CRYSTALLIZATION KINETICS OF NA-7.4(ALSIO4)(6)(CO3)(0.7)CENTER-DOT-4H(2)O, AN INTERMEDIATE PHASE BETWEEN CANCRINITE AND SODALITE, GROWN UNDER LOW-TEMPERATURE HYDROTHERMAL CONDITIONS

During low temperature hydrothermal alkaline conversion of kaolinite i n the presence of sodium carbonate a disordered phase crystallizes sho wing structural features between the sodalite and cancrinite minerals. A detailed study of its formation field and the kinetics of crystalli zation is the first step for further investigations of this zeolite-li ke material. The crystallization kinetics was investigated in the earl y stage of the reaction for times unto 192 h at a temperature of 353 K . Besides X-ray powder diffraction Al-27 MAS NMR has been found to be the preferred method to follow the reaction kinetics because a signal assigned to six-coordinated aluminium of the starting kaolinite can be clearly distinguished from a second resonance, being typical for alum inium in four-coordinate position of an aluminosilicate framework stru cture. Important conclusions can also be drawn from the experiments fo r the synthesis of ideal ordered cancrinites.