Xu,J., Fan,C. and Teng,H.H.(2012): Calcite dissolution kinetics in view of Gibbs free energy, dislocation density, and pCO2. Chemical Geology, 322-323, 11-18.

『ギブズの自由エネルギーと転位密度と二酸化炭素分圧を考慮した方解石溶解カイネティックス』


Abstract
 The dissolution of calcite in a full range of saturation conditions is investigated to explore the kinetic effect of Gibbs free energy, dislocation density, and the presence of atmospheric pCO2. Experiments are carried out in a mixed-flow reactor at room temperature (25℃) in both closed and open (to air) settings, and calcite samples are prepared by fragmentation and milling to generate different defect densities. Experimental observations show a highly nonlinear dependence of the dissolution rates on the Gibbs free energy; however, the kinetics does not seem to be affected by the samples' dislocation density, nor the presence of atmospheric pCO2 at any saturation condition. Fitting the conventional transition state model (TST) to the observed rate - free energy relationship indicates that, though the TST rate equation is sufficient to describe the dissolution kinetics near and far from equilibrium, it clearly overestimates the dissolution rate when the system sits in between. These results suggest that: (i) the classic TST model may not be sufficient to depict the relation between dissolution rate and Gibbs free energy once solution saturation falls from its extreme. (ii) The steps associated with increased crystal defects may be overwhelmed by those regenerated at corners and edges of calcite particles through layer-by-layer dissolution along the cleavage directions. (iii) The presence of CO2 in ambient environments bears little importance to calcite dissolution possibly due to the slow response of aqueous HCO3- to pCO2 change at low CO2 partial pressure conditions.

Keywords: Mineral dissolution; Calcite; Transitional state theory; Kinetics; Dislocation density』

1. Introduction
2. Theoretical consideration
 2.1. Calcium carbonate dissolution kinetics
 2.2. Etch pit formation
3. Material and methods
 3.1. Mixed-flow reactor system
 3.2. Sample and solution preparation
 3.3. Imaging by fluid contact AFM
4. Experimental results and dissolution
 4.1. Calcite dissolution rates as a function of ΔG
 4.2. Role of dislocation density in calcite dissolution
 4.3. Effect of atmospheric carbon dioxide on calcite dissolution
5. Conclusions
Acknowledgments
Appendix A. Supplementary data
References


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