Synthesis and characterization of the basic and non-basic members of the cancrinite-natrodavyne family

Abstract

Hydrothermal synthesis has been studied in the system Na 2O-SiO 2-Al 2O 3-CO 2-H 2O at temperatures of 400 and 770 K and pressures of 10 and 150 MPa. The initial concentrations of carbonate and sodium hydroxide were varied, but the Si/Al ratio was always 1.0. Beside cancrinite hydrate and basic cancrinite two new members of the natrodavyne family could be obtained as non-basic and basic species: Na 8[AlSiO 4] 6CO 3·4H 2O and Na 6[AlSiO 4] 6(CO 3) 0.75 (OH) 0.5·3.4H 2O. X-ray powder diffraction, IR spectroscopy and thermoanalytical investigations clearly indicate structural differences as compared with the cancrinites. Stacking disorder of the interconnected six-membered rings of the aluminosilicate framework is to be expected for the new phases. Whereas the X-ray powder patterns of both species resemble the (cubic) sodalites, polarization microscopy reveals their hexagonal symmetry. From this point of view the disordered natrodavyne can be considered as an intermediate phase between sodalite and cancrinite. Thermoanalytical investigations and high temperature X-ray powder diffraction indicate a significant expansion of the framework for both new dehydrated species, in contrast to cancrinite. Structural phase transitions at 960 K for Na 8[AlSiO 4] 6CO 3 and 795 K for Na 8[AlSiO 4] 6(CO 3) 0.75(OH) 0.5 were revealed by differential scanning calorimetry indicating a high degree of temperature-induced orientational disorder and dynamics of the CO 2− 3 guest anions.