> I have another problem that I hope you can help me with: I have mixed a groundwater with 129 ppm SO4-2 (specified as S) with Fe_metal and observed that the water reaches saturation with respect to several Fe oxide and sulfide phases and as well as As2S3. As expected, pe drops like a rock. Magenite has the greatest degree of saturation of the Fe phases. > Allowing magnetite, pyrite, and orpiment to "exist" shows that only magnetite ppts, leaving pH high but perhaps unrealistically high (12+) so that HS- is trivial relative to other S species and the sulfides do not form. > Do I need to define reactions for the other S species and the orpiment, pyrite, etc.? If I understand you correctly, no, you do not need to define additional reactions. The current definitions allow for equilibrium among all of the aqueous species. Reactions defining the minerals can be written in terms of any of the aqueous species. > Also, can you comment on reaction precipitation sequence? I can't comment. Equilibruim with iron metal would put you outside the stability field for water, so you need to be careful how you define your system. I think the program is probably doing what you are asking it to do if it converges to a solution. You just need to think about what is plausible. David David Parkhurst (dlpark@xxxxxxxx) U.S. Geological Survey Box 25046, MS 413 Denver Federal Center Denver, CO 80225
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