Application to Mineral-Water Reactions

To illustrate applications of reaction path calculations to natural systems, we will draw heavily from the papers of Helgeson and his co-authors. The notation used below is consistent with Helgeson s as much as possible, to facilitate reference to his articles. 

Helgeson et al. (1969) assumed that the only important aqueous species in this system are K, AP, Al(OH), Al(OH), H4Si04, and HsSiO. If you perform a speciation calculation of the kind we have been discussing on a solution having K, Al and Si in the proportions 1 1 3 (as in K-feldspar) at very low concentration, you find that the dominant species are K+, Al(OH)J, and H4Si04. Essentially, Al(OH)J must dominate the Al species to maintain a charge balance with K. Therefore the dissolution reaction of K-feldspar can be approximated by 

To find out whether the solution has become saturated with another mineral, the solubility products of all minerals in the system considered (i.e., all minerals which contain any combination of the elements in the system) must be compared against the corresponding Ion Activity Product (lAP) in the solution after each increment of dissolution. This can be literally hundreds of minerals in large model systems. In the relatively simple K-feldspar case, there are only a few minerals that could possibly form. The first of these, according to the data used by Helgeson et al. (1969), is gibbsite. The solubility product for gibbsite is 

Saturation in gibbsite occurs when its activity product exceeds the solubility product for gibbsite. 

Because the ratio (aK /oH+) is fixed, but Si02 continues to increase as K-feldspar 

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