CONTROLS OF BURGERS VECTORS ON ETCH PIT FORMATION: AN EXAMPLE ON CALCITE SURFACES

Mineral dissolution plays a central role in natural weathering and controls various geochemical processes at both global and local scales. It is one of the most intensively studied subjects in the broad field of mineral-water interactions. To date, it is generally understood that the dissolution processes are controlled by chemical reactions that occur at the solid-fluid interfaces. One of the crucial pieces of evidence leading to this understanding is the ubiquitous occurrence of etch pits on the surfaces of naturally weathered and laboratory dissolved minerals.  While extensive studies were carried out to explore the roles of etch pits in mineral dissolution, few concerned the physiochemical controls on pit formation. Yet, the dissolution theory predicts that the development of surface pitting is a function of solution saturation and the modulus of the Burgers vectors.

This study attempts to look into the effect of the Burgers vectors on etch pit nucleation using calcite as a model system. Experiments were conducted on {10ī4} cleavage faces at various under-saturations to monitor the nucleation of etch pits. The goal of the experiments is to determine how the magnitude of the Burgers vectors controls the critical under-saturation at which etch pits form. Preliminary experimental results show that, whereas shallow pits nucleate at Ω ≈ 0.4 ~ 0.5 where Ω is defined as the ratio of ionic activity product to solubility product, dislocations with the Burgers vectors greater than the thickness of one {10ī4} monolayer open up at conditions much closer to equilibrium. This is consistent with the dislocation theory’s prediction that the critical under-saturation for etch pit nucleation is exponentially proportional to the reciprocal of the Burgers vectors. These results may explain why etch pits are widely observed on samples collected from natural settings that are often close to equilibrium conditions.