PNNL

Abstract

We studied the impact of aqueous Mn(II) on the (1014) surface of calcite using AFM and a flow-through cell. These experiments show equivalent interactions of Mn with the obtuse and acute steps on calcite, similar to results with Ca ions and in marked contrast to results with carbonate and Sr ions. Little impact on dissolution rate is seen until a threshold level is reached, whereupon near complete frustration of dissolution occurs for a further four-fold increase in Mn concentration. These nonlinear results can be explained using a simple terrace-ledge-kink model that incorporates site-blocking and works equally well with metal ion or carbonate ion pair concentrations. With even higher concentrations of Mn, a new lath-shaped phase forms on the calcite surface oriented along the [221] direction, having a uniform width of 150-220 nm, variable length, and a height of only 2.5 nm. Spectroscopic evidence (EPR, XPS, XANES) suggests that Mn(II) is present, but the exact composition is unknown. Assuming a Mn-substituted carbonate forms, the direction of growth is along the direction of greatest lattice mismatch to calcite. This barrier to growth is apparently overcome by a decrease in lattice stiffness. The cross-sectional morphology of the heteroepitaxial layer is described well by a glued wetting layer model.