Experimental determination of the dissolution rates of calcite, aragonite, and bivalves

The dissolution rates of calcite, aragonite, and ground clam, cockle, and mussel shells were measured at 25 °C as a function of reactive fluid saturation state. All experiments were performed in mixed-flow reactors using a pH-4 HCl inlet solution. Reactive solution pH ranged from 5.1 to 9.8, and the chemical affinity of the dissolving carbonates ranged from 0 to 47 kJ/mol in the experiments. BET surface area-normalized dissolution rates for calcite are of the same order of magnitude as those of aragonite. In contrast, geometric surface area-normalized calcite dissolution rates are ∼30% lower than corresponding aragonite rates. The dissolution behaviour of the biogenic samples depends on their composition and the surface area used to normalize rates. In all cases, measured BET-normalized dissolution rates of shells are approximately one order of magnitude lower than corresponding mineral dissolution rates. In contrast, measured geometric surface area-normalized bivalve dissolution rates are equal to within uncertainty of those of aragonite or calcite. Geometric surface area-normalized dissolution rates (rgsa) of both aragonite and crushed clam and cockle shells, which are composed of aragonite, can be described within uncertainty using: r gsa mol / cm 2 / s = ( 2.69 ± 0.5 ) × 10 − 10 ( 1 − Ω ) 0.86 ± 0.11 where Ω stands for the saturation state of the dissolving carbonate. Similarly, rgsa of calcite can be described using: r gsa mol / cm 2 / s = ( 1.82 ± 0.2 ) × 10 − 10 ( 1 − Ω ) 1.25 ± 0.16 where rgsa for mussel shells, which are composed of ∼90% calcite and ∼10% aragonite, are similar to those of calcite, but display a complex variation with chemical affinity due to the presence of two minerals. Consistent with previous studies, rgsa is found to be accurately described as a function of saturation index independent of pH at neutral to basic conditions.