The crystal structure of franzinite, the ten-layer mineral of the cancrinite group

Franzinite, ideally [(Na,K)30Ca10][Si30Al30O120](SO4)10•2H2O, a feldspathoid belonging to the cancrinite group, has a ten-layer stacking sequence. Its structure was solved in the space group P321 to an R value of 5.96%. Cell parameters are a 12.916(1), c 26.543(3) Å. The framework, characterized by the stacking sequence ABCABACABC, contains regular alternations of two “cancrinite” and two “sodalite” cages along [0 0 z], and of two “sodalite” and one “losod” cages along [⅔ ⅓ z] and [⅓ ⅔ z]. The Si:Al ratio is equal to 1, and the framework has a perfectly ordered Si,Al distribution, as was found in liottite and afghanite, the 6- and 8-layer cancrinite-like minerals, respectively. The cages host a complex distribution of extra-framework cations (Ca, Na, K), (SO4)2– anions, and H2O molecules. The “losod” cage contains two sulfate groups aligned along z and separated by a triplet of cations. Two additional triplets of cations are located around the two sulfate groups. The two bases of the cage are occupied by calcium atoms, which make favorable bond-distances with the apical atoms of oxygen of the SO4 tetrahedra within the cage. The two “cancrinite” cages share a common base and contain a segment of the ...Na–H2O…Na–H2O.... chain, which is a characteristic feature of the cancrinite, vishnevite, and pitiglianoite structures. The “sodalite” cages host one sulfate group that is always disordered and displaced from the three-fold axis. The (Ca, K, Na) cations are distributed among various split sites, each one with partial occupancy, showing marked similarity with the “sodalite” cages of the (SO4)2–- and S2–-bearing minerals with sodalite-type structure.