Dissolution calcite

Dissolution of a mineral occurs when the crystal lattice breaks down and it separates into its component ions in water. Minerals most affected are salts, sulfates, and carbonates. For example, calcite dissolution is described by... 

Berner, R.A. Morse, J.M. 1974. Dissolution kinetics of calcium carbonate in seawater. IV. Theory of calcite dissolution, American Journal of Science, 274,108-134. 

Sjoberg, E.L. Rickard, D.T. 1984. Calcite dissolution kinetics surface speciation and the origin of the variable pH dependence, Chemical Geology, 42, 119-136. 

Carbon dioxide has a dominant effect on the dissolution of carbonate minerals, such as calcite and dolomite (Table 2.1). If a carbonate mineral dissolves in water that is equilibrated with a constant source of CO, then the concentration of dissolved carbonic acid remains constant and high. However, when calcite dissolution is accompanied by consumption of carbonic acid and a continuous source of CO is not maintained, the reaction proceeds further to achieve equilibrium. 

Figure 8.33 Reaction mechanism contribution to the total rate of calcite dissolution reaction as a function of pH and Pco2 25 °C. From L. N. Plummer, T. M. L. Wigley, and D. L. Pankhurst (1978), American Journal of Science, 278, 179-216. Reprinted with permission of American Journal of Science.

Sjoberg E. L. (1976). A fundamental equation for calcite dissolution kinetics. Geochim. Cosmochim. Acta, 40 441-447. 

Before proceeding, a few general comments will be helpful. In most cases at least one of the four carbonate system analytical quantities (total alkalinity, total CO2, carbon dioxide partial pressure, and pH) will be known. The equilibrium equations relating these quantities, along with those for water and calcite, will be used frequently in calculations. Also, in cases where calcite dissolution and... 

A number of factors have been investigated that influence calcite dissolution in relatively simple systems. These include the important variables of temperature, pH, and the partial pressure of CO2. The equations describing the net dissolution rate of carbonate minerals in simple experimental solutions differ somewhat, depending on experimental conditions and the interpretations placed on the experimental results by various investigators (cf., for example, Plummer et al., 1978 Chou et al., 1989). 

Mg2+ influences calcite dissolution rates the same way, but not to the same extent as Ca2+. The inhibition effects of Mg2+ can be described in terms of a Langmuir adsorption isotherm. Sjoberg (1978) found he could model results for the combined influences of Ca2+ and Mg2+ in terms of site competition consistent with ion exchange equilibrium. The inhibition effects of Mg2+ in calcite powder runs increase with increasing Mg2+ concentration and as equilibrium is approached. 

Phosphate has been found to be an extremely strong inhibitor of carbonate reaction kinetics, even at micromolar concentrations. This constituent has been of considerable interest in seawater because of its variability in concentration. It has been observed that phosphate changes the critical undersaturation necessary for the onset of rapid calcite dissolution (e.g., Berner and Morse, 1974), and alters the empirical reaction order by approximately a factor of 6 in going from 0 to 10 mM orthophosphate solutions. Less influence was found on the log of the rate constant. Walter and Burton (1986) observed a smaller influence of phosphate on calcite... 

Studies of phosphate inhibition of calcite dissolution both in the presence and absence of Ca2+ and Mg2+ have shown that the inhibiting effect of phosphate increases with increasing Ca2+ concentration. Results in solutions with Mg2+ were less reproducible, indicating the same trend, but not as strong an inhibition relation (Sjoberg, 1978), and a decrease in apparent equilibrium with increasing phosphate,... 

The influence of heavy metals on calcium carbonate reaction rates has not been extensively studied. Experiments have shown that many metals exhibit inhibitory effects on calcite dissolution. Ions tested by Terjesen et al. (1961), in decreasing order of effectiveness, were Pb2+, La3+, Y3+, Sc3+, Cd2+, Cu2+, Au3+, Zn2+, Ge4+, and Mn2+, and those found to be about equal were Ni2+, Ba2+, Mg2+, and Co2+. The general trend follows the solubility of the metal carbonate minerals, with the exception of Zn2+ and the "about equal" group whose solubilities are all greater than calcite. 

In addition to calcite, aragonite and occasionally magnesian calcites may also reach the ocean floor. They may play an important role even in sediments where they do not accumulate, by contributing carbonate ion to pore waters because of their dissolution, thus causing less calcite dissolution to occur than would be the case in their absence. This process is enhanced by the benthic process of bioturbation, whereby organisms cause a physical mixing of sediments down to... 

Baumann J., Buhmann D., Dreybrodt W. and Schultz H.D. (1985) Calcite dissolution kinetics in porous media. Chem. Geolog. 53, 219-228. 

Buhmann D. and Dreybrodt W. (1985a) The kinetics of calcite dissolution and precipitation in geologically relevant situations of Karst areas. 1. Open System. Chem. Geol. 48, 189-211. 

Plummer L.N., Parkhurst D.L. and Wigley T.M.L. (1979) Critical review of the kinetics of calcite dissolution and precipitation. In Chemical Modelling--Speciation, Sorption, Solubility and Kinetics in Aqueous Systems (ed. E. Jenne), pp. 537-573. American Chemical Society, Washington, D.C. 

Reddy M.M., Plummer L.N. and Busenberg E. (1981) Crystal growth of calcite from calcium bicarbonate solutions at constant Pc02 and 25°C A test of the calcite dissolution model. Geochim. Cosmochim. Acta 45,1281-1291. 

Sjoberg E.L. (1978) Kinetics and mechanism of calcite dissolution in aqueous solutions at low temperatures. Acta Univ. Stockholmiensis, Stockholm Contr. Geology 332, 1-92. 

Sjoberg, E. L., "Kinetics and Mechanism of Calcite Dissolution in Aqueous Solutions at Low Temperatures," Stockholm Contributions in Geology, 32(1), 1981. 

Boudreau, B.P., and Canfield, D.E. (1993) A comparison of closed- and open-system models for porewater pH and calcite dissolution. Geochim. Cosmochim. Acta 57, 317-334. 

FIG. 3.3. Influence of pH on the rates of calcite dissolution and precipitation (left, adapted from Chou et al.7 with the permission of Elsevier Science Publishers), and an initial-rate plot for calcite precipitation indicating the applicability of Eq. 3.9 (right, data from W. P. Inskeep and P. R. Bloom,8 adapted with permission. Copyright by Pergamon Press Ltd., Headington Hill Hall, Oxford 0X3 OBW, England.)... 

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