The surface microtopography of hematite over the course of dissolution in oxalic and citric acids was examined by in-situ and ex-situ atomic-force microscopy. In-situ imaging of the basal-plane surface of a centimeter-scale natural hematite sample immersed in 2 mM citric acid demonstrated that the basal-plane surface was relatively unreactive; rather, dissolution occurred along step edges and via etch-pit formation. Ex-situ imaging of synthetic hematite particles following batch dissolution in 1 mM oxalic acid showed similar dissolution features on basal-plane surfaces; in addition, etching along particle edges was apparent. The presence of etch features is consistent with a surface-controlled dissolution reaction. The results are in agreement with previous investigations suggesting that the basal-plane surface is relatively unreactive with respect to ligand exchange. Both in-situ and ex-situ imaging of particle surfaces can provide valuable information on the roles of surface structures and microtopographic features in mineral dissolution.
