Molecular-scale mechanisms of crystal growth in barite

Abstract

Models of crystal growth have been defined by comparing macroscopic
growth kinetics with theoretical predictions for various
growth mechanisms. The classic Burton–Cabrera–Frank (BCF)
theory predicts that spiral growth at screw dislocations will
dominate near equilibrium. Although this has often been
observed, such growth is sometimes inhibited, which has
been assumed to be due to the presence of impurities. At
higher supersaturations, growth is commonly modelled by twodimensional
nucleation on the pre-existing surface according to
the ‘birth and spread’ model. 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
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 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.