Carbonate minerals dissolution rates

Walter L.M. and Burton E.A. (1986) The effect of orthophosphate on carbonate mineral dissolution rates in seawater. Chem. Geol. 56, 313-323. 

Small differences in mineral/water ratios in the reservoir can result in large differences in carbonate mineral dissolution rates. 

The rate at which metastable phases dissolve or are replaced is an important problem in carbonate diagenesis. Carbonate mineral assemblages persist metastably in environments where they should have altered to stable assemblages. The question is "what are the time scales of these alterations" They are certainly variable ranging from a few thousand to a few hundreds of millions of years. Even calcites in very old limestones show chemical and structural heterogeneities, indicating that the stabilization of these phases is not complete. Unfortunately, it is difficult, but not impossible, to apply directly the lessons learned about carbonate mineral dissolution and precipitation in the laboratory to natural environments. 

The protocol involving NaOAc-HOAc at pH 5 was first proposed and used by Jackson (1958) to remove carbonates from calcareous soils to analyze soil cation exchange characteristics (Grossman and Millet, 1961). Other researchers used HOAc for the extraction of metals from sediments and soils (Nissenbaum, 1972 Mclaren and Crawford, 1973). Tessier et al. (1979) first used the NaOAc-HOAc solution at pH 5 to dissolve the carbonate fraction from sediments. Since then, the NaOAc-HOAc buffer has been widely used as a specific extractant for the carbonate phase in various media (Tessier et al., 1979 Hickey and Kittrick, 1984 Rapin et al., 1986 Mahan et al., 1987 Han et al., 1992 Clevenger, 1990 Banin et al., 1990). Despite its widespread use, this step is not free from difficulties, and further optimization is required in its application. Questions arise with regard to this step in the elemental extraction from noncalcareous soils, the dissolution capacity and dissolution rates imposed by the buffer at various pHs, and the possibility that different carbonate minerals may require different extraction protocols (Grossman and Millet, 1961 Tessier et al., 1979). 

ABSTRACT Atmospheric carbon dioxide is trapped within magnesium carbonate minerals during mining and processing of ultramafic-hosted ore. The extent of mineral-fluid reaction is consistent with laboratory experiments on the rates of mineral dissolution. Incorporation of new serpentine dissolution kinetic rate laws into geochemical models for carbon storage in ultramafic-hosted aquifers may therefore improve predictions of the rates of carbon mineralization in the subsurface. 

But within the pH range of natural waters, the dissolution (and precipitation) of carbonate minerals is surface controlled i.e., the rate of dissolution is rate determined by a chemical reaction at the water-mineral interface. Fig. 8.1 gives the data on the dissolution rates of various carbonate minerals in aqueous solutions obtained in careful studies by Chou and Wollast (1989). 

Rate of Dissolution of Carbonate Mineral Matrix in Oil Shale by Dilute Acids... 

Y-intercept and the slope, respectively, and are listed in Table I. The results of the dissolution rate determination method (method 2) are presented in Figure 3. As can be seen, the maximum removable mineral (P0) by dilute acid is independent of the size of the shale particles. However, the carbonate fraction in the shale mineral matrix is very close to this figure. This could mean that the accessibility of the leaching agent to the leachable materials in shale is complete in the size ranges studied in this experiment—but at different rates. This could also indicate that the carbonate deposit sites are not isolated but can, perhaps, be thought of as interconnected by minerals built of the dilute acid-resistant minerals. 

Ks do not match with each other. This is partly the result of the effects of the specific surface which was different in the two methods. However, the mechanisms of the dissolution kinetics seem to be identical. The reaction rate of the acid with carbonate mineral would be controlled by diffusion of the reactant into and the products out of the pores. Therefore, the availability of only the contact surface is not adequate. The type of surface in terms of relevant diffusion model and the closest theory to that model, such as film, penetration, or any other, should also be specified. 

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. 

A number of factors have been investigated that influence calcite dissolution in relatively simple systems. These include the important variables of temperature, pH, and the partial pressure of CO2. The equations describing the net dissolution rate of carbonate minerals in simple experimental solutions differ somewhat, depending on experimental conditions and the interpretations placed on the experimental results by various investigators (cf., for example, Plummer et al., 1978 Chou et al., 1989). 

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. 

The influence of heavy metals on calcium carbonate reaction rates has not been extensively studied. Experiments have shown that many metals exhibit inhibitory effects on calcite dissolution. Ions tested by Terjesen et al. (1961), in decreasing order of effectiveness, were Pb2+, La3+, Y3+, Sc3+, Cd2+, Cu2+, Au3+, Zn2+, Ge4+, and Mn2+, and those found to be about equal were Ni2+, Ba2+, Mg2+, and Co2+. The general trend follows the solubility of the metal carbonate minerals, with the exception of Zn2+ and the "about equal" group whose solubilities are all greater than calcite. 

A basic concept is that a given carbonate mineral will not dissolve in a solution that is supersaturated with respect to that mineral or precipitate from a solution undersaturated with respect to that mineral. If a solution is undersaturated with respect to all carbonate minerals, they may all dissolve with their relative dissolution rates determined by grain size, microstructure, and solution composition, among other factors. The idea that under universally undersaturated conditions mineral solubility may not simply control dissolution rates, even for grains of the same size, was confirmed by Walter and Morse (1985). They observed that relative dissolution rates in seawater could not be normalized directly to total surface areas, but rather depended strongly on microarchitecture (Figure 7.6). 

The numerator of the right side is the product of measured total concentrations of calcium and carbonate in the water—the ion concentration product (ICP). If n = 1 then the system is in equilibrium and should be stable. If O > 1, the waters are supersaturated, and the laws of thermodynamics would predict that the mineral should precipitate removing ions from solution until n returned to one. If O < 1, the waters are undersaturated and the solid CaCOa should dissolve until the solution concentrations increase to the point where 0=1. In practice it has been observed that CaCOa precipitation from supersaturated waters is rare probably because of the presence of the high concentrations of magnesium in seawater blocks nucleation sites on the surface of the mineral (e.g., Morse and Arvidson, 2002). Supersaturated conditions thus tend to persist. Dissolution of CaCOa, however, does occur when O < 1 and the rate is readily measurable in laboratory experiments and inferred from pore-water studies of marine sediments. Since calcium concentrations are nearly conservative in the ocean, varying by only a few percent, it is the apparent solubility product, and the carbonate ion concentration that largely determine the saturation state of the carbonate minerals. 

The dissolution rates of the minerals of calcium carbonate have been shown in laboratory experiments to follow the rate law ... 

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