From structure topology to chemical composition. XI. Titanium silicates: crystal structures of innelite-1Tand innelite-2Mfrom the Inagli massif, Yakutia, Russia, and the crystal chemistry of innelite

Abstract

AbstractThe crystal structures of two polytypes of innelite, ideally Ba4Ti2Na2M2+Ti(Si2O7)2[(SO4) (PO4)]O2[O(OH)] where M2+= Mn, Fe2+, Mg, Ca: innelite-1T,a5.4234(9),b7.131(1), c 14.785(3) Å, α 98.442(4), β 94.579(3), γ 90.009(4)°,V563.7(3) Å3, space groupP1̄,Dcalc= 4.028 g/cm3, Z = 1; and innelite-2M,a5.4206(8),b7.125(1),c29.314(4) Å, 0 94.698(3)°,V1128.3(2) Å3, space groupP2/c,Dcalc.=4.024 g/cm3,Z =2, from the Inagli massif, Yakutia, Russia, have been refined toRvalues of 8.99 and 7.60%, respectively. Electron-microprobe analysis gave the empirical formula for innelite as (Ba3.94Sr0.06)Σ4.00(Na2.16Mn0.382+Fe2+0.17Mg0.15Ca0.10☐0.04)Σ3(Ti2.97Nb0.02Al0.02)Σ3.01Si4.01(S1.02P0.81☐0.17)Σ2H1.84O25.79F0.21which is equivalent to (Ba3.94Sr0.06)Σ4.00(Ti1.97Nb0.02Al0.02)Σ2.01[(OH0.99F0.21)Σ1.20O0.80], calculated on the basis of 26 (O + F) anions, with H2O calculated from structure refinement. The crystal structure of innelite is a combination of a TS (titanium silicate) block and an I (intermediate) block. The TS block consists of HOH sheets (H-heteropolyhedral, O-octahedral) and exhibits linkage and stereochemistry typical for Ti-disilicate minerals of Group III (Ti = 3 a.p.f.u.): two H sheets connect to the O sheet such that two (Si2O7) groups link to thetransedges of a Ti octahedron of the O sheet. The I block containsTsites, statistically occupied by S and P, and Ba atoms. In the structures of innelite-1Tand innelite-2M, TS blocks are related by an inversion centre and a cyglide plane, respectively. HRTEM images show a coherent intergrowth of the two polytypes, together with an as-yet unidentified ∼10 Å phase.