The Na_2−nH_n[Zr(Si₂O₇)]∙mH₂O Minerals and Related Compounds (n = 0–0.5; m = 0.1): Structure Refinement, Framework Topology, and Possible Na⁺-Ion Migration Paths

The Na_2−nH_n[Zr(Si₂O₇)]∙mH₂O family of minerals and related compounds (n = 0–0.5; m = 0.1) consist of keldyshite, Na₃H[Zr₂(Si₂O₇)₂], and parakeldyshite, Na₂[Zr(Si₂O₇)], and synthetic Na₂[Zr(Si₂O₇)]∙H₂O. The crystal structures of these materials are based upon microporous heteropolyhedral frameworks formed by linkage of Si₂O₇ groups and ZrO₆ octahedra with internal channels occupied by Na⁺ cations and H₂O molecules. The members of the family have been studied by the combination of theoretical (geometrical–topological analysis, Voronoi migration map calculation, structural complexity calculation), and empirical methods (single-crystal X-ray diffraction, microprobe analysis, and Raman spectroscopy for parakeldyshite). It was found that keldyshite and parakeldyshite have the same fsh topology, while Na₂ZrSi₂O₇∙H₂O is different and has the xat topology. The microporous heteropolyhedral frameworks in these materials have a 2-D system of channels suitable for the Na⁺-ion migration. The crystal structure of keldyshite can be derived from that of parakeldyshite by the Na⁺ + O²⁻ ↔ OH⁻ + □ substitution mechanism, widespread in the postcrystallization processes in hyperagpaitic rocks. View Full-Text