New insight into the crystal structure of davyne (Na,Ca,K)8(Si,Al)12O24(SO4,Cl)3
Davyne, [ideally (Na,Ca,K)8(Si,Al)12O24(SO4,Cl)3, space group P63 or P63/m, a~12.8 and c~5.36 Å] is a member of the so-called “cancrinite group” of minerals. Its crystal structure is built from layers of six-membered rings of Si/Al-tetrahedra with open-cavities in which the extra-framework content (cations, anions and minor H2O) lies. Despite the structure model of davyne previously reported being consistent, at least for the tetrahedral framework, the X-ray experiments left several open questions about the tetrahedral Si/Al-ordering and the extra-framework population: 1) a different configuration of the SO4 groups and of the -Ca-Cl- chains is reported in the previous studies, 2) Na and K have been modeled as ordered in two mutually exclusive sites (only 0.9 Å apart), 3) the main Cl site shows an uncommon vibrational parameters with a pronounced anisotropy (U11~0.1 Å2). The aim of the present study is a reinvestigation of the crystal structure of a natural H2O-bearing davyne (3 wt% H2O) at ambient conditions by means of single-crystal neutron diffraction, in order to define: 1) A reliable description of the Si/Al-ordering at the tetrahedral sites, and so the actual symmetry
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NENERT Gwilherm and GATTA Giacomo Diego. (2013). New insight into the crystal structure of davyne (Na,Ca,K)8(Si,Al)12O24(SO4,Cl)3. Institut Laue-Langevin (ILL) doi:10.5291/ILL-DATA.5-11-403
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