Changes in the crystal structure of marialite during heating

The crystal structure of marialite (PAM-1)Na4Al3Si9O24Cl with 4.6% of meionite (the Pamirs, Central Asia) was refined by the Rietveld method at temperatures ranging within 25-600 deg. C. The X-ray diffraction data were collected on a STOE powder diffractometer (λMoKα1,4.00 deg. <2θ<48.98 deg. ). The sample was placed into a rotating 0.3-mm-diameter capillary. The number of Bragg reflections was about 500 (for each X-ray data set measured at 200, 400, and 600 deg. C). All the calculations were performed using the WYRIET program (version 3.3) within the sp. gr. I4/m; Rwp=0.047-0.031; RF=0.036-0.023, s=1.53-0.64. Unlike the cations and anions located in large cavities of the marialite structure, strongly dependent on heating, the silicon-aluminum-oxygen framework of marialite changed only slightly upon heating to 600 deg. C. The volume (V0) and the unit-cell parameter a increase, whereas the parameter c slightly decreases with an increase of temperature. Such temperature dependence of the parameter c is observed for the first time