The algorithm for choosing the stable set of phases is not exactly the sequential process you describe. Hopefully, it should be at most a two step process, regardless of the number of linearly dependent mineral compositions in EQUILIBRIUM_PHASES. When PHREEQC is iterating to find the solution to the algebraic set of equations (Newton-Raphson), it is using a variant of the simplex algorithm that allows not only equalities, but sets of inequalities and linear objective functions. In a nutshell, the algorithm tries to drive SI's of all minerals in EQUILIBRIUM_PHASES to zero (objective functions try to make SI=0) or less than zero (inequality constraints require SI <= 0). If, at the end of the Newton-Raphson, all minerals have SI = 0 or have SI < 0 and zero moles, then the problem is solved. SI < 0 and moles > 0 should not occur, because the SI can be made smaller by dissolving more of the mineral. However, for example, if you include both calcite and aragonite with positive moles in EQUILIBRIUM_PHASES, it is possible that at the end of the Newton Raphson iterations, calcite SI = 0 and aragonite SI < 0, but both still have positive moles in the system. In this case, PHREEQC will add all of the aragonite to solution (setting moles of aragonite to zero) and solve the system again. The second time through, it should put all of the CaCO3 into calcite, so that SI calcite = 0, SI aragonite < 0, moles calcite > 0, but this time, moles aragonite = 0. If there are multiple minerals with SI