Distribution of cations and water molecules in the heulandite-type framework

The crystal structures of a Mg-rich clinoptilolite from Kamloops Lake [a=17.668(4)Å b=17.986(4)Å c=7.417(3)Å β=116.33(2) ⁰, Z=Na_2.87K_0.37Mg_0.80Ca _0.70Sr_0.08Ba_0.15Al_6.80Si _29.20O₇₂+23.9H₂O]. and a partia11y Rb-exchanged heulandlte [a=17.699(4)Å b=17.927(6) Å c=7.422(2) Å β=116.47(2)^o, Z=Ca_3.31Sr_0.l0Rb _1.45 K_0.25Al_8.45Si_27.49O ₇₂+20.5H₂O] have been determined, with the space group C2/m, based on diffraction intensities collected with a single-crystal diffractometer. The result of the former has established the location of Mg at the M(4) site defined by Koyama and Takéuchi [1] and that of the latter showed that the Rb atoms are essentially located at the M(3) site with-the occupancy 35%. Inspite of this situation, the Rb-exchanged heulandite became amorphous after heating it at 500°C for 16 h, showing that the location of cations at M(3) is not necessarily the factor of thermal stability of the heulandite framework. The water molecules, W(4) and W(5), in this heulandite are displaced from the twofold axis. Both structures combined with a survey of structures reported for clinoptilolite and heulandite have revealed general features on the distribution of cations and water molecules in the heulandite framework.