Dissolution and Crystal Growth of Carbonates

We already touched on some aspects of carbonate surface chemistry e.g., in Chapter 3.4. We have already illustrated some of the factors that affect surface charge and the point of zero charge, pHpzc, in Chapter 3.5, and have discussed certain elementary aspects of CaC03 nucleation in Chapter 6.5 and of coprecipitation (and solid solution formation) in Chapter 6.7. 

Dissolution of carbonates can only occur if the solution is thermodynamically undersaturated, pH is an important variable affecting the saturation ratio (Appendix 8.1 gives a brief review of the CaC03 solubility characteristics in open and closed systems). 

At very low pH, the rate of dissolution is so fast that the rate is limited by the transport of the reacting species between the bulk of the solution and the surface of the mineral (Berner and Morse, 1974). The rate can then be described in terms of transport (molecular or turbulent diffusion) of the reactants and products through a stagnant boundary layer, 8. The thickness of this layer depends on the stirring and the local turbulence. (See Chapter 5 for a discussion of transport vs surface controlled processes.) 

But within the pH range of natural waters, the dissolution (and precipitation) of carbonate minerals is surface controlled i.e., the rate of dissolution is rate determined by a chemical reaction at the water-mineral interface. Fig. 8.1 gives the data on the dissolution rates of various carbonate minerals in aqueous solutions obtained in careful studies by Chou and Wollast (1989). 

The dissolution rate for calcite and aragonite have been described in terms of the following rate law (Plummer et al., 1978 Busenberg and Plummer, 1986 Chou and Wollast, 1989.  

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