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

Elena Sokolova and Fernando Cámara

CNR – Istituto di Geoscienze e Georisorse, unità di Pavia, Via Ferrata 1, I–27100 Pavia, Italy

The crystal structure of delindeite, ideally Ba₂ Na₂ Ti₃ (Si₂O₇)₂ O₂ (OH)₂ (H₂O)₂, a 10.6452(9), b 13.713(1), c 21.600(2) Å, β 93.804(3)°, V 3146.15(5) ų, space group C2/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 R₁ value of 6.4% on the basis of 3559 unique reflections (F_o 4F) collected on a Bruker AXS SMART APEX diffractometer with a CCD detector and MoK radiation. Electron-microprobe analyses gave (wt.%): SiO₂ 27.08, Al₂O₃ 0.29, Nb₂O₅ 1.98, TiO₂ 24.33, Fe₂O₃ 1.07, FeO 0.40, MnO 0.19, BaO 33.20, Na₂O 4.63, K₂O 1.37, F 1.41, H₂O 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 Fe²⁺_0.05 Mn²⁺_{0.02 1.42})_3.00 (Ti_2.70 Fe³⁺_0.12 Nb_0.13Al_0.05)_3.00 (Si₂O₇)₂ [O_1.53(OH)_0.47]_2.00 [(OH)_1.34 F_0.66] _2.00 (H₂O)_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 <Si–O> 1.622 Å and three [6]-coordinated Ti-dominant sites. Two sites, M(1) and M(2), are coordinated by five O atoms and one OH group, <M(1,2)–O,OH)> 1.974 Å; they give Ti_1.88Nb_0.07Al_0.05; the M(3) site, of composition Ti_0.82Fe³⁺_0.12Nb_0.06, is coordinated by four O atoms and two OH groups, <M(3)–O,OH> 1.964 Å. There are four partially occupied sites: the Na(1) and Na(2) sites are occupied by Na with minor Fe²⁺ and Mn²⁺ at 70%, <Na(1,2)–O,OH,H₂O> 2.31 Å; the [8]-coordinated Na(3) site of composition (Na_0.700.30, <Na(3)–O,H₂O> 2.52 Å), and the [7]-coordinated K site of composition (K_0.180.82, <K–O> 2.90 Å), are at a short distance, 1.25 Å. The M(1,2) octahedra and (Si₂O₇) groups constitute the H sheet that is completely ordered. In the C sheet, M(3) octahedra and H₂O groups [O(19)] are long-range ordered, whereas Na(1–3) and K atoms and three H₂O 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), H₂O [O(22)]; (2) K; (3) 2 H₂O [O(20) and O(21)]. Two H₂O sites [O(20) and O(21)] correspond to two cation sites in the close-packed O sheet of octahedra (O sheet). The substitution H₂O + 2Na⁺ + 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 (Si₂O₇) groups with M(3) octahedra. The second or intermediate I block includes the [10]-coordinated Ba(1) and Ba(2) sites of composition Ba_0.92K_0.08 and Ba_1.00, <Ba(1)–O> 2.961 and <Ba(2)–O> 2.873 Å. The HCH and I block, which is a layer of Ba atoms, alternate along c. The ideal formula of delindeite is Ba₂ Na₂ Ti₃ (Si₂O₇)₂ O₂ (OH)₂ (H₂O)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.

Keywords: delindeite, Ti disilicate minerals, structure refinement, electron-microprobe data, structure topology, chemical formula, TS block.

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