Seawater magnesian calcite precipitation

Table 3.1. Laboratory precipitation of magnesian calcite from seawater (Berner, 1978).

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. 

Mucci A. (1986) Growth kinetics and composition of magnesian calcite overgrowths precipitated from seawater Quantitative influence of orthophosphate ions. Geochim. Cosmochim. Acta 50, 2255-2265. 

Mucci A., Morse J.W. and Kaminsky M.S. (1985) Auger spectroscopy analysis of magnesian calcite overgrowths precipitated from seawater and solutions of similar composition. Amer. J. Sci. 285, 289-305. 

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 stoichiometric solubility constants for calcite and aragonite (see Section 7.03.3). The most difficult problem is that, although the solubility of pure calcite is sought in experiments with seawater solutions, extensive magnesium co-precipitation can produce magnesian calcites. The solubility of these magnesian calcites differs from that of pure calcite. Thus, it is not possible to measure the solubility of pure calcite directly in seawater. 

MUCCI, A. 1987. Influence of temperature on the composition of magnesian calcite overgrowths precipitated from seawater. Geochimica et Cosmochimica Acta, 51, 1977-1984. 

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