A geochemical and mineralogical investigation of calcite growth and its interactions with divalent cadmium, manganese, and zinc /

The incorporation of divalent metals into calcite grown from low-temperature (~25°C) aqueous solutions is quantified by partition coefficients (DMe). However, DMe values are commonly derived from bulk analyses of calcite seeds, a practice that underestimates the effect of differential incorporation at symmetrically nonequivalent sectors (i.e, sector zoning) and limits the application of DMe in diagenetic and environmental studies. This study sheds some light on the effect of sector zoning on DMe by investigating the growth kinetics and morphology of calcite and the incorporation trends of divalent Cd, Mn and Zn.Despite their similar ionic radii, Mn and Zn were found to exhibit opposite surface site preferences over a large range of chloride concentration. Zn was preferentially incorporated at geometrically less constrained surface sites of (101¯4) faces of calcite when coprecipitated from stepwise diluted solutions. The anomalous incorporation trend of Zn is not due to the prevalence of "large" aqueous complexes (e.g., ZnCln2-n ) but is probably controlled by its electronic configuration. The increase in Cl incorporation into calcite observed in parallel with its stepwise dilution in solution correlates also with an increase in surface roughness, suggesting that the latter is a factor contributing to impurity incorporation during crystal growth.Investigation of the spontaneous nucleation and growth morphology of calcite from aqueous solutions prepared with different (Ca2+):( CO2-3 ) activity ratios and NaCl and NH4Cl concentrations demonstrated the potential role that the amine functional group in organic molecules may play in influencing biomineralisation and the role of the (Ca2+):( CO2-3 ) activity ratio as an abiotic habit modifier.Cd2+, Mn2+ and Zn2+ respectively inhibited the growth of the (101¯0), (011¯2) and (0001) surfaces of calcite and were preferentially incorporated in the stabilized corresponding growth sectors. DMe values unaffected by growth kinetics approach DMe,ideal when calculated from analyses of the stabilized growth sectors and may represent equilibrium values. DCd,eq = 2235 +/- 400 at {101¯0}, DMn,eq = 97 +/- 6 at {011¯2} and DZn,eq = 190 +/- 32 at (0001). While at {101¯4}, D Cd and DMn reached only 500 +/- 120 and 21 +/- 3.4, respectively. These findings demonstrate that equilibrium partitioning is attained only when (1) calcite growth reaches stoichiometric saturation and "morphological equilibrium" with the aqueous solution and (2) if metal incorporation occurs entirely at surface sites offering optimal geometry and coordination for ion incorporation. A conceptual model is proposed to quantify the contribution of sector zoning on DMe values derived from bulk analyses of precipitates