The crystal structure of sacrofanite, the 74 Å phase of the cancrinite-sodalite supergroup

Sacrofanite, a = 12.903(2) , c = 74.284(8) , space group P6̄2c, belongs to the cancrinite-sodalite supergroup of minerals, and displays a 28-layer stacking sequence along the c axis. Its stacking sequence is ABCABACACABACBACBACABABACABC..., where A, B and C stand for the positions of six-member rings of tetrahedra in each layer. It corresponds to the Zhdanov symbol |12(8)21|12(8)21|, and gives rise to a framework with topological symmetry P63/mmc. The ordering of Si and Al in the tetrahedral sites reduces the symmetry to P6̄2c. The members of this supergroup of minerals belong to the wider ABC-6 family, where also double rings of tetrahedra may occur. They share many structural features with zeolites, showing structural cages hosting extra-framework ions as well as H2O molecules. The crystal structure of sacrofanite has been modelled on the basis of High Resolution Transmission Electron Microscopy (HRTEM) images. The resulting model has been successfully refined by using both single-crystal synchrotron radiation and laboratory data. The refinements converged to R = 0.083 for 4228 unique reflections, and to R = 0.096 for 15,795 unique reflections, respectively. The resulting framework is formed by eight cancrinite and four sodalite cages superimposed along [0, 0, z, whereas one cancrinite, four sodalite, two losod, and one liottite cages occur along [1/3, 2/3, z and [2/3, 1/3, z. The structural formula of sacrofanite, as obtained from the refinement and by crystal chemical considerations, is (Na61K19Ca32) ∑=112(Si84Al84O336)(SO 4)26Cl2F6·2H2O.