Diagenesis carbonate dissolution

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. 

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. 

Walter L.M. (1986) Relative efficiency of carbonate dissolution and precipitation during diagenesis A progress report on the role of solution chemistry. J. Sediment. Petrol. 56, 1-11. 

Carbonate reservoirs are usually affeoted to varying degree by diagenesis. However the process of dissolution and replacement is not limited to carbonates. Feldspar for instance is another family of minerals prone to early alterations. 

In carbonate diagenesis V we deal usually with a combination of low supersaturation and absence of mechanical agitation. Homogeneous nucleation will certainly not occur. The important factors to be investigated are heterogeneous nucleation and rates of growth and dissolution of crystals. 

One of the most important aspects of carbonate diagenesis is the net movement of carbonate minerals. This mass transfer can be accomplished on a small scale by diffusive transport or on a large scale by the flow of subsurface waters. It is the basic process by which secondary porosity is created and cementation occurs. In most cases, it involves carbonate mineral dissolution at one site and precipitation at another. While this can be simply accomplished where mineralogic transformations from a metastable phase to a more stable phase are involved, more complex mechanisms may be required within mineralogically homogeneous carbonate bodies. 

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