Carbonate groups in davyne: Structural and crystal-chemical considerations

We have refined the crystal structure of a specimen of davyne from Mt. Vesu vius, Italy, characterized by the presence of a significant amount of carbo nate. The crystals have P6(3) symmetry, with cell parameters a 12.6916(9), c 5.3333(5) Angstrom. The chemical formula, obtained through electron-micro probe analysis, is (Na4.58K0.67Ca2.56)(Si6.06Al5.94O23.94)Cl-2.35(CO3)(0.88 )(SO4)(0.19). According to its structural and chemical features, it may be classified as carbonate-bearing davyne, as it represents an intermediate ph ase between davyne sensu stricto and a hypothetical CO3 end-member. The car bonate groups are located inside the large channel as in cancrinite sensu s tricto; 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. Becau se of the short C-C contacts, a deviation from stoichiometry may be expecte d. This fact, as well as a deficiency of cations, were observed in the stru ctural refinement and confirmed by the electron-microprobe data.