Synthesis and crystal structure of carbonate cancrinite Na8[AlSiO4]6CO3(H2O)3.4, grown under low-temperature hydrothermal conditions

The synthesis of cancrinite in the system Na2O-SiO2-Al2O3-Na2CO3-H2O was studied under low-temperature hydrothermal conditions in the 353 K<T<473 K interval. The aim was to reveal the suitable range for the crystallization of pure-phase carbonate cancrinite with the ideal composition Na-8[AlSiO4](6)CO3(H2O)(2) without cocrystallization of sodalite or intermediate disordered phases between cancrinite and sodalite. It was found that cancrinite formation reacts very sensitive on the temperature within the autoclaves whereas the concentration of reactants and the alkalinity of the hydrothermal solution have a much lower influence on the phase formation. Thus the temperature of crystallization of carbonate cancrinite without any by-products should not remain below 473 K. At the lower reaction temperature of 353 K the formation of a disordered intermediate phase between the cancrinite and the sodalite structure has been obtained in every case, independent of the template concentrations and the base. Some problems to detect this in a typical powder product mixture are discussed. Besides the Si-29 and Al-27 MAS NMR characterization of the products, the crystal structure refinement of pure carbonate cancrinite of ideal composition Na-8[AlSiO4](6)CO3(H2O)(3.4), has been carried out from X-ray powder data using the Rietveld method: P6(3), a=1271.3(1) pm, c=518.6(1) pm, R-WP=0.073, R-F=0.0116 for 347 structure factors and 45 variable positional parameters. (C) 1999 Elsevier Science B.V. All rights reserved.