Mineralogical changes which occur during the reaction of copper(II) chloride with carbonate in aqueous solution have been investigated, and compared with those involving copper(I) chloride, nantokite, CuCl, and copper artifacts coated with malachite, Cu₂CO₃(OH)₂. The reactions observed and the products obtained are seen to have implications with respect to the use of aqueous sodium carbonate solutions to stabilize archaeological copper objects. The frequent observation of the mineralogically rare species chalconatronite, Na₂Cu(CO₃)₂.3H₂O, on the surface of treated objects is readily explained by known reaction chemistry. Its formation can be minimized by washing the object after treatment.