SYNTHESIS AND CHARACTERIZATION OF THE BASIC AND NON-BASIC MEMBERS OF THE CANCRINITE NATRODAVYNE FAMILY

Hydrothermal synthesis has been studied in the system Na2O-SiO2-Al2O3-CO2-H2O 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: Na8[AlSiO4]CO3.4H2O and Na6[AlSiO4]6(CO3)0.75 (OH)0.5.3.4H2O. 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 Na8[AlSiO4]6CO3 and 795 K for Na8[AlSiO4]6(CO3)0.75(OH)0.5 were revealed by differential scanning calorimetry indicating a high degree of temperature-induced orientational disorder and dynamics of the CO3(2-) guest anions.