The complete series of the otavite-calcite solid has been synthesized by counter diffusion of CdClz + CaClz and CaCO] through a column of silica hydrogel. In this system there is a strong preferential partitioning of Cd in the solid phase, and only a very narrow range of aqueous phase compositions can coexist in equilibrium with intermediate solid solutions, i.e., the (Cd, Ca)C03 solid solution tend to crystallize from aqueous solutions in a bimodal way. At high supersaturations, the range of aqueous solutions from which intermediate solid solutions can nucleate expands, but the bimodal effect remains. This bimodal behaviour accounts for the development of high gradient composistional zoning and oscillatory zoning during the growth process.