High-pressure diffraction study of zeolites stilbite and stellerite

The high-pressure behavior of zeolites of the stilbite-stellerite isomorphous series is studied by X-ray powder diffraction using synchrotron radiation in the penetrating (water-containing) medium in a high-pressure cell. At the initial stage (< 2 GPa), orthorhombic stellerite |Ca3.77(H2O)28| [Al7.54Si28.46O72] shows lower compressibility than monoclinic stilbite |Ca4.00Na1.7(H2O)30| [Al9.7Si26.3O72]. Single crystal XRD does not display any significant differences in the behavior of stellerite in the penetrating and nominally non-penetrating media. At the first stage of the compression, the number of H2O molecules increases from 28 to 32 due to the filling of statistically occupied positions. With further compression no changes are observed. At around 1 GPa stellerite undergoes a second-order phase transition with a decrease in symmetry from Fmmm to F2/m. The compressibility of the powder sample of stellerite is considerably lower than that of the single crystal. These differences may be attributed to kinetic effects of additional hydration induced by the pressure.