Microscopic effects of carbonate, manganese, and strontium ions on calcite dissolution

Abstract

Aqueous dissolution of the (10 1 4) surface of calcite was observed at pH near 9 by using an atomic force microscope equipped with a fluid cell. The influences of carbonate (CO 3 2−), strontium (Sr 2+), and manganese (Mn 2+) ion concentrations on the rates of step motion were measured. Carbonate ions were shown to have a step-specific effect on calcite dissolution. At low levels (<1 μM) of CO 3 2−, the retreat rate of the more structurally open [ 4 41] + steps was faster than the retreat rate of the structurally confined [ 4 41] − steps, leading to anisotropic dissolution. Increasing the CO 3 2− level to as high as 900 μM decreased the rate of retreat of both steps, but the [ 4 41] + step was slowed to a much greater extent changing the degree of dissolution anisotropy. This decrease in step velocity at high CO 3 2− levels was attributed to a corresponding increase in the back reaction (i.e., precipitation) as the solution approached saturation with respect to calcite. Strontium cations were also shown to have a step-specific effect on calcite dissolution similar to that of CO 3 2−. Manganese cations, on the other hand, slowed the rate of retreat of the [ 4 41] − step to a greater extent than Sr 2+. The influence of impurity metal sorption on dissolution is examined in terms of sorption at kinks and the dissolution behavior is explained in terms of a terrace-ledge-kink site-blocking model. Evidence is given to support the hypothesis that ion-pairs formed in solution are the primary growth units for calcite.