Carbonate minerals equilibrium relations

Figure 6.1 shows equilibrium relations between stable (A) and metastable (B) carbonate minerals. The boundaries between stability fields are most easily obtained by consideration of the mass action equations representing mineral compatibilities in terms of the variables shown in Figure 6.1. For example, consider the phase boundaries in Figure 6.1 A the reactions and the equilibrium constants for these boundaries in terms of the ratio of activity of Ca2+ to activity of Mg2+ and PCO2 are 3s follows ...

Because of the great depth of the ocean, the most important physical property determining the solubility of carbonate minerals in the sea is pressure. The pressure dependence of the equilibrium constants is related to the difference in volume, AV, occupied by the ions of Ca and in... 

It is useful to construct a graph relating carbonate mineral solubilities to CO2 pressure. This can be done for calcite starting with equilibrium constant expression (6.2) above. If done rigorously, the derivation accounts for the effects of ion activity coefficients and the presence of CaHCOI and CaCOf ion pairs and of CaOH. Considering all complexation, the exact charge-balance equation for a pure water in which calcite is dissolving is... 

Injection of HCl from the hydrothermal weathering into marine water as well as dissolution of atmospheric SO2 steadily transformed the ocean s pH value from alkaline to acid. About 1 billion years ago the primitive soda Ocean was transformed into present salt sea. On other side, the changes in pH and possibly Eh have significantly altered the marine environment. This related mainly to carbonate system, which switched from sodium- to calcium- and magnesium-dominated mineral equilibrium about 1 billion years ago (Degens, 1989). 

The relation between zinc content in carbonate and many physicochemical variables such as /02, temperature, pH, and so on was derived on the basis of the equilibrium between coexisting carbonate and sphalerite (Figs. 1.95 and 1.96). It is predicted theoretically that zinc content increa.ses with increasing /oj in the oxidized sulfur species and oxidized carbon species region (Fig. 1.95). This estimate is consistent with mineral assemblage containing the siderite. 

There are three important minerals used by organisms to form hard tissues such as bones and shells. The most widespread of these is calcium carbonate, an important structural component in animals ranging from Protozoa to Mollusca and Echinoder-mata. It is also a minor component of vertebrate bones. Its widespread use is probably related to the generally uniform distribution of dissolved calcium bicarbonate. Animals employing calcium carbonate are most abundant in fresh waters containing large amounts of calcium and magnesium ("hard water") and in warm, shallow seas where the partial pressure of carbon dioxide is low (e.g., the formation of coral reefs by coelenterates). The successful precipitation of calcium carbonate depends upon the equilibrium ... 

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