Calcite aqueous solutions

We have studied the dissolution kinetics of calcite in stirred CO2 water systems at CO2 partial pressures between 0.0003 and 0.97 atm and between 5° and 60°C, using pH-stat and free drift methods (J ) Our results suggest a mechanistic model for reactions at the calcite-aqueous solution interface that has broad implications to the controls on calcite dissolution and precipitation under diverse chemical and hydrodynamic conditions. 

If calcite is in equilibrium with the aqueous solution, the relationship between pH and the concentration of Ca " " can be obtained from reaction (2-10). 

Fig. 2.14. The variation of concentration of with concentration of CP in aqueous solution in equilibrium with a given mineral assemblage at 250°C. I Equilibrium curve based on albite-sericite-Na-montmorillonite-quartz-aqueous solution equilibrium and Na-K-Ca relationship obtained by Fournier and Truesdell (1973). 2 Equilibrium curve based on albite-K-feldspar-aqueous solution equilibrium and Na-K-Ca relationship obtained by Fournier and Truesdell (1973). 3 Wairakite-albite-sericite-K-feldspar-quartz. 4 Calcite-albite-sericite-K-feldspar-quartz (/jjhjCO, = 10 ). 5 Calcite-albite-sericite-Na-montmorillonite-quartz (mH2C03 = 10 ). 6 Ca-montmorillonite-albite-sericite-Na-montmorillonite-quartz. 7 Calcite-albite-sericite-K-feld-spar-quartz (mnjCOj = 10 ). 8 Calcite-albite-sericite-Na-montmorillonite-quartz (mHjCOj = 10 ). 9 Ca-montmorillonite-albite-sericite-K-feldspar-quartz. 10 Anhydrite = 10 ). (Shikazono, 1976)...

Glasner A., Weiss D. The crystallization of calcite from aqueous solutions and the role of zinc and magnesium ions. I. Precipitation of calcite in the presence of Zn2+ ions. J Inorg Nucl Chem 1980 42 655-663. 

Kaushansky P.,Yariv S. The interactions between calcite particles and aqueous solutions of magnesium, barium or zinc chloride, Appl Goechem 1986 1 607-618. 

Usually, however, the distribution coefficients determined experimentally are not equal to the ratios of the solubility product because the ratio of the activity coefficients of the constituents in the solid phase cannot be assumed to be equal to 1. Actually observed D values show that activity coefficients in the solid phase may differ markedly from 1. Let us consider, for example, the coprecipitation of MnC03 in calcite. Assuming that the ratio of the activity coefficients in the aqueous solution is close to unity, the equilibrium distribution may be formulated as (cf. Eq. A.6.11)... 

It is doubtful that formation and dissolution of any mineral in low temperature aqueous solutions has been more fully investigated than the magnesian caicite. This mineral is a preponderant carbonate phase, mostly of biogenic origin, in seawater. Fig. 8.8 gives some data on the solubilities of Mg-calcites as a function of MgC03 content. 

Bischoff, W. D., F. T. Mackenzie, and F. C. Bishop (1987), "Stabilities of Synthetic Magnesian Calcites in Aqueous Solution Comparison with Biogenic Materials," Geochim. Cosmochim. Acfa51, 1413-1423. 

In precipitation studies (4 7, 4 ) it has been shown that, below a certain Mg/Ca concentration ratio in the aqueous solution, the rate of nucleation of calcite was faster than that of aragonite. Above that Mg/Ca ratio the order was reversed. This was explained by the effect of Mg2+ ions on the interfacial tension between the solution and precipitate, which apparently is larger for calcite than for aragonite (49). At still higher Mg/Ca ratios dolomite can be formed (50). Such low temperature precipitates of dolomite contain ordering defects. The number of defects increases when precipitation proceeds in a shorter time interval or at lower temperatures C51 ). 

According to Equation 48 calcite should precipitate from waters having a Mg/Ca ratio below a certain value, while dolomite should precipitate from waters having a Mg/Ca ratio above that critical value. This rule is obeyed under conditions of precipitation from very slightly supersaturated aqueous solutions like those occurring in certain areas of the ocean. Ocean water is close to equilibrium with both calcite and dolomite (53). 

Experimental and empirical studies have demonstrated that certain doubly charged cations of ionic radius less than calcium (e.g., Mn2 +, Zn2+, Fe2+, Cd2 +, and Co 2+) can be extensively incorporated into calcite precipitated from aqueous solution at... 

