The crystal structure of offretite (hexagonal, a= 13.291, c= 7.582/~,) from Mt. Simiouse, France, has been refined in the space group P~m2 to a residual of 0.108. The frame structure proposed by Bennett & Gard [Nature, Lond. (1967) 214, 1005 was confirmed. It comprises alternating double and single (7,:r,½) respectively, forming columns with two types of rings of six tetrahedra centred on (0,0,0) and 1 2 cavity, one similar to those found in cancrinite, the other in the centre of the double ring. The columns are cross-linked to enclose channels ca. 6.9 A wide, parallel to e, and cavities similar to those in gmelinite, with 'windows' 3.6 A diameter. Each cancrinite-type cavity contains a K + ion that cannot be replaced with an NH + ion at 80°C. During synthesis, the cancrinite cage probably collects round the K ion to form a precursor of erionite or offretite. A few (0.14) double rings contain a Ca ion. Most (0.82) gmelinite-type cavities contain an Mg ion coordinated to five water molecules. A feasible arrangement was deduced for the Ca ions and water molecules that occupy seven-tenths of the volume of the channels. 5.4 out of 15.2 H20 per unit cell were not located; they are probably in random sites or are mobile. (Si, AI)-O distances suggest that most of the 5.2 A1 atoms are located in the 12 tetrahedra of the double ring; this implies a high degree of local ordering, and the space group P~m2 is probably an average one. Disorder comprising intergrowth with erionite is discussed as this places limits on refinement, and also constricts the channels. Consideration of the unit-cell dimensions of zeolites with frames comprising rings of six tetrahedra showed that the ratio nac-1, where n is the number of rings in the c-period, can be used to indicate the presence of double rings only, single rings only, or both, in frame structures of this type.
