The rare mineral otavite CdCO3 was found at the abandoned Pb-Zn mine of Montevecchio (SW Sardinia, Italy). Otavite occurs in the oxidised gossan, in association with aurichalcite. SEM imaging shows that otavite is composed of lenticular hollow structures containing aggregates of tiny rhombohedral crystals. Oxidation of primary Cd-bearing sulphides (sphalerite and greenockite) releases up to 24 ppm of Cd to drainage waters of the Montevecchio area. These waters are generally saturated with respect to otavite. Therefore, this mineral might play an important role in controlling Cd mobility in the environment.

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 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 Cd2+/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 in a ratio which deviates from the equilibrium distribution. Experimental nucleation data can be fitted to a curve X CdCO3 (X 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 Cd,aq)eq and X CdCO3 (X Cd,aq)ef confine the reaction path corresponding to the growth process. Here, these curves are used to account for the observed zoning patterns.