Hydrothermal Single Crystal Growth and Structural Investigation of the Nepheline and Kalsilite Stuffed Tridymite Species

Abstract

A series of high-quality single crystals of the formula Na_xK_1−xAlSiO₄ were synthesized using a high temperature hydrothermal method. This enabled the detailed single crystal study of four examples of this class of compounds, namely KAlSiO₄, Na_0.10K_0.90AlSiO₄ Na₃KAl₄Si₄O₁₆ and NaAlSiO₄. The potassium-containing species all had fully ordered AlO₄ and SiO₄ tetrahedral sites that led to formation of polar acentric structures. In contrast NaAlSiO₄ displayed the unusual feature of an exceptionally large and complex unit cell along with complete disordering of the Al and Si sites. This led to the formation of a centrosymmetric structure, that is also a new polymorph of the NaAlSiO₄ composition. The polymorphism of hydrothermal KAlSiO₄ was also examined in light of the crystal’s synthetic and thermal histories. The study also revealed a structural sensitivity toward the degree of Na/K substitution in the lattice. The strong tendency to form polar acentric structures makes understanding these structures of great interest. These detailed structures resolved a considerable degree of previous structural ambiguity within this nominally simple class of compounds.

Graphical Abstract

Structural subtleties are examined in the nepheline–kalsilite series of Na_xK_1−xAlSiO₄, revealing changes in the resulting structure according to synthetic method, thermal history, and alkali metal substitution.