Kinetics of Precipitation and Dissolution Reactions

A reasonable assumption about the rate of mineral precipitation or dissolution might be that the rate is proportional to the degree of super- or under-saturation. Expressing this idea mathematically, we would say that the rate R is 

The proportionality factor would include, at a minimum, the rate constant for the reaction, and the relative surface area of the mineral, i.e., the mineral surface area per unit volume of solution. We might also add a term, v, to account for cases in which the dissolution of one mole of solid phase gives rise to v moles of the component we are measuring. For example, if we measured the solubility of Mg2SiC 4 by measuring aqueous Mg, v = 2. 

The Geochemist s Workbench In program react, part of The Geochemist s Workbench , equation (11.29) is implemented by using the kinetic command to set the variables (key words) required. The format is 

For example, the rate constant for the dissolution of albite is 10-12 26 mol m-2 s-1, or 10-16-26mol cm-2 s-1 (Lasaga, 1998, Table 1.5). This is entered in react as 

3 molal Ca++ =. 05 molal Cl- =. 3 molal HC03- =. 02 molal 10 free grams Kaolinite 10 free grams Quartz 10 free grams Muscovite react 10 grams Albite 

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These computations describe the tendency of a water sample to be saturated, but they do not necessarily demonstrate whether mineral dissolution or precipitation is taking place. For dissolution to take place, the mineral must be present and it must dissolve at a rate that is fast enough relative to the flow rate of the water to affect the water chemistry (Berner, 1978). Likewise for a mineral to precipitate it must do so at a fast enough rate. The kinetics of precipitation and dissolution reactions must be applied to get a realistic interpretation of water-rock interactions. 

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