Growth surface topography of kaolinite-group minerals and their simulation based on the regular sequence of enantiomorphic layers

Abstract

The regularities of manifestation of polytypism in a number of kaolinite-group minerals (kaolinite, dickite, halloysite, and nacrite) have been investigated by transmission electron microscopy and vacuum decoration. Growth patterns of elementary layers with a thickness of 7 A, individual for each polytype, were observed on (001) faces of microcrystals. Specific features of the growth patterns of polytypes have been revealed by comparing them with simulated patterns constructed based on a packing of regularly alternating right- and left-handed (enantiomorphic) kaolinite layers. The new approach to the consideration of the polytypism of kaolinite minerals is substantiated by the absence of symmetry elements in the 7-A-thick layer, which determines their structure; the formation of enantiomorphic forms of kaolinite; the presence of grazing-reflection planes in the growth patterns; and the structure of polytypes with a two-layer period. Packing of enantiomorphic layers may yield eight structures, two of which correspond to the right- and left-handed forms of kaolinite, one is for dickite, two are for halloysite, and three are for nacrite. It is shown that the simulated and real growth patterns of these minerals are in good correspondence.