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PHREEQC question

Hi Dr. Parkhurst-

I have a question about the way in which PHREEQC decides mineral precipitation order.  I have read over the documentation and your online FAQ, and haven't seen the answer, so I thought I'd send you an email.

In some of our runs we have different minerals with similar composition (for instance, iron oxyhydroxides, or calcite/aragonite) that are all supersaturated, and that we allow to precipitate in EQUILIBRIUM PHASES. 

How does PHREEQC decide which mineral to precipitate first?  From working with it, I think that it chooses the mineral that is *most* supersaturated first, and brings about precipitation to take that saturation index to 0.  Then in subsequent steps it considers whether the other (similar-composition minerals) are still supersaturated, and if so, again chooses to precipitate the mineral with the highest SI.  And on down the line...  Does this explanation sound right to you?

Thanks very much, we like PHREEQC a lot!

Annett :)

Annett B. Sullivan
Environmental Sciences Division
Oak Ridge National Laboratory
P.O. Box 2008, MS-6036
Oak Ridge, TN 37831
phone: 865-574-6367  fax: 865-576-8543

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