TY - JOUR

T1 - The crystal structure of mineral magadiite, Na2Si14O28(OH)2â 8H2O

AU - Marler, Bernd

AU - Krysiak, Yaşar

AU - Grosskreuz, Isabel

AU - Gies, Hermann

AU - Kolb, Ute

N1 - Funding Information: The authors are very grateful to K. Beneke and G. Lagaly, Kiel, Germany, for kindly providing natural magadiite samples. Also, many thanks to K. Kuroda and M. Koike, Tokyo, Japan, for suggestions regarding the structure solution. Moreover, the authors thank S. Grabowski for recording the NMR spectra. The work was funded by the Deutsche Forschungsgemeinschaft with grant number MA 6641/3-1. Yaşar Krysiak is very grateful for the support from the Czech Science Foundation, project number 19-08032S.

PY - 2022/11/1

Y1 - 2022/11/1

N2 - Magadiite from Lake Magadi was structurally analyzed based on Xâ ray powder diffraction data. The idealized chemical composition of magadiite is Na16[Si112O224(OH)16]â 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.

AB - Magadiite from Lake Magadi was structurally analyzed based on Xâ ray powder diffraction data. The idealized chemical composition of magadiite is Na16[Si112O224(OH)16]â 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.

KW - characterization

KW - layer silicate

KW - Rietveld

KW - Sodium silicate

KW - structure determination

UR - http://www.scopus.com/inward/record.url?scp=85141910930&partnerID=8YFLogxK

U2 - 10.2138/am-2022-8156

DO - 10.2138/am-2022-8156

M3 - Article

AN - SCOPUS:85141910930

VL - 107

SP - 2101

EP - 2110

JO - American mineralogist

JF - American mineralogist

SN - 0003-004X

IS - 11

ER -