MnCO3-ZnCO3 solid-solutions from Broken Hill (NSW), Australia: Geochemical and infrared spectroscopic investigations.

A nearly complete rhodochrosite-smithsonite (MnCO3-ZnCO3) solid-solution series with minor amounts of Pb2+, Cu2+, Ca2+ and Mg2+, encrusting goethite/coronadite, is found in the oxidized zone of the Broken Hill ore body. 13C-values between -10.0 and -12.3 º/°° (PDB), and 18O-values between 26.5 and 28.1 º/°° (SMOW) indicate the influence of organic carbon and confirm formation under low temperature conditions, respectively.

Based on thermodynamic data and assuming equilibrium precipitation, element ratios of the fluids depositing the rhodochrosite-smithsonite solid-solutions are estimated (25 ºC). Since Pb2+ and Cu2+ are incorporated into the lattice of rhombohedral carbonates, the stabilities of hypothetical calcited-structured PbCO3 and CuCO3 (SVERJENSKY & MOLLING 1992) are used in these calculations. The aqueous environment is calculated to have been depleted in Pb2+ and Cu2+ with respect to Mn2+, and highly variable with respect to Zn2+/Mn2+, Ca2+/Mn2+ and Mg2+/Mn2+ ratios. Estimated log-values of the Pb2+/Zn2+ ratios range between -1 and -3. A Lippmann phase diagram (LIPPMANN 1980) is constructed for the system MnCO3 – ZnCO3 – H2O assuming the solids to form ideal solid-solutions.

Based on the model of 2 – 4 covariance (BÖTTCHER et al. 1992), infrared spectroscopy is applied to investigate the effects of cationic substitution on the positions of absorption bands. Frequency shifts in the 2 and 4 vibrational bands of the carbonate ion are observed in the MnCO3 – ZnCO3 mineral series as a function of cationic substitution. Taking into account analysis of end-member carbonates, a 2 – 4 -plot is constructed, reflecting mineral compositions in the ternary systems MnCO3 – ZnCO3 – CaCO3 and MnCO3 – ZnCO3 – MgCO3.