We have refined the crystal structure of a specimen of davyne from Mt. Vesuvius, Italy, characterized by the presence of a significant amount of carbonate. The crystals have P6(3) symmetry, with cell parameters a 12.6916(9), c 5.3333(5) Angstrom. The chemical formula, obtained through electron-microprobe 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 phase between davyne sensu stricto and a hypothetical CO3 end-member. The carbonate groups are located inside the large channel as in cancrinite sensu stricto; 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 expected. This fact, as well as a deficiency of cations, were observed in the structural refinement and confirmed by the electron-microprobe data.
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