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

A synthetic kenyaite sample possessing the chemical composition Na-16[Si160O320(OH)(16)]center dot 64H(2)O 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 a(0) = 10.080(1) angstrom, b(0) = 79.383(8) angstrom, c(0) = 10.604(1) angstrom. 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 chi(2) = 3.1, R-F = 0.020 and R-Bragg = 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 angstrom (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 Q(4) to Q(3) 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 [110]. The equatorial water molecules of the octahedra are hydrogen bonded to terminal silanol groups with O center dot center dot center dot O distances of 2.5 angstrom to 2.6 angstrom. 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 center dot center dot center dot O) approximate to 2.5 angstrom) between the terminal O atoms (silanol/siloxy groups) confirmed by H-1 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.

Automated summary

A synthetic kenyaite sample with a specific chemical composition was structurally analyzed using X-ray powder data, confirming its lattice parameters and crystal structure. The study revealed the characteristics of kenyaite's thick silicate layers, similarity to RUB-6, as well as differences in cation intercalation and layer stacking compared to other related structures. Physico-chemical characterization techniques further supported the structural findings of kenyaite.