The crystal structure of a K-poor Ca-rich silicate with the nepheline framework, and crystal-chemical relationships in the compositional space (K,Na,Ca,□)₈(Al,Si)₁₆O₃₂

The crystal structure of a K-poor Ca-rich tectosilicate (a = b = 9.982, c = 8.364 Å, space group P6₃) with composition close to Ca₁□₁Na₆Al₈Si₈O₃₂ has been refined using 364 independent reflections collected by a single-crystal X-ray diffractometer. The anisotropic refinement yielded a final R factor equal to 0.037 for the observed reflections. The tetrahedral framework is the same as that of nepheline, but the distribution of the Ca, Na, and K cations is quite unusual: the large A cavity is here statistically occupied by (Ca + Na + K + □), with one position at the K position of ordinary nephelines but occupied by (C+Na+minor K+□), and a second position labelled Ca' and occupied by (Ca+Na+□) displaced along the z axis at approximately 1.25 Å from the K position. A review of the crystal structures in the compositional space (K,Na,Ca,□)₈(Al,Si)₁₆O₃₂, which includes kalsilite, nepheline, lisetite, anorthite, tridymite and many polymorphs thereof, is also presented. This reveals that compositions close to the K₂Na₆-, Na₂Na₆-, Na₂Na₆- and Ca₁□₁Na₆- end-members distinguish themselves by having respectively "centred', "off-centred' and "displaced' cations in the large cation site. -Authors