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.

Récupéré en direct depuis OpenAlex et désinversé. Les résumés ne sont pas conservés dans cette base de données : les index inversés représentent 8,6 Go des 9,3 Go de texte de la base, et le serveur dispose de 13 Go libres.