A reexamination of the crystal structure of the zeolite offretite

The crystal structure of a natural offretite from Poia Creek, Adamello, Italy (schematic formula KCaMgAl5Si13O36:17H(2)O) has been refined to a residual wR of 0.038. The offretite framework can be described by a sequence AABAAB ... of 6-membered rings of tetrahedra. The (SI,AI) distribution (29% of Al in T1 and 37% of Al in T2) is consistent with the disorder found in tetrahedral sites for all zeolites describable by sequences of 6-rings. K is at the center of the cancrinite cage and is coordinated to 6 framework oxygen atoms. Mg, placed at the center of the gmelinite cage, is coordinated to 6 water molecules; 4 of these are disordered on 9 possible positions relative to two sites with multiplicity 6 and 3, respectively. Ca atoms alternately occupy two sites near each other, at the center of the wide channel parallel to parameter c, and are octahedrally coordinated to the same water molecules. The strong differences in the T-O distances found between hydrated and dehydrated offretite are here explained in terms of interactions between extraframework ions and framework oxygens.