CALCITE SURFACE-STRUCTURE OBSERVED AT MICROTOPOGRAPHIC AND MOLECULAR SCALES WITH ATOMIC-FORCE MICROSCOPY (AFM)

Atomic force microscopy (AFM) was used to study calcite cleavage surfaces in air and under aqueous solution at the microtopographic and molecular scales. Surfaces freshly fractured and imaged in air showed wide, flat terraces with steps only a few monolayers high. Exposure to distilled water promoted development of etch pits with differential dissolution rates on inequivalent sides. The pits became broad and shallow with apparent approach to equilibrium after about fifteen minutes. Exposure of the freshly fractured surfaces to air for more than a couple of hours resulted in the appearance of narrow and deepening, irregular-sided pits. These observations indicate a highly dynamic surface capable of incorporating adsorbed material into the near-surface bulk. Molecular resolution images revealed a regular array of corrugations that were interpreted to represent the covalent carbonate groups. The unit cell dimensions of the lattice were the same as those expected for the cleavage plane of bulk calcite but the two-dimensional surface symmetry was not conserved. Apparent displacement of every second carbonate row probably results from frictional forces as the tip scans over rows of alternating orientation; likewise, it may be an expression of surface hydration species adsorbed to these alternate rows and/or the adsorbed species may enhance the effect of the frictional forces. A 2 X 1 structure is observed on surfaces of pure calcite imaged by AFM in air and under water and supports observations made previously using low energy electron diffraction (LEED) in vacuum. The 2 x 1 pattern may be caused by slight twisting of some surface carbonate groups in order to stabilize ''dangling bonds'' and surface charge after cleavage and surface hydration. However, exact interpretation of row-pairing and the 2 X 1 structure is difficult because of the convolution of information in the AFM images resulting from topography, elastic surface deformation, and electrostatic forces.