The crystal structure of calcium mordenite has been studied at 150, 300, 450, and 20 °C by single-crystal X-ray diffraction methods. The cation distribution changes with temperature although the framework is not affected. One calcium ion, in the small channel, is bonded to six framework oxygens and to two water molecules. This ion does not move on heating, but the water occupancy is reduced to that of the calcium ion. The second calcium ion, in the large channel, is bonded at room temperature to seven water molecules. As the temperature rises, water is lost and the cation becomes distributed over two pairs of sites, in both of which it is bonded to framework oxygens of the wall of the large channel and up to 150 °C also to water molecules. The site in the small channel has the best coordination for calcium, and its occupancy increases as the temperature rises.

Publication costs assisted by the Katholieke Universiteit LeuvenTwo crystal structures of dehydrated NH4-exchanged chabazite crystals were studied by use of single-crystal X-ray diffraction methods after dehydration at 320 "C, and at 600 "C following a steaming period. No significant differences are observed between the two structures; there is no preference of the protons for a single oxygen species nor any indication of A1 removal from the framework after steaming. A strong correlation between T-0-T bond angles and T-0 distances in adjacent tetrahedra shows an identical slope with that of dehydrated H mordenite. The framework oxygens, all pointing either to the hexagonal prisms or to the centers of the framework eight-rings, prohibit the free vibration of OH bonds in the large cavity. This explains the 30 cm-' decrease of the HF bands as compared to the value predicted from the framework composition.

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