Carbonate groups in davyne: structural and crystal-chemical considerations

The crystal structure of a specimen of davyne from Mt. Vesuvius (Italy) characterized by the presence of a significant amount of carbonate has been refined. The crystals have P63 symmetry with cell parameters of a = 12.6916(9) Å, c = 5.3333(5) Å. The chemical formula, obtained through electron-microprobe analysis, is (Na4.58K0.67Ca2.56)(Si6.06Al5.94O23.94)Cl2.35(CO3)0.88(SO4)0.19. According to its structural and chemical features, it may be classified as a carbonate-bearing davyne, as it represents an intermediate phase between davyne s.s. and a hypothetical CO3 end-member. The carbonate groups are located inside the large channel as in cancrinite s.s; their position is influenced by the presence of a limited number of SO4 groups. In fact, the triplets of oxygen atoms of the carbonate groups may alternatively be involved in the tetrahedral coordination of SO4. Because of the short C-C contacts a deviation from stoichiometry may be espected. This fact, as well as a deficiency of cations, was observed during the structural refinement and confirmed by the electron microprobe analysis.