Aragonite dissolution kinetics

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. 

Morse J.W., deKanel J. and Harris J. (1979) Dissolution kinetics of calcium carbonate in seawater. VII The dissolution kinetics of synthetic aragonite and pteropod tests. Amer. J. Sci. 279, 482-502. 

Calcium carbonate is accumulating in deep ocean sediments, in which the overlying water is undersaturated with respect to both aragonite and calcite, and sediment marker levels closely correspond to unique saturation states. This indicates that dissolution kinetics play an important role in determining the relation between seawater chemistry and calcium carbonate accumulation in deep ocean basins. It is, therefore, necessary to have knowledge of the dissolution kinetics of calcium carbonate in seawater if the accumulation of calcium carbonate is to be understood. 

Morse, J.W., de Kanel, J., and Harris, K. The dissolution kinetics of calcium carbonate in seawater VII. The dissolution kinetics of synthetic aragonite and pteropods, Amer. Jour. Sci. (in press). 

Dissolution kinetics of single carbonates such as calcite, aragonite, and magnesite exhibit simple dependence with respect to a limited number of reactants, specifically, H +, H2C03, and H20. It is thus easy to identify the elementary steps leading to the formation of the surface activated complex and the nature of the products of the detachment process following the decomposition of the activated complex. 

Dissolution kinetics of a simple component close to saturation and the mechanism of the backward precipitation reaction are still subject to controversy. Minerals such as calcite and aragonite are known to reach rapidly a dissolution equilibrium when placed in closed aqueous systems. According to simple and classical thermodynamical concepts, this requires that each forward... 

Busenberg, E. Plummer, L.N. 1986. A comparative study of the dissolution and crystal growth kinetics of calcite and aragonite. U S. Geological Survey Bulletin, 1578, 139-168. 

Busenberg, E., and L. N. Plummer (1986b), "A Comparative Study of the Dissolution and Crystal Growth Kinetics of Calcite and Aragonite", in F. A. Mumpton, Ed., Studies in Diagenesis, U.S. Geol. Surv. Bull. 1578, pp. 139-168. 

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. 

Plummer. L.N. and Wigley, T.N.L. Critical review of the kinetics of calcite dissolution and precipitation, in Jenne, E.A., ed., "Chemical Modeling. Speciation, Sorption, Solubility and Kinetics in Aqueous Systems," Amer. Chem. Soc. Symp. Series, Washington, D.C. (this volume), de Kanel, J. and Morse, J.W. The chemistry of orthophosphate uptake from seawater onto calcite and aragonite, Geochim. Cosmochim. Acta 1335-1340 (1978). 

Dissolution or precipitation reactions are generally slower than reactions among dissolved species, but it is quite difficult to generalize about rates of precipitation and dissolution. There is a lack of data concerning many geo-chemically important solid-solution reactions kinetic factors will be discussed later (Chapter 13). Frequently, the solid phase formed incipiently is metastable with respect to a thermodynamically stable solid phase. Examples are provided by the occurrence under certain conditions of aragonite instead of stable calcite or by the quartz oversaturation of most natural waters. This oversaturation occurs because the rate of attainment of equilibrium between silicic acid and quartz is extremely slow. 

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