Concentric zoning patterns in crystallizing (Cd,Ca)CO3 solid solutions from aqueous solutions

Otavite-calcite solid solutions have been synthesized to investigate the fa ctors which control the patterns of compositional zoning. The equilibrium p artitioning of otavite between the aqueous and the solid phase can be descr ibed by means of a function X-CdCO3(X-Cd,X- (aq))(eq), which relates the so lid mole fraction and the aqueous activity fraction of Cd. According to thi s function, there is a strong preferential partitioning of Cd towards the s olid. A set of experiments starting with parent solutions with different Cd 2+/Ca2+ ratios was carried out. In these experiments nucleation takes place at high supersaturations and the Cd2+ and Ca2+ ions tend to be laid down i n a ratio which deviates from the equilibrium distribution. Experimental nu cleation data can be fitted to a curve X-CdCO3(X-Cd,X- aq)(ef) of the same functionality as that for equilibrium. After nucleation, supersaturation decreases as the crystals grow. During th is process, substituting ions are not incorporated into the solid in the sa me 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-Cd,X- aq)(eq) and X-CdCO3(X-Cd,X- aq)(ef) confine the reaction path corresponding to the growth process. Here, these curves are used to account for the observed zo ning patterns.