The improvement of enzyme like MIP is currently another area of intense research. Beside the use of the MIP themselves as catalysts, they may also be applied as enhancer of product yield in bio-transformation processes. In an exemplary condensation of Z-L-aspartic acid with L-phenylalanine methyl ester to Z-aspartame, the enzyme thermolysin was used as catalyst. In order to shift the equilibrium towards product formation, a product imprinted MIP was added. By adsorbing specifically the freshly generated product from the reaction mixture, the MIP helped to increase product formation by 40% [130]. MIP can also be used to support a physical process. Copolymers of 6-methacrylamidohexanoic acid and DVB generated in the presence of calcite were investigated with respect to promotion of the nucleation of calcite. Figure 19 (left) shows the polymer surface with imprints from the calcite crystals. When employing these polymers in an aqueous solution of Ca2+ and CO2 the enhanced formation of rhombohedral calcite crystals was observed see Fig. 19 (right) [131]. 

Figure 17. Plot of Li isotopic composition vs. temperature of growth for synthetic calcite crystallized from a solution containing Li from L-SVEC (Marriott et al. 2004). The results are most consistent with temperature not being a significant control on mass fractionation of Li during crystallization from aqueous solution, thus essentially eliminating Li isotopes as a paleotemperature proxy in marine carbonates.

Figure 8.18 Relationship between pH of aqueous solution and fco2 of gaseous phase in equilibrium with water and calcite. Equilibrium pH is in correspondance with zero on ordinate axis.

The third and the most common type is complex phase transformations, including the following (i) some components in a phase combine to form a new phase (e.g., H2O exsolution from a magma to drive a volcanic eruption the precipitation of calcite from an aqueous solution, Ca + + COf calcite the condensation of corundum from solar nebular gas and the crystallization of olivine from a basaltic magma), (ii) one phase decomposes into several phases (e.g., spinodal decomposition, or albite jadeite + quartz), (iii) several phases combine into one phase (e.g., melting at the eutectic point, or jadeite +... 

In the context of transition-state theory, for ions and molecules in the liquid to attach to the crystal, they must first become an activated complex. The same is true for detachment (Figure 4-5). For clarity of discussion, use calcite (Cc), assumed to be pure CaCOs, growth from an aqueous solution as an example. The reaction is... 

It is observed that in the case of simultaneous saturation of two or more phases, the phase that forms first is often the least stable, or the most disordered, especially at room temperatures. For example, in aqueous solutions, opal (disordered) often forms but the more stable quartz rarely forms. Over a very long time, opal may "mature" to become quartz. The same is true for the formation of calcite (as compared dolomite), and analbite (as compared to albite). From the vapor phase, phosphorous vapor condenses first to yellow phosphorus (high entropy), instead of the more stable red phosphorous (low entropy)... 

A number of adsorption studies have been conducted for the adsorption of transition and heavy metals on carbonates. Mn2+ adsorption has been the most intensely studied mechanism. This is possibly because of its importance to carbonate mineral behavior in marine sediments (see Chapter 4). Studies of Mn2+ interaction with the surface of calcite in dilute aqueous solutions indicated that Mn2+ uptake is approximately balanced by Ca2+ release (McBride, 1979). It was suggested that the calcium release may not be the result of direct displacement, but rather results from the following reactions ... 

Carbonate minerals are among the most chemically reactive common minerals under Earth surface conditions. Many important features of carbonate mineral behavior in sediments and during diagenesis are a result of their unique kinetics of dissolution and precipitation. Although the reaction kinetics of several carbonate minerals have been investigated, the vast majority of studies have focused on calcite and aragonite. Before examining data and models for calcium carbonate dissolution and precipitation reactions in aqueous solutions, a brief summary of the major concepts involved will be presented. Here we will not deal with the details of proposed reaction mechanisms and the associated complex rate equations. These have been examined in extensive review articles (e.g., Plummer et al., 1979 Morse, 1983) and where appropriate will be developed in later chapters. 

Recently, Chou et al. (1989) studied the dissolution kinetics of various carbonate minerals in aqueous solution. Figure 2.10 illustrates the experimental results for aragonite, calcite, witherite, dolomite, and magnesite. These data can be fit by rate equations, an example of which is shown in equation 2.28 for calcite. 

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.

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