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

Franzinite, ideally [(Na,K)(30)Ca-10][Si30Al30O120](SO4)(10). 2H(2)O, a fel dspathoid belonging to the cancrinite group, has a ten-layer stacking seque nce. 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) Angstrom. The framework, ch aracterized by the stacking sequence ABCABACABC, contains regular alternati ons of two "cancrinite" and two "sodalite" cages along [0 0 z], and of two "sodalite" and one "losod" cages along [2/3 1/3 z] and [1/3 2/3 z]. The Si: Al ratio is equal to 1, and the framework has a perfectly ordered Si,Al dis tribution, as was found in liottite and afghanite, the 6- and 8-layer cancr inite-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 aro und the two sulfate groups. The two bases of the cage are occupied by calci um atoms, which make favorable bond-distances with the apical atoms of oxyg en 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 piti glianoite structures. The "sodalite" cages host one sulfate group that is a lways disordered and displaced from the threefold axis. The (Ca, K, Na) cat ions are distributed among various split sites, each one with partial occup ancy, showing marked similarity with the "sodalite" cages of the (SO4)(2-)- and S2--bearing minerals with sodalite-type structure.