Dyhydration Behaviour and Thermal Stability of (Na, Ca)-Exehanged Gonnardite

Natural gonnardite (Na₅Ca₂(Al₉Si_nO₄₀).12 H₂0), belonging to the fibrous zeolites group, was cation-exchanged with Na and Ca to give a series of Na^+∗∗∗Ca²⁺ exchanged forms. The Na_zO/(Na₂O+CaO+K₂O) ratios of the obtained products were in the range from 82.84% to 6.88% (Ca-form). Dehydration behaviour and thermal stability of these products as well as natural three gonnardite and one tetranatrolite were examined by high-temperature X-ray diffraction and thermal analysis (TG-DTA). The lattice parameters a and c were almost unchanged in the whole Na^+∗∗∗Ca^z+ exchanged series. Ca-gonnardite, of which occurrence in nature was very seldom, showed two endotherms due to dehydration at 222°C and 350°C. This indicates that the Ca-gonnardite essentially dehydrates in two steps like scolecite. It transformed directly into amorphous phase at about 360°C by the destruction of the framework structure due to dehydfation. This destruction temperature was about 240°C and lower by 70°C than those of tetranatrolite and gonnardite. In the framework of the gonnardite type, the Ca-exchange makes the hydrated phase more stable, while it lowers the thermal stability. This is because in the Ca-form water molecules occupy vacant sites produced by 2 Na^+∗∗∗Ca²⁺ exchange, and that the framework structure is, more easily susceptible to distortion and hence to thermal destruction on heating.