ordered, whereas Na(1–3) and K atoms and three H 2 O groups [O(20–22) with 46, 44 and 50% occupancy] are long-range disordered. Three SRO (short-range order) arrangements have been considered for the C sheet: (1) Na(1), Na(2), 2 Na(3), H 2 O [O(22)]; (2) K; (3) 2 H 2 O [O(20) and O(21)]. Two H 2 O sites [O(20) and O(21)] correspond to two cation sites in the close-packed O sheet of octahedra (O sheet). The substitution H 2 O + 2□↔Na + + 2O 2− describes the relation between the composition of the C and O sheets. Linkage of H and C sheets occurs via common vertices of M(1,2) octahedra and (Si 2 O 7 ) groups with M(3) octahedra. The second or intermediate I block includes the [10]-coordinated Ba (1) and Ba (2) sites of composition Ba 0.92 K 0.08 and Ba 1.00 , 2.961 and 2.873 A. The HCH and I block, which is a layer of Ba atoms, alternate along c . The ideal formula of delindeite is Ba 2 Na 2 Ti 3 (Si 2 O 7 ) 2 O 2 (OH) 2 (H 2 O) 2 , Z = 8. The crystal structure reported here explains why the TS block in delindeite exhibits topology different from that of the titanium disilicate minerals, where the central sheet is trioctahedral.