The effect of Mn2+ on calcite growth kinetics was investigated in CaCl2 solutions using a constant composition experimental technique which permitted maintenance of a constant degree of calcite supersaturation and constant Mn2+/Ca2+ during overgrowth precipitation. Calcite growth rates were determined at various temperatures (10, 25, 50°C) and degrees of calcite supersaturation (Ωc from 1.3 to 10). Mn2+ is a fairly strong inhibitor of calcite precipitation, reducing calcite growth rates by up to an order of magnitude. At 25°C, precipitation inhibition increased with increasing Mn2+/Ca2+ ratio in the solution and with decreasing Ωc. An order of magnitude change in either PCO2 or Mca at the same solution Mn2+/Ca2+ ratio did not significantly affect the degree of calcite growth rate inhibition or the functional dependence of degree of inhibition on Ωc. Inhibition appears to depend on the Mn2+/Ca2+ ratio rather than on absolute concentration of Mn2+; with fixed Mn2+/Ca2+ ratio, a factor of 10 change in MMn did not substantially alter calcite growth kinetics. At 10 and 50°C, the degree of inhibition by Mn was similar to results at 25°C for the same Mn2+/Ca2+ ratio. The degree of inhibition is related to the mol% MnCO3 in the calcite overgrowth, suggesting that the inhibitory effects of Mn2+ on calcite growth kinetics in these solutions are related to Mn2+ adsorption and uptake in the calcite crystal lattice. Prior kinetic investigations in seawater suggest an increasing Mg-calcite precipitation rate with increasing Mn2+/Ca2+ solution ratios, rather than the growth inhibition we report. Mn2+ interacts quite differently with the calcite surface during precipitation from low vs. high Mg2+/Ca2+ solutions. If the Mg2+/Ca2+ ratio in the solution is sufficiently low to preclude formation of Mg-calcite, growth rates will be inhibited by Mn2+. Thus, in diagenetic systems with waters of low Mg concentration (most meteoric and sedimentary basin waters), Mn2+ can be a strong inhibitor of calcite precipitation. Low-Mg calcites containing > 2 mol% MnCO3 likely have experienced significant inhibition during precipitation. © 1990.
