The crystal structure of CO 3 -cancrinite was analyzed by applying a generalized structure factor expression of the Gram -Charlier expansion to the least -squares refinement performed by using the X -ray intensities measured on single crystals at 173, 293, 473, and 673 K. No satellite reflection or diffuse scattering was observed. The space group is P6 3 with unit cell a = 1.2604(3) and c = 0.51144(8) nm at 173 K, and a = 1.2709(4) and c = 0.5202(6) nm at 823 K. The temperature behavior for the dehydration and the unit cell dimensions were considerably different from those previously reported for CO 3 -cancrinite, which shows superstructure reflections. Two position vectors were assigned to each of the framework atom sites or the CO 3 site for examining its asymmetric probability density function-one for the free energy minimum and another for the potential energy minimum. Two polarization vectors were obtained for a unit cell from those position vectors and ion charges. The rigid ion contributions to the pyroelectric coefficients p (σ) (measured under a constant stress σ) and the secondary coefficient p (2) were finally estimated from the temperature dependences of the polarization vectors.Keywords: Cancrinite, Asymmetric thermal vibrations, Pyroelectricity, Spontaneous polarization INTRODUCTIONCancrinite is a tectosilicate mineral belonging to a polar space group P6 3 (Grundy and Hassan, 1982), which allows the crystal to polarize electrically along the hexagonal c -axis. Figure 1 illustrates the structure of cancrinite in projections along [001] and [210]. Symmetrically independent atom sites obtained from the split -site refinement in the present study are shown in Figure 1a; the z -coordinates are rounded off and given in parentheses. Some measurements of the pyroelectric coefficients have been reported for CO 3 -cancrinite at temperatures 4.2 -500 K by Drozhdin et al. (1975) and Kozlov et al. (1989). These authors found that the temperature dependence of the pyroelectric coefficient p (σ) , which was measured under a constant mechanical stress σ, is non -monotonic, including sign changes below 100 K, and has some polarization relaxations. Most of these studies have pointed out the importance of water molecules in the absolute values of p (σ) , although the interpretation of temperature behaviors is not unique. Drozhdin et al. (1975) considered two cases, wherein the primary and secondary (the piezoelectric effect due to thermal expansions) effects are predominant, whereas Kozlov et al. (1989) considered the temperature dependence of the primary effect as the reason for the sign change. In a more general interpretation, another possibility has been pointed out on the basis of the compensation between the two values of the changes in polarization-one value for the aluminosilicate framework and another for one or more mobile structural elements possessing nonzero intrinsic dipole moments (Galitskii, 1981). Many compounds contain molecules or complexes in channels or voids in their structures, which ar...