In situ powder diffraction studies of temperature induced transformations in minerals

Synchrotron radiation XRPD is a powerful tool for the in situ investigation of phase transformation in crystalline materials. Diffraction studies in time-resolved or temperature-resolved conditions allow complete characterization of the temperature induced structural changes and of the process kinetics. Transformations involving fast reactions take particular advantage of the brilliance and tunability of synchrotron X-rays. A few case studies of phase and structure changes in minerals nicely demonstrate applications of in situ studies. The kinetics of the dehydroxylation process of natural kaolinites exhibiting different degrees of stacking fault densities has been studied using dynamic and isothermal synchrotron XRPD data collected at the SRS, Daresbury. The kinetic path and the activation energies of kaolinite dehydroxylation and of subsequent mullite crystallization at high temperature show a marked dependence on the original structural state of the kaolinite material. The crystallization process of zeolite Na-LTA from activated kaolinite precursors has been characterized using time-resolved isothermal diffraction experiments at the NSLS, BNL. The nucleation and growth processes are related to the temperature of activation of the precursor material, and the process invariably implies a stage where nucleation of metastable Na-LTA and thermodynamically stable SOD an in competition. The correct choice of the activated material and of the crystallization temperature are crucial in order to optimize the reaction yield. Finally, dynamical structure analysis will be illustrated by discussing the dehydration processes in the natural zeolites laumontite and stilbite. The structure response to temperature has been studied with temperature-resolved XRPD data collected at the NSLS, BNL. Analysis of the powder patterns by Rietveld method allows complete characterization of the water molecules-cation-framework oxygen atoms interaction during the thermally driven release of the water molecules from the zeolitic channels. In stilbite, dynamic breaking of the T-O-T bonds is experimentally evidenced for the first time, and it is caused by the increased coordination of the Ca cations to framework oxygens. (C) 1997 Elsevier Science B.V.