Inhibition of calcite growth rates by Mn 2+ in CaCl 2 solutions at 10, 25, and 50°C

Abstract

The effect of Mn 2+ on calcite growth kinetics was investigated in CaCl 2 solutions using a constant composition experimental technique which permitted maintenance of a constant degree of calcite supersaturation and constant Mn 2+/Ca 2+ during overgrowth precipitation. Calcite growth rates were determined at various temperatures (10, 25, 50°C) and degrees of calcite supersaturation (Ω c from 1.3 to 10). Mn 2+ 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 Mn 2+/Ca 2+ ratio in the solution and with decreasing Ω c. An order of magnitude change in either PCO 2 or M ca 2+ at the same solution Mn 2+/Ca 2+ ratio did not significantly affect the degree of calcite growth rate inhibition or the functional dependence of degree of inhibition on Ω c. Inhibition appears to depend on the Mn 2+/Ca 2+ ratio rather than on absolute concentration of Mn 2+; with fixed Mn 2+/Ca 2+ ratio, a factor of 10 change in M Mn 2+ 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 Mn 2+/Ca 2+ ratio. The degree of inhibition is related to the mol% MnCO 3 in the calcite overgrowth, suggesting that the inhibitory effects of Mn 2+ on calcite growth kinetics in these solutions are related to Mn 2+ adsorption and uptake in the calcite crystal lattice. Prior kinetic investigations in seawater suggest an increasing Mg-calcite precipitation rate with increasing Mn 2+/Ca 2+ solution ratios, rather than the growth inhibition we report. Mn 2+ interacts quite differently with the calcite surface during precipitation from low vs. high Mg 2+/Ca 2+ solutions. If the Mg 2+/Ca 2+ ratio in the solution is sufficiently low to preclude formation of Mg-calcite, growth rates will be inhibited by Mn 2+. Thus, in diagenetic systems with waters of low Mg concentration (most meteoric and sedimentary basin waters), Mn 2+ can be a strong inhibitor of calcite precipitation. Low-Mg calcites containing > 2 mol% MnCO 3 likely have experienced significant inhibition during precipitation.