SUMMARYThe 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 ZnχCa1‐χCO3as 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)2which has a higher stability than ZnCO3. The CaCO3surface 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 CuχCa1‐XCO3, but CuCO3is not found naturally because of much greater stability of Cu(OH)2, and so there is no independent value for the solubility of a CuCO3end‐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 h