Dolomite, dissolution

Fig. 50 Calcite solubility and incongruent solution calcite-dolomite (calcite -precipitation and dolomite dissolution)...

Luttge A., Winkler U., and Lasaga A. C. (2003) An interferometric study of the dolomite dissolution a new conceptual model for mineral dissolution. Geochim. Cosmochim. Acta 67, 1099-1116. 

The rates of dissolution of carbonates and aluminosilicates as a function of pH are generalized in Fig. 2.11. Calcite and dolomite dissolution rates are generally 10 to 1 O -fold faster than rates for the silicates and decrease with pH up to saturation with the carbonates, usually between pH 8 and 10. Dissolution rates among the silicates range widely and are greatest for rapidly weathered minerals such as nepheline and olivine and slowest for quartz, muscovite (illite) and kaolinite, important products of chemical weathering in soils, discussed in more detail in Chap. 7. 

The rates of dissolution of various carbonates were studied in [200], The kinetics of magnesite dissolution was studied in [201] and that of dolomite dissolution in [202,203],... 

The relative dissolution rates between calcite and dolomite were also estimated for Bonanza field by examining the ratio of excess Ca to excess Mg in the post-treatment produced water. This ratio is 2.6 (Fig. 8). Assuming pure calcite (CaCOj) and pure dolomite (CaMg(C03)2>, this could only have resulted from the dissolution of 1.6 mol of calcite and 1 mol of dolomite, a calcite/dolomite dissolution ratio of 1.6. Assuming an impure calcite composition, the Ca/Mg ratio would result in a calcite/ dolomite dissolution ratio of approximately 1.9. Even though calcite is a minor component in the Tensleep reservoir, its relative contribution to the post-COj injection water is great. This is also despite the fact that the dolomite is more finely crystalline than the calcite, and would thus be expected to have a larger surface area exposed to the carbonated water. 

At very high PCO2, dolomite dissolution rates approach those of calcite. 

The amount of carbon observed in the Shullsburg waters was less than the modeled carbon in a closed system. An increase in the amount of sulfide oxidation implies an increase in the amount of carbonate dissolution in a 1 1 stoichiometric ratio (Equation 2). When calcite precipitation, driven by dolomite dissolution, is included in reaction modeling, the relationship between sulfate and carbon becomes nonlinear (Figure 3). Even for this nonlinear model, there is a discrepancy between measured carbon concentrations and modeled closed system carbon concentrations. 

The overall trend calculated in carbon isotopes is toward heavier isotope ratios for carbonate in solution after sulfide oxidation and neutralization. Dolomite dissolution introduces heavier carbon ((5 C = -17oo i )- Calcite precipitation reverses this trend by preferentially removing = 11.8 to 27oo Ht urc, where e, is the isotopic fractionation between calcite precipitated (p) and (s) the solution), but the mass transfer for this step tended to be small. The effect of CO outgassing above pH = 5.5 is to make the in solution heavier by preferentially removing light carbon = 0.5 to -9.07oo where is the isotopic... 

If the above constants of calcite and dolomite dissolution are used,... 

Steefel Lichtner (1994) highlighted the need to take flow geometries into account when assessing the effects of host-rock alterations. They modelled diffusive and advective transport processes along a hyperalkaline fluid-filled fracture in marl and also perpendicular to it between fracture and matrix. Dolomite dissolution was found to result in increased permeability parallel to the fracture, and diffusion was responsible for the precipitation of a calcite front in the wall rock, thus isolating the fracture physically and chemically from the rock matrix. This may reduce the effective buffering and sorption capacity of the rock. The mechanisms which affect the transport properties of a host rock are shown in this work to depend on many different factors and may be far more complex than can easily be modelled or simulated in a laboratory. 

Our interpretation of Fig. 24 is that there should be a significant dolomite dissolution event in the Gippsland Basin at a present-day depth from approximately 1200 to 2800 m. Over this depth interval, the log (AP/K) values are negative and dolomite typically is undersaturated. Points A, B, C, and D correspond to layers presently at 1500, 2800, 3600, and 4800 m depth. The next step in this discussion is to evaluate the quality of the porosity anomaly prediction for the 1200 to 2800 m present-day depth interval. 

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