The reactions of copper and zinc with calcium carbonate surfaces

The reaction of copper and zinc were studied by adding the metal nitrate to a 4g:50ml suspension of calcite and water which had been equilibrated for two days, and then equilibrating for a further three days. Zinc behaved in a similar manner to cadmium in forming a surface-solid solution of Zn_χCa_1-χCO₃ as a result of adsorption, and the pIAP of the equilibrium solutions were close to those expected from the Thorstenson & Plummer equation. However, the continuity from adsorption to precipitation was broken by the formation of Zn₅(OH)₆(CO₃)₂ which has a higher stability than ZnCO₃. The CaCO₃ surface appears to constrain the adsorbed Zn to conform to a carbonate structure despite its lower stability than the hydroxy carbonate.
The adsorption data for Cu can also be explained in terms of a surface-solid solution of Cu_χCa_1-XCO₃, but CuCO₃ is not found naturally because of much greater stability of Cu(OH)₂, and so there is no independent value for the solubility of a CuCO₃ end-member if a surface-solid solution is formed. Secondary reactions are more likely to occur for Cu, especially close to the adsorption-precipitation boundary. Continuity from adsorption to precipitation again was not found. Theory predicts that Cu is less likely to form a surface-solid solution than Zn.
Upper Chalk (calcite with 4% other minerals, mostly quartz) behaved in a similar way but with increased adsorption due to its higher surface area.