MOLECULAR-SCALE MECHANISMS OF CRYSTAL-GROWTH IN BARITE

Models of crystal growth have been defined by comparing macroscopic gr owth kinetics with theoretical predictions for various growth mechanis ms(1,2). The classic Burton-Cabrera-Frank (BCF) theory(3) predicts tha t spiral growth at screw dislocations will dominate near equilibrium. Although this has often been observed(2,4), such growth is sometimes i nhibited(4,5), which has been assumed to be due to the presence of imp urities(6). At higher supersaturations, growth is commonly modelled by two-dimensional nucleation on the pre-existing surface according to t he 'birth and spread' model(7), In general, the morphology of a growin g crystal is determined by the rate of growth of different crystallogr aphic faces, and periodic-bond-chain (PBC) theory(8,9) relates this mo rphology to the existence of chains of strongly bonded ions in the str ucture. Here we report tests of such models for the growth of barite c rystals, using a combination of in situ observations of growth mechani sms at molecular resolution with the atomic force microscope(10,11) an d computer simulations of the surface attachment of growth units. We o bserve strongly anisotropic growth of two-dimensional nuclei with morp hologies controlled by the underlying crystal structure, as well as st ructure-induced self-inhibition of spiral growth. Our results reveal t he limitations of both the BCF and PBC theories in providing a general description of crystal growth.