Otavite-calcite solid solutions have been synthesized to investigate the factors which control the patterns of compositional zoning. The equilibrium partitioning of otavite between the aqueous and the solid phase can be described by means of a function X CdCO3 (X (sub Cd,aq) ) eq , which relates the solid mole fraction and the aqueous activity fraction of Cd. According to this function, there is a strong preferential partitioning of Cd towards the solid. A set of experiments starting with parent solutions with different Cd (super 2+) /Ca (super 2+) ratios was carried out. In these experiments nucleation takes place at high supersaturations and the Cd (super 2+) and Ca (super 2+) ions tend to be laid down in a ratio which deviates from the equilibrium distribution. Experimental nucleation data can be fitted to a curve X CdCO3 (X (sub Cd,aq) ) ef of the same functionality as that for equilibrium. After nucleation, supersaturation decreases as the crystals grow. During this process, substituting ions are not incorporated into the solid in the same stoichiometric proportion as in the aqueous solution. Therefore, crystal and fluid compositions tend to vary as growth proceeds and this evolution is registered as a compositional zoning. The curves X CdCO3 (X (sub Cd,aq) ) eq and X CdCO3 (X (sub Cd,aq) ) ef confine the reaction path corresponding to the growth process. Here, these curves are used to account for the observed zoning patterns.