Synthesis of sodium zeolites from a natural halloysite

The kinetics of hydrothermal crystallisation of sodium zeolites from a natu ral mixture of halloysite and amorphous silica with Si/Al approximate to 4 was investigated. The sample collected at Scarpara (Tuscania, Italy) is the final product of an intense hydrothermal alteration process on the pre-exi sting leucitic tufities. In order to enhance its reactivity in the NaOH sol ution, the sample was thermally activated at 600 degreesC for 1 h. The hydr othermal crystallisation sequence of zeolites formed in the range 90-150 de greesC has been followed using real-time synchrotron powder diffraction. Th e reaction kinetics of Na-X. Na-P and analcime were analysed using a model developed for the study of the kinetic data from X-ray diffraction experime nts. Na-X and Na-P cocrystallize with an autocatalytic nucleation at lower isothermal temperatures and with a heterogeneous nucleation at higher isoth ermal temperatures. Na-X tends to dissolve before Na-P, which in turn trans forms into analcime. This work is part of a general project on the kinetics of formation of zeolites from clay precursors which is important for eithe r engineering and production of valuable industrial materials and for the i nterpretation of poorly understood processes of formation of zeolites in na tural hydrothermal environments.