14:15 〜 14:30
[SMP44-09] Symmetry reduction of analcime with Al/Si ordering
キーワード：アナルサイム、単結晶X線回折、Al/Si秩序配置
Analcime is a sodium aluminosilicate hydrate (NaAlSi2O6·H2O) with the ANA type of zeolite framework. It occurs widely in hydrothermal and diagenetic environments. Symmetry of analcime is well known to be changed with distribution of framework cations and extra-framework cations. Naturally occurring analcime generally exhibits cubic symmetry space group Ia-3d, which is the maximum topological symmetry, but it can crystallize in at least three different symmetries; tetragonal space group I41/acd, orthorhombic space group Ibca, and monoclinic space group I2/a. However, crystallization conditions affecting the symmetry change have not been fully understood yet. In the study, we hydrothermal synthesized single crystals of analcime and hydrothermally re-heated under various heating time. Single crystals obtained from the different processes were refined by using single-crystal X-ray diffraction method.
Single crystals ranging in size from 50 to 120 μm were grown from gels of Al2(SO4)3 and Na2SiO3. They show deltoidal icositetrahedron habit with well-developed 24 equivalent {2 1 1} crystal faces. Single crystals grown from gels possess cubic Ia-3d symmetry, in which Si and Al are totally disordered over the framework T sites. Single crystals of analcime hydrothermally reheated for 24h, however, exhibit tetragonal I41/acd symmetry. The tetragonal analcime shows a weak site preference of Si for T1 site and Al for T2 site. Single crystals of analcime hydrothermally reheated for 48h display orthorhombic Ibca symmetry. In the orthorhombic analcime, Si and Al are strongly ordered over the T sites. Si is preferentially distributed into T11 and T12 sites whereas Al is into T2 site. The crystal structural analysis revealed continuous symmetry reduction from cubic Ia-3d to orthorhombic Ibca through tetragonal I41/acd depending on heating time. On the other hand, Na atoms are equally distributed over the extra-framework sites during the symmetry reduction. The result of the study clearly shows the heating time significantly influences the Al/Si ordering over the framework T sites rather than the ordering of extra-framework cations. The symmetry reduction in analcime would be useful for understanding of petrological and geochemical history of rocks.
Single crystals ranging in size from 50 to 120 μm were grown from gels of Al2(SO4)3 and Na2SiO3. They show deltoidal icositetrahedron habit with well-developed 24 equivalent {2 1 1} crystal faces. Single crystals grown from gels possess cubic Ia-3d symmetry, in which Si and Al are totally disordered over the framework T sites. Single crystals of analcime hydrothermally reheated for 24h, however, exhibit tetragonal I41/acd symmetry. The tetragonal analcime shows a weak site preference of Si for T1 site and Al for T2 site. Single crystals of analcime hydrothermally reheated for 48h display orthorhombic Ibca symmetry. In the orthorhombic analcime, Si and Al are strongly ordered over the T sites. Si is preferentially distributed into T11 and T12 sites whereas Al is into T2 site. The crystal structural analysis revealed continuous symmetry reduction from cubic Ia-3d to orthorhombic Ibca through tetragonal I41/acd depending on heating time. On the other hand, Na atoms are equally distributed over the extra-framework sites during the symmetry reduction. The result of the study clearly shows the heating time significantly influences the Al/Si ordering over the framework T sites rather than the ordering of extra-framework cations. The symmetry reduction in analcime would be useful for understanding of petrological and geochemical history of rocks.