Dolomite saturation index

Fig. 34 Dolomite saturation index of complete and incomplete water analyses (calculated with PHREEQC after data by Merkel 1992)...

In fact, the choice of CO2 fugacity has little effect on the mineralogical results of the mixing calculation. In the model, the critical property of the Fountain fluid is that it is undersaturated with respect to calcite, so that calcite dissolves when the fluid mixes into the Lyons. Because we assume equilibrium with dolomite and magnesite, the saturation index (log Q/K) of calcite is fixed by the reaction... 

Hydrochemical analyses should be as complete and correct as possible because they are the basic prerequisite of a reliable hydrogeochemical model. They represent the essential information and errors propagate from them to the final result. Fig. 32 to Fig. 34 show an example of the saturation index calculation for calcite and dolomite, of the C02 equilibrium partial pressure, and of the consequences an incomplete analysis may have. The following analysis is given (pH = 7.4, temp. = 8.1°C, conductivity = 418 pS/crn, concentrations in mg/L) ... 

In order to make the saturation indices of carbonates roughly comparable, one can normalize their SI values to the same mole number of carbonate groups or of cations. Thus, if 5/ is the saturation index of dolomite, and AT,/ and lAP y its solubility and ion activity products, we write... 

Calcium carbonate (such as calcite, siderite, dolomite, and aragonite) is typically present in marine deposits and Pleistocene sediments deposited in river-banks and practically absent in deposits from ancient rivers and deposits of Miocene age. Calcium is generally the dominant exchangeable cation (56-82%) followed by magnesium (6-26%). An important parameter for behavior during aquifer transport is the calcite saturation index (SI = log[Ca ][C03 J/KcacOj)-... 

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