Crystal structures with a chabazite framework. I. Dehydrated Ca-chabazite

The crystal structure of dehydrated Ca-chabazite (composition Cal.95A13.gSis.lO2~ ) was determined from the [001] projection of the rhombohedral cell (R'3m: a = 9.37 A, a = 92 ° 01').Each corner of the rhombohedral cell is occupied by a ditrigonal prism of twelve linked tetrahedra with an Si, A1-O tetrahedron lying at each vertex of the prism.The resulting three-dimensional network contains large ellipsoidal cavities linked together by apertures to form a honeycombed structure.The apertures are bounded by 8-membered rings of tetrahedra and have an approximately rectangular free area of 3.1 by 4-4 A. Three peaks for Ca atoms were located, the electron density corresponding to 0"6 (1-fold) and 0"35 (2-fold) atoms.The total (1.3) is less than the 1.95 found by chemical analysis but slight evidence for a possible 12-fold position occupied on the average by one/sixteenth of a Ca atom was found.The charge distribution in the structure is poor, as would be expected for such an unstable structure as this.Dehydration causes a major change in the Si, A1-O framework.