『Abstract
The dissolution
rates of calcite, aragonite, and ground clam, cockle, and mussel
shells were measured at 25℃ 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 (r gsa)
of both aragonite and crushed clam and cockle shells, which are
composed of aragonite, can be described within uncertainty using:
r gsa /(mol/cm2/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, r gsa of calcite can be
described using:
r gsa /(mol/cm2/s)＝（1.82±0.2）×10-10（1−Ω）1.25±0.16
where r gsa 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, r gsa is found to
be accurately described as a function of saturation index independent
of pH at neutral to basic conditions.
Keywords: Biogenic carbonates; Mineral dissolution; Surface area;
Calcite; Aragonite』
1. Introduction
2. Theoretical background
2.1. Standard states
2.2. Calcium carbonate dissolution kinetics
2.3. Calculation of pH
3. Materials and samples
4. Results
5. Discussion
5.1. Is BET or geometric surface area the best proxy of reactive
surface area for biogenic carbonates?
6. Conclusions
Acknowledgments
References