The crystal structure of mineral magadiite, Na2Si14O28(OH)2∙8H2O

Abstract

Magadiite from Lake Magadi was structurally analyzed based on X-ray powder diffraction data. The idealized chemical composition of magadiite is Na16[Si112O224(OH)16]∙64H2O per unit cell. The XRD powder diffraction pattern was indexed in orthorhombic symmetry with lattice parameters a0 = 10.5035(9) Å, b0 = 10.0262(9) Å, and c0 = 61.9608(46) Å. The crystal structure was solved from a synthetic magadiite sample in a complex process using 3D electron diffraction combined with model building as presented in an additional paper. A Rietveld refinement of this structure model performed on a magadiite mineral sample in space group F2dd (No. 43) converged to residual values of RBragg = 0.031 and RF = 0.026 confirming the structure model. Physico-chemical characterization using solid-state NMR spectroscopy, SEM, TG-DTA, and DRIFT spectroscopy further confirmed the structure. The structure of magadiite contains two enantiomorphic silicate layers of, so far, unknown topology. The dense layers exhibit no porosity or micro-channels and have a thickness of 11.5 Å (disregarding the van der Waals radii of the terminal O atoms) and possess a silicon Q4 to Q3 ratio of 2.5. 16 out of 32 terminal silanol groups are protonated, and the remaining groups compensate for the charge of the hydrated sodium cations. Bands of edge-sharing [Na(H2O)6/1.5] octahedra are intercalated between the silicate layers extending along (110) and (110). The water molecules are hydrogen bonded to terminal silanol groups with O···O distances of 2.54–2.91 Å. The structure of magadiite is slightly disordered, typical for hydrous layer silicates (HLS), which possess only weak interactions between neighboring layers. In this respect, the result of the structure refinement represents a somewhat idealized structure. Nevertheless, the natural magadiite possesses a higher degree of structural order than any synthetic magadiite sample. The structure analysis also revealed the presence of strong intra-layer hydrogen bonds between the terminal O atoms (silanol/siloxy groups), confirmed by 1H MAS NMR and DRIFT spectroscopy. The surface zone of the silicate layers, as well as the interlayer region containing the [Na(H2O)6/1.5] octahedra, are closely related to the structure of Na-RUB-18.

Bridging the Gap between Zeolites and Dense Silica Polymorphs: Formation of All‐Silica Zeolite with High Framework Density from Natural Layered Silicate Magadiite

“…In‐plane crystallinity of the layers was also confirmed by electron diffraction (ED) analysis (Figure S13b,d). The Rietveld analysis result also confirms that the structure of the mag layer being part of the framework is preserved and therefore also support the validity of the previously determined crystal structure of magadiite [9a,b] . The nitrogen adsorption/desorption measurements of cal‐NMF(rf)‐H‐nMag (Figure 4a) also reveal uptake at very low relative pressure originating from micropores (Similar uptake is confirmed for cal‐NMF(rf)‐H‐sMag (Figure 4b)).…”

“…Thus, the utilization of dense layered silicates with thick layers is expected to yield new zeolites with both microporosity and retention of dense layers through topotactic condensation. The recent success in the structural analysis of magadiite [9a,b] has enabled us to investigate the formation of the aforementioned zeolite.…”

“…The reason for the difficulty of topotactic condensation of magadiite is that the crystal structure of magadiite itself was unknown before the publication, [9a] in which magadiite with poor crystallinity was used as the starting material. Based on the very recently determined crystal structures of synthetic and natural magadiites, [9a,b] natural magadiite exhibits better crystallinity than synthetic magadiites. These findings indicate that natural magadiite has a well (but not perfectly) ordered structure.…”

Synthesis and real structure of RUB-11, a novel high-density silica zeolite based on magadiite layers

“…A condensation process involving the silanol groups of the silicate layers and monomeric silicic acid may finally generate the framework of RUB-11. Using this type of reaction, several microporous materials have been obtained by Ikeda et al 37 So called “pillared lamellar silicates”, named APZ-1, APZ-2, APZ-3 and APZ-4, were prepared by thermal acid treatment of the layered silicates PLS-1, PLS-3, PLS-4, and PREFER, which consist of ferrierite type silicate layers. These layer silicates could be converted into APZ materials representing new open-framework microporous materials by pillaring with SiO 2 (–OH) 2 fragments.…”

“…A condensation process involving the silanol groups of the silicate layers and monomeric silicic acid may finally generate the framework of RUB-11. Using this type of reaction, several microporous materials have been obtained by Ikeda et al 37 So called ''pillared lamellar silicates'', named APZ-1, APZ-2, APZ-3 and APZ-4, were Fig. 9 The average structure of RUB-11 in two projections.…”