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,D)8(Al,Si)16032
Rossi, Giuseppe; Oberti, Roberta; Smith, David C.
published: Mar 21, 1989
ArtNo. ESP147050101009, Price: 29.00 €
Abstract The crystal structure of a K-poor Ca-rich tectosilicate (a = b = 9.982, c = 8.364 Å, space group F63) with composition close to Ca1D1Na6Al8Si8O32 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+D), with one position at the K position of ordinary nephelines but occupied by (Ca+Na+minor K + D ) , and a second position labelled Ca' and occupied by (Ca+Na+D) 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,D)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- and Ca1D1Na6- end-members distinguish themselves by having respectively "centred", "off-centred" and "displaced" cations in the large cation site.