Mistry, M.; (2005) The surface structure and growth of natural zeolites. Doctoral thesis , University of London.

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Abstract

A systematic study has been carried out in order to investigate two aspects of zeolite chemistry: to determine the external surface structure and to establish the mechanism of crystal growth. The natural zeolite edingtonite (EDI) was chosen as a model material and the surface structures were studied by finding the most stable termination for each face. For each of the four morphologically important faces the surface structure was found to minimise the number of broken bonds created upon cleaving the surface. In the absence of experimental data, the crystal morphologies were used as a proxy indicator of relative growth rate of different faces. Qualitative inspection indicates that only the equilibrium morphology is able to reproduce the habit of the natural crystal. However, quantitative analysis with the crystal habit observed in SEM images reveals a discrepancy in aspect ratios. By predicting the crystal morphology by using a newly proposed, less demanding method of calculating the number of broken bonds upon cleaving the surface per surface area (Nbb/SA), one is able to accurately reproduce the crystal morphology with the correct aspect ratio. The utility of the NBb/SA method has been tested for a sample of zeolites, results show the Nbb/SA technique has correctly predicted the growth rate and crystal habit of merlinoite, analcime, natrolite, and zeolite A. The test on thomsonite was able to reproduce the correct order of stability for each face, however the aspect ratios were not precisely predicted. By the use of atomistic and ab-initio methods, this research has also investigated the growth mechanism of EDI. The total and condensation energies of a range of possible solution fragments have been calculated and the most stable cluster along with the mechanism of formation has been suggested. The energetics of clusters along with the surface structural analysis has shown strong evidence that characteristic building units control the growth and aggregation of EDI, which dictates the extended crystal structure and the rate at which these building units condense onto the surface determines the crystal shape.

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