MAPPING OF CHEMICAL INHOMOGENEITIES AT CARBONATE MINERAL SURFACES

The composition of mineral-water interfaces plays a significant role in governing/indicating the chemistry of the local aqueous environment. Using Lateral Force Microscopy (LFM), we have studied several model systems exhibiting nanometer scale chemical inhomogeneity on atomically flat surfaces of calcite and dolomite. Calibrated LFM measurements show distinct contrast between impurity cation-bearing overgrowths and the original carbonate substrate. However, the relatively large variations in surface cation composition designed in these experiments rarely yielded changes in the lateral force of greater than 20% relative to the lateral force experienced on the substrate. From the observed increase in lateral force on Ca, Cd, and Sr-enriched monolayers, we designed investigations to test the mechanism responsible for the friction contrast. Initial results suggest that the vertical compliance of the impurity-laden monolayer is lower than the substrate surface, giving rise to a larger contact area between the LFM tip and monolayer. These experiments also revealed negligible difference in the tip-surface adhesion, pointing to vertical compliance as the determining mechanism for friction contrast in these systems. With the relatively poor sensitivity of the LFM to large surface compositional variations, the applicability of LFM in quantitative compositional mapping of mineral surfaces is quite limited. New approaches that rely on surface chemical modification to enhance the sensitivity as well as selectivity of the lateral force approach will be discussed in light of the changes in approaches to mineral surface science over the past 50 years.