Atomic force microscopy of synthetic barite microcrystals

Prismatic ({210}, {001}) and rounded prismatic ({hk0}, {210}, {001}) synthetic barites were prepared with and without hydrochloric acid. The microcrystals were studied with the use of mid- and high-resolution atomic force microscopy (AFM), scanning electron microscopy (SEM), and computational modeling. The gross morphology of barite prepared in acid solution was observed to transform toward {210}-prismatic through a series of {hk0} faces including {310} and {410}. The gross morphology of crystals prepared without acid was fairly constant, but AFM measured a transformation in surface texture from an irregular corrugation to one with regular and lathe-like steps where the long axis of the surface features were aligned parallel to the c-axis. The overall rounding of the {hk0} face is due to both variation in the density of steps along the b- and c-axes and also to non-orthogonality of the faces of the microsteps. Microstep faces were typically not {210}, but were a variety of partly ordered faces similar to models of {310}, {410}, {610}, {810}, and {100}. A greater fraction of {100} or near-{100} facets were measured near the center of the {hk0} face and single-ion-layer steps were also observed. Compared to {210}, the AFM lattice images of the {hk0} faces were very disordered. Moreover, on {hk0} a smaller lattice distance and longer correlation length were measured parallel to the c-axis when compared to the b-direction. Together, these observations support the following proposed model for the growth of rounded faces on synthetic barite. High rates of oriented homoepitaxial secondary nucleation occur near the center of the microcrystal and on step faces that are {100} or near-{100} faces. These features are charged or contain linear surface features which can rapidly generate nuclei via alignment of oligomeric barium sulfate polyions formed from the solution. Once formed, a growth step tends to ripen toward {210} through a series of {hk0} microfacets which are similar in surface energy but are separated by small kinetic barriers.