Magnesian calcites solubility

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

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

In most experiments to calculate solubilities the magnesian calcites have been treated as solids of fixed compositions of one component, whereas they are actually a series of at least two-component compounds forming a partial solid solution series. 

Plummer, L. N., and F. T. Mackenzie (1974), "Predicting Mineral Solubility from Rate Data Application to the Dissolution of Magnesian Calcites", Amer. J. Sd. 274, 61 -83. 

The solid carbonate can also present several potential difficulties in solubility studies. These can be broken down into two major areas heterogeneity in composition, and excess free energy associated with lattice strain or defects and surface free energy. The problem of solid heterogeneity presents itself in most sedimentary carbonates, and is especially important in biogenic carbonates such as magnesian calcites. The problem of lattice strain and high defect density is most... 

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. 

It will be shown later that biogenic magnesian calcites are structurally disordered and chemically heterogeneous aragonite also shows some evidence of structural disorder. Both these factors play a role in determining the "solubilities" of these phases and in their reactivity in natural systems. 

Natural carbonate minerals do not form from pure solutions where the only components are water, calcium, and the carbonic acid system species. Because of the general phenomenon known as coprecipitation, at least trace amounts of all components present in the solution from which a carbonate mineral forms can be incorporated into the solid. Natural carbonates contain such coprecipitates in concentrations ranging from trace (e.g., heavy metals), to minor (e.g., Sr), to major (e.g., Mg). When the concentration of the coprecipitate reaches major (>1%) concentrations, it can significantly alter the chemical properties of the carbonate mineral, such as its solubility. The most important example of this mineral property in marine sediments is the magnesian calcites, which commonly contain in excess of 12 mole % Mg. The fact that natural carbonate minerals contain coprecipitates whose concentrations reflect the composition of the solution and conditions, such as temperature, under which their formation took place, means that there is potentially a large amount of information which can be obtained from the study of carbonate mineral composition. This type of information allied with stable isotope ratio data, which are influenced by many of the same environmental factors, has become a major area of study in carbonate geochemistry. 

Figure 3.7. Solubilities of the magnesian calcites as a function of MgCC>3 content, expressed in terms of -log IAPMg.caicjte. The solid curve represents the general trend of dissolution experiment results.

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.

Now consider the dissolution of mineral Cao.85 Mgo.15 CO3 in pure water with one atmosphere CO2. If this mineral acted like a solid solution phase, it would dissolve congruently along reaction path AA until it reached at A saturation with a calcium-rich magnesian calcite less soluble than itself. We will see later in this... 

Solubilities of annealed skeletal carbonates were estimated by Land (1967) by allowing the system to reach a steady state pH and by determining dissolved Ca2+ and Mg2+ concentrations. Dissolution of the magnesian calcites in Land s experiments appeared to be congruent in the sense that the steady state solution had a molar Ca2+/Mg2+ ratio similar to that of the solid. 

Bertram (1989) showed, in dissolution studies of magnesian calcites, that their solubilities decrease with increasing temperature. This trend and its relationship to solubility vs. temperature trends for calcite and aragonite are discussed in Chapter 7. 

Precipitation experiments Mucci and Morse (1984) reported solubility data for several magnesian calcites produced from precipitation experiments (see Figures 3.7 and 3.14). In their experiments overgrowths of magnesian calcite were precipitated on calcite seeds in a pH-stat which maintained constant solution composition. The overgrowth compositions were determined by atomic absorption spectrophotometry and X-ray diffraction. The Mg contents of the overgrowths varied only with the Mg Ca ratio in solution, and the overgrowths were shown to be... 

The above discussion generates an obvious question What are the "correct" values for the relative solubilities of the magnesian calcites Bischoff et al. (1987) offer an explanation of the three curves in Figure 3.7 that has bearing on natural processes affecting calcites discussed later in this book. They suggest that each of the curves is applicable to the reactivity of magnesian calcites, but under different theoretical and environmental constraints ... 

The possible presence of water in the structure of the magnesian calcite solid solution must affect its free energy of formation and thus its solubility. Inspection... 

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