Magnesian calcites equilibrium

Thus, larger solid/water ratios such as are encountered in pore waters of sediments lead to smaller MgC(>3 contents in the equilibrium magnesian calcites although in either case the magnesium content of the solid increases. Wollast and Reinhard-Derie presented data to support the theory from the standpoint of dissolution and some of our results for the precipitation case... 

Mucci and Morse (1983) found no statistically significant dependence of the partition coefficient for Mg in calcite precipitated from seawater on reaction rate over a range of seawater supersaturations from about one half ( 2 =3) to close to three times ( 2=17) that of typical surface seawater. However, their experiments had durations of from only a few hours to days. The compositions of magnesian calcites grown very near equilibrium ( 2=1.2) over periods of several months were determined by Mucci et al. (1985). They found excellent agreement with the results of Mucci and Morse (1983) even though different experimental techniques were used. The rate independence of the partition coefficient of Mg in calcite, therefore, has been found to be independent of reaction rate over about 12 orders of magnitude in seawater. 

Figure 3.8. Schematic relations for equilibrium between magnesian calcite of fixed composition or a magnesian calcite solid solution series and aqueous solution. SS is a saturation curve and VV is a solubility curve. Tie lines are hypothetical. See text for discussion.

Berner R.A. (1978) Equilibrium kinetics, and the precipitation of magnesian calcite from seawater. Amer. J. Sci. 278, 1435-1477. 

Garrels R.M. and Wollast R. (1978) Discussion of Equilibrium criteria for two-component solids reacting with fixed composition in an aqueous phase-example The magnesian calcites. Amer J. Sci. 278, 1469-1474. 

Figure 1 The equilibrium lAP for magnesian calcites of biogenic and synthetic origin versus magnesium concentration (after Morse and Mackenzie, 1990).

Consider first a formal (equilibrium) approach to the solubility of magnesian calcite and compare its solubility with that of CaC03 (calcite or aragonite)... 

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