Order-Disorder in the Structures of Lithium Aluminosilicate Minerals by XRD and Multinuclear NMR

Abstract

The crystal structures of the lithium aluminosilicate minerals of the Li2O–Al2O3–SiO2 (LAS) system (Li1−xAl1−xSi1+xO4 system for 0.0 ≤ x ≤ 1.0), and bikitaite were determined by X-ray diffraction (XRD) in literature, suggesting several possible lattice models for each of the crystallized phases, because of the intrinsic experimental difficulties of this technique. Here, we correlate powder XRD patterns with Rietveld refinement of cell parameters and magic angle sample spinning multinuclear magnetic resonance (NMR) spectra, including 29Si, 27Al, 7Li, and 6Li spectroscopy at 7.05 T, 9.4 T, and 20 T. The aim is to select appropriate lattice models from short-range order schemes in the lithium aluminosilicate phases, from natural minerals and synthetic crystals from the crystallization of amorphous gel precursors by a ceramic route and also by hydrothermal high-pressure experiments. Solid solutions were found in α-quartz and α-cristobalite up to x ≥ 0.75, and in β-eucryptite and β-spodumene for 0.0 ≤ x ≤ 1.0, when the ceramic synthesis is at work. The local structures of these intermediate members of the β-eucryptite and β-spodumene solid-solution series have 29Si NMR spectra consistent with the Loewenstein’s rule, i.e., they have short-range order but are strictly non-periodic structures. However, β-eucryptite LiAlSiO4 end-member has a short-range structure compatible with the long-range order of the P6422 symmetry, when the crystallization is produced at hydrothermal conditions. The local structure of α-spodumene LiAlSi2O6 is consistent with the C2/c model. α-eucryptite LiAlSiO4 shows a short-range structure as that suggested by the R-3 lattice model. Petalite LiAlSi4O10 has a local structure compatible with the P2/a space group. Finally, the 29Si NMR spectra of bikitaite LiAlSi2O6·H2O indicate a short-range structure well-suited with the P1 symmetry. These results are consistent with the Ostwald‘s rule of stages, forming a order-disorder sequence of increasing long-range order from the starting fully disordered solid gels, through crystalline pseudoperiodic structures in non-stoichiometric solid solution crystals that respect the Lowenstein’s rule, up to fully ordered crystals with short-range structures from NMR close to the long-range structures by XRD, as in the stoichiometric compounds found in some natural minerals.