Saturation index for the water in granite terrain and limestone terrain

[Manoj Subramanian]

I am working on surface water interaction. In that the samples were collected from granite and its adjacent limestone terrain. i need to calculate the SI in both the solutions. When i tried it in phreeqc i observe only few mineral phases like calcite, an-hydrate, halite etc. It doesn't give SI values for K-feldspar. Na-feldspar and other phases. I need to calculate the SI for the phases like feldspar, quartz, dolomite, etc which is shown in output results from phreeqc.

[dlparkhurst]

To calculate saturation indices, you need analytical data for all the major elements in a mineral. For an alumino-silicate mineral, that means you need analyses of aluminum (and silica). However, accurate analyses of aluminum are difficult. Concentrations are low and there is the issue of colloids versus dissolved aluminum. So except in acid mine water with high aluminum concentrations, water analyses lack reliable aluminum concentrations and SIs for alumino-silicate minerals cannot be calculated.

If you want to assume that the aluminum concentation is determined by equilibrium with kaolinite or gibbsite (for example), then PHREEQC does have the capability to calculate aluminum concentration given an assumption of equilibrium with a mineral, and then you would get SIs for other alumino-silicate minerals.

To compound the issue, the thermodynamic data for alumino-silicate minerals is not as well established as more soluble minerals.