The crystal structure of synthetic kenyaite, Na2Si20O40(OH)2·8H2O

Abstract

A synthetic kenyaite sample possessing the chemical composition Na16[Si160O320(OH)16]·64H2O per unit cell was structurally analysed using X-ray powder data. The powder pattern was indexed based on space group symmetry Fdd2 (No. 43) with lattice parameters a0 ​= ​10.080(1) Å, b0 ​= ​79.383(8) Å, c0 ​= ​10.604(1) Å. The crystal structure was solved by model building based on the unit cell parameters, the results of a general characterisation of the material and the comparison with the closely related structures of RUB-6, Na-RUB-18 and magadiite. A Rietveld refinement of this structure model converged to residual values of χ2 ​= ​3.1, RF ​= ​0.020 and RBragg ​= ​0.025, confirming the structure model. Physico-chemical characterisation using solid-state NMR spectroscopy, SEM, TG-DTA, and DRIFT spectroscopy further confirmed the structure. The structure of kenyaite contains thick silicate layers with a thickness of 15.9 ​Å (ignoring the van der Waals radii of the terminal oxygen atoms). The dense layers possess the same topology as the layers of RUB-6, exhibit no porosity and have a silicon Q4 to Q3 ratio of 4.0. 16 out of 32 terminal silanol groups are protonated while 16 siloxy groups (≡Si–O–) compensate the charge of the sodium cations. RUB-6 and kenyaite differ, however, with respect to the cation intercalated and to the stacking arrangement of layers. Bands of edge-sharing [Na(H2O)6/1.5] octahedra are intercalated between the silicate layers extending along [110] and [1¯10]. The equatorial water molecules of the octahedra are hydrogen bonded to terminal silanol groups with O⋯O distances of 2.5 ​Å to 2.6 ​Å. The structure of kenyaite is slightly disordered typical for hydrous layer silicates which possess only weak interactions between neighbouring silicate layers. In this respect, the result of the structure refinement represents a somewhat idealised structure. The structure analysis also revealed the presence of strong intra-layer hydrogen bonds (d(O⋯O) ≈ 2.5 ​Å) between the terminal O atoms (silanol/siloxy groups) confirmed by 1H MAS NMR spectroscopy. The surface zone of the silicate layers as well as the inter-layer region containing the edge-sharing [Na(H2O)6/1.5] octahedra are closely related to the structures of Na-RUB-18 and magadiite.