Crystals of (Cd,Ca)CO3 have been synthesized under controlled conditions by the counter-diffusion of Cd2+,Ca2+ and CO3= ions through a column of silica hydrogel. Although in nature intermediate phases of (Cd,Ca)CO3 have been not observed to exist, the system is of interest from the point of view of the "sorption" (and subsequent solid solution formation) of Cd2+ at the surface of carbonate minerals. A set of 15 experiments, starting with mother solutions of different concentrations ratios was carried out. The composition of the solids was analyzed by electron microprobe. The method allows the establishment of
the influence of the supersaturation on the distribution of material between solid and aqueous phases. With this aim, the equilibrium calculations and the effective composition of the nucleating crystals were compared. Moreover, backscattered electron images of crystal sections were obtained in order to observe the compositional evolution during the growth process.
The extremely low solubility of otavite compared to calcite implies a strong preferential partitioning of cadmium in the solid phase. As a consequence, in precipitating calcium-rich solid solutions, the aqueous-phase must be very poor in Cd2+. Moreover, only a narrow range of aqueous-phase compositions can coexist in equilibrium with solid mole fractions in the range 0.1 to 0.9. Within this range, small changes in the fluid composition imply drastic changes in the solid equilibrium composition, which switches from one endmember to the other by modifying by some thousandths the value of the aqueous activity fractions. Supersaturation tends to cushion this bimodal tendency, by extending the range of aqueous solutions from which intermediate solid solutions can nucleate, but the overall effect remains. This behaviour is related to the appearance of high gradient compositional zoning. The crystalline individuals develop sharp compositional zoning when the fluid composition passes through a certain critical range during the growth process. Finally, the crystals of (Cd,Ca)CO3 show oscillatory zoning for a wide range of crystallization conditions. The overall sequence is similar in all the crystals of a given experiment, but there is no correlation between specific zoning patterns for two nearby crystals of the same precipitate. The oscillatory behaviour develops autonomously, and it could be accounted for by chemical self-organization during the growth process (Ortoleva et al., 1987; Putnis et al., 1992).
Ortoleva, P., Merino, E., Moore, C. & Chadam, J., Am. J. Sci. 287, 979-1007 (1987).
Putnis, A., Fernández-Díaz, L. & Prieto, M., Nature 358, 743-745 (1992).