Surface imaging of a natural mineral surface using scanning-probe microscopy

Abstract

The {0 0 1} surface of natural BaSO 4 was examined in air and aqueous solution using atomic force microscopy (AFM). On the atomic scale, the arrangement of ions on the {0 0 1} cleavage plane is the same (within experimental error) as in the bulk crystal. Fourier transform maps of the atomic scale AFM images yielded surface lattice parameters of a=5.54±0.04 Å and b=8.84±0.03 Å, indicating that AFM images can be used to obtain crystallographic data of surfaces and to assign surface symmetry as compared with bulk structure. On the micrometre-scale we observed crystallographically orientated monosteps one unit cell (along the c-axis) in height and polysteps as well as etch figures with intrinsic 2 1 screw axis symmetry after dissolution in water on an overall heterogeneous {0 0 1} cleavage plane. In addition to etch pit formation, half-unit-cell-high steps following certain crystal directions ([2 1 0] and [0 1 0]) could be observed, emphasizing the structural control of the dissolution process. Prolonged exposure of a cleaved barite surface to ambient air led to significant surface rearrangement due to the formation and reprecipitation of surface-hydration species within an adsorbed layer of water. Atomistic models of the {0 0 1} cleavage plane, crystallographically oriented step edges along [2 1 0] and [0 1 0] and etch figure boundaries are reported. From these an understanding of the growth and dissolution processes of barite was derived in addition to an explanation of observed step retreat velocities based on surface charging and energetic arguments.