Crystal chemical models for the cancrinite-sodalite supergroup: the structure of a new 18-layer phase

The framework of the minerals belonging to the cancrinite-sodalite supergroup is very flexible and adapts itself to a wide range contents of extra-framework anions, giving rise to complex structures with different stacking sequences of layers that contain six-membered rings of tetrahedra. The knowledge of the crystal chemistry of these minerals was the key for the structure solution of a new phase showing a sequence with 18 layers. Geometrical and chemical constraints were imposed to the positions of the six-member rings so limiting the number of possible non-equivalent sequences of the 18 layers. The more probable sequences were single out on the basis of the chemical data and used as starting model vs. single crystal X-ray intensity data. The new 18-layer phase, ideal chemical formula (Na, K, Ca)(72)(Si54Al54O216)(SO4)(16)(Cl, H2O)(8), Z = 1, space group P3, a = 12.904(2) angstrom, c = 47.802(4) angstrom, displays a stacking sequence ABCABACACABABACABC, Zdhanov symbol (8)211(2)112. The examined crystal was found in the sample MMUR (Museo di Mineralogia dell'Universita di Roma "La Sapienza") 24320, associated with sacrofanite, which is the 28-layer member of the supergroup. The sample was collected at Sacrofano, (Latium, Italy) in a volcanic ejected block, which originated from a pneumatolitic-pyroclastic explosion