Mineral precipitation and dissolution in aqueous solution: in-situ microscopic observations on barite (001) with atomic force microscopy

Abstract

Crystal growth and dissolution mechanisms on the barite (001) surface have been observed in-situ in aqueous solution as a function of the saturation state at molecular scales using Atomic Force Microscopy (AFM). On freshly cleaved barite (001) surfaces, cleavage steps with a step height of 7 Å occur, representing the height of a unit cell in the c direction. Growth and dissolution occurs via the advance and retreat of steps with a step height of half a unit cell (3.5 Å). In supersaturated BaSO 4 solution, monolayer step growth increases linearly with the BaSO 4 concentration. Step velocities of one BaSO 4 half-unit cell layer are faster in [100] relative to [−100], whereas in the underlying BaSO 4 half-unit cell layer, step velocities are faster in [−100] than in [100]. A 2 1 screw axis parallel to [001] causes the directional growth in opposite directions in different half-unit cell layers. At low to moderate supersaturations, up to about 80 μM BaSO 4, spiral growth is the dominant growth mechanism, whereas at higher BaSO 4 concentrations, surface nucleation dominates the growth process. In the presence of a crystal growth inhibitor (NTMP, nitrilotri(methylenephosphonic) acid), the nucleation rate as well as step growth is retarded depending on the inhibitor concentration. The morphology of monolayer step edges becomes irregularly curved and jagged, thus suggesting that NTMP molecules attach preferentially to step edges. In pure water, shallow etch pits form with a morphology defined by monolayer steps parallel to [010] and [110]. All etch pits within one BaSO 4 layer point in the same direction, whereas etch pits in the underlying BaSO 4 layer point in the opposite direction, as a result of the 2 1 screw axis. Chelating agents, such as EDTA, ethylenediaminetetraacetic acid, dissolve barite effectively by the formation of Ba-EDTA surface complexes and their desorption resulting in an increased etch pit formation rate. Different types of etch pits could be distinguished: (i) shallow etch pits are defined by steps parallel to [010] and [110], (ii) whereas deep etch pits are defined by steps parallel to [100] and [010]. Step velocities of retreating monolayer steps parallel to [110] retreat faster than steps parallel to [010] resulting in an elongated etch pit morphology. With increasing EDTA concentration from 0.1 mM to 100 mM, the velocity of steps parallel to [110] is reduced by one order of magnitude. Etch pit morphology as well as monolayer step kinetics suggest that EDTA molecules attach to the barite (001) surface and detach a BaEDTA 2− complex preferentially from steps parallel to [110].