Calcite solubility

Walter, L. M., and J. W. Morse (1984a), "Magnesian Calcite Solubilities A Reevaluation", Geochim. Cosmochim. Acta 48, 1059-1069. 

Abyssal clays are found in greater abundance on the western side of the Atlantic Ocean than on the eastern side. This is due to bottom topography that restricts the flow of North Atlantic Deep Water and Antarctic Bottom Water to the western side of the basin. The lower temperature of the western waters causes the CCD to be somewhat shaUower than on the east side of the basin as calcite solubility increases with decreasing... 

Heterogeneous equilibria and calcite solubility. If river water in Box 8-2 is saturated with calcite (CaC03), fCa2+] is governed by the following equilibria ... 

A classic example of metastability is surface-seawater supersaturation with respect to calcite and other carbonate minerals (Morse and Mackenzie 1990 Millero and Sohl 1992). The degree of calcite supersaturation in surface seawater varies from 2.8- to 6.5-fold between 0 and 25 °C (Morse and Mackenzie 1990). In Fig. 3.18, experimental calcite solubility (metastable state) is approaching model calcite solubility (stable state) at subzero temperatures. In Table 5.1, the difference in seawater pH, assuring saturation or allowing supersaturation with respect to calcite, is 0.38 units. Moreover, in running these calculations, it was necessary to remove magnesite and dolomite from the minerals database (Table 3.1) because the latter minerals are more stable than calcite in seawater. But calcite is clearly the form that precipitates... 

He S, Morse JW (1993) The carbonic acid system and calcite solubility in aqueous Na-K-Ca-Mg-Cl-S04 solutions from 0 to 90°C. Geochim Cosmochim Acta 57 3533-3555... 

Figure 2.6. Change in calcite solubility with particle size, assuming cubic shape and a = 85 ergs cm-2.

During the past decade there have been four major studies of calcite solubility in dilute solutions, three of which also included aragonite (Jacobsen and Langmuir, 1974 Berner, 1976 Plummer and Busenberg, 1982 Sass et al., 1983). These studies were aimed at more accurate and precise determinations of the thermodynamic solubility product for these minerals. Earlier work on this topic was well summarized by Jacobsen and Langmuir (1974). 

In the previous chapter, the fact that stoichiometric and apparent constants have been widely used in seawater systems was discussed. Berner (1976) reviewed the problems of measuring calcite solubility in seawater, and it is these problems, in part, that have led to the use of apparent constants for calcite and aragonite. The most difficult problem is that while the solubility of pure calcite is sought in experimental seawater solutions, extensive magnesium coprecipitation can occur producing a magnesian calcite. The magnesian calcite should have a solubility different from that of pure calcite. Thus, it is not possible to measure pure calcite solubility directly in seawater. 

Let us examine this problem using more recently published constants. The first question is what value should be used for the aragonite to calcite solubility ratio The value obtained from the study of Plummer and Busenberg (1982) is 1.38, whereas Sass et al. (1983) found a ratio of 1.51. A value of 1.45 0.07 is consistent with most measurements and their uncertainty. 

There is one rather nasty twist in the ion pair evaluation where a negative value for free carbonate ion concentration results, because the initial value which must be used for the carbonate ion activity coefficient is much larger than it is under the new conditions. This demands some maneuvers with both the calcite solubility constant and the calcium carbonate ion pair association constant. These will not be gone into here. 

Bertram M. (1989) Temperature effects on magnesian calcite solubility and reactivity Application to natural systems. M.S. thesis, Univ. Hawaii. 

Nagy K.L. and Morse J.W. (1990) Temperature dependence of calcite solubility of NaCl solutions from 25° to 89°C at 1 atm. Geochim. Cosmochim. Acta (in press). 

Different factors have an influence on the calcite solubility. First of all, like for gypsum (chapter 4.1.1.4) the formation of the CaC03° complex is endothermic (AH(CaC03°) = +3.5), while the mineral dissolution is exothermic (AH(CaC03(s)) = -2.3). Thus, the maximum solubility occurs at some medium temperature, at which the formation of the CaC03° complex is already increased and the decreasing solubility of CaC03(s) does not predominate yet. 

Furthermore, calcite solubility does not only depend on temperature but also on P(C02). 

That means, the higher the P(C02), the more CaC03 can be dissolved. Yet, the solubility of C02 as gas in water depends on the temperature The higher the temperature the lower the gas solubility. Consequently, initially, the calcite solubility increases with temperature due to the endothermic reaction of CaC03° complexation, but with increased temperature the exothermic reaction of CaC03(s) dissolution and the significantly reduced C02 dissolution decrease the total calcite solubility. 

Fig. 50 Calcite solubility and incongruent solution calcite-dolomite (calcite -precipitation and dolomite dissolution)...

Comparison of the calcite solubility in an open and a closed system... 

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