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Chabazite [idealized formula Ca2Al4Si8O24·12H2O] is one of the most widespread natural zeolites. A consensus on positions of water molecules and exchangeable cations in chabazites had not been obtained, although attempts to determine their positions have been made intensively. However, our previous study (Nakatsuka et al., 2007) successfully determined their positions at room temperature, in a natural chabazite [Ca1.57Na0.49Al3.39Si8.55O24·12.47H2O] from Komuro, Oh-hito, Shizuoka Prefecture, Japan, using a single crystal X-ray diffraction method. Consequently, we revealed the presences of five water sites (OW1-OW5) and four exchangeable-cation (Ca1Ca4) sites. However, locations of hydrogen atoms in chabazites could not be determined. Determining their positions is quite important for gaining insights into the interactions between water molecules and framework and between water molecules. X-ray diffraction study at low temperature is useful for reducing thermal vibrations of atoms and should make the determination of hydrogen positions easy. Here, we perform the single crystal X-ray diffraction study at 123 K using the same sample as that of our previous study (Nakatsuka et al., 2007) and try to determine hydrogen positions. Moreover, we discuss the crystallographic configurations of water molecules and exchangeable cations at low temperature. In the present study, we found five water sites and four exchangeable-cation sites, as well as the case of room temperature (Nakatsuka et al., 2007). Three (OW1, OW3, OW4) of five water sites found at 123 K are the same as those at room temperature, whereas the remaining two water sites (OW2, OW5) are different from those at room temperature. At present, we are examining hydrogen positions on difference Fourier map.

Structural behaviour and thermal phase transitions of borosilicates with large cations