Heteroepitaxial growth of a manganese carbonate secondary nano-phase on the (1 0(1)over-bar 4) surface of calcite in solution

Heteroepitaxy of a manganese carbonate phase with nano-meter dimensions on the (1014) surface of calcite (CaCO3) using an AFM has been observed in solution during dissolution of calcite when the ion activity product of Mn2+ and CO32- nears the solubility limit of MnCO3. Growth rate observations at different Mn concentrations, coupled with X-ray photoelectron spectroscopy and electron paramagnetic resonance measurements, suggest that the resulting phase is Mn0.5Ca0.5CO3. These islands, while growing many microns in length along the [22 1] direction, have a uniform width in the range of 120-240 nm and a uniform height of approximately 2.7 nm, corresponding to nine atomic layers. The islands cease growing when they encounter step edges and have been observed to dissolve when undercut by a growing etch pit. Comparison of the crystal lattices of calcite and Mn0.5Ca0.5CO3 indicates that [221] is the direction of preferred growth. A glued-wetting-layer model with a condition of constant surface chemical potential has been used to model the observed cross section of the heteroepitaxial layer. Although not all the required parameters are accurately known, the model accurately depicts the measured profiles of the islands. (C) 2002 Elsevier Science B.V. All rights reserved.