Crystallization kinetics of Na_7.4(AlSiO₄)₆(CO₃)_0.7·4H₂O, an intermediate phase between cancrinite and sodalite, grown under low temperature hydrothermal conditions

Abstract

During low temperature hydrothermal alkaline conversion of kaolinite in the presence of sodium carbonate a disordered phase crystallizes showing structural features between the sodalite and cancrinite minerals. A detailed study of its formation field and the kinetics of crystallization is the first step for further investigations of this zeolite-like material. The crystallization kinetics was investigated in the early stage of the reaction for times up to 192 h at a temperature of 353 K. Besides X-ray powder diffraction²⁷Al 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 aluminium in four-coordinate position of an aluminosilicate framework structure. Important conclusions can also be drawn from the experiments for the synthesis of ideal ordered cancrinites.