Molecular-scale mechanisms of crystal growth in barite

Models of crystal growth have been defined by comparing macroscopic growth kinetics with theoretical predictions for various growth mechanisms(1,2). The classic Burton-Cabrera-Frank (BCF) theory(3) predicts that spiral growth at screw dislocations will dominate near equilibrium. Although this has often been observed(2,4), such growth is sometimes inhibited(4,5), which has been assumed to be due to the presence of impurities(6). At higher supersaturations, growth is commonly modelled by two-dimensional nucleation on the pre-existing surface according to the 'birth and spread' model(7), In general, the morphology of a growing crystal is determined by the rate of growth of different crystallographic faces, and periodic-bond-chain (PBC) theory(8,9) relates this morphology to the existence of chains of strongly bonded ions in the structure. Here we report tests of such models for the growth of barite crystals, using a combination of in situ observations of growth mechanisms at molecular resolution with the atomic force microscope(10,11) and computer simulations of the surface attachment of growth units. We observe strongly anisotropic growth of two-dimensional nuclei with morphologies controlled by the underlying crystal structure, as well as structure-induced self-inhibition of spiral growth. Our results reveal the limitations of both the BCF and PBC theories in providing a general description of crystal growth.