Thermally induced phase transformations of Ca-exchanged LTA and FAU zeolite frameworks: Rietveld refinement of the hexagonal CaAl2Si2O8 diphyllosilicate structure

Thermally induced phase transformations of Ca-exchanged LTA and FAU zeolites are followed in the range from room temperature to 1500 degrees C. Both frameworks collapse into amorphous intermediate products after heating between 800 and 900 degrees C. Prolonged heating of the intermediate product obtained from the Ca-LTA zeolite over 900 degrees C induces recrystallization of CaAl2Si2O6 phases (hexagonal CaAl2Si2O8 phase and anorthite) into a mixture, The crystal structure of the hexagonal CaAl2Si2O8 phase [a(0) = 5.1154 (Angstrom), c(0)= 14.767(9) Angstrom] is refined by the Rietveld method from X-ray powder diffraction data to final agreement factors R(P) = 8.99%, R(WP) = 11.87%, and R(B) = 4.31%. The refined hexagonal Ca-diphyllosilicate phase is unstable in prolonged calcination, and at 1050 degrees C it can be transformed polymorphously into an highly ordered anorthite,,,. Prolonged heating of the amorphous intermediate product obtained from Ca-FAU zeolite over 900 degrees C results directly in formation of a less ordered anorthite(FAU) Ca(0.88)rectangle(0.12)Al(1.77)Si(2.23)O(8.00) framework with vacant Ca2+ sites.