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) (sub 8) (Al, Si) (sub 16) O (sub 32)

The crystal structure of this silicate has been refined using 364 independent reflections collected by a single-crystal X-ray diffractometer which yielded a final R factor of 0.037. The tetrahedral framework is the same as that of nepheline but the distribution of the Ca, Na and K cations is unusual. A review of the crystal structures in the compositional space (K,Na,Ca,square) (sub 8) (Al,Si) (sub 16) O (sub 32) is also presented. This reveals that compositions close to the K (sub 2) Na (sub 6) -, Na (sub 2) Na (sub 6) - and Ca,square,Na (sub 6) -end- members distinguish themselves by having respectively 'centred', 'off-centred' and 'displaced' cations in the large cation site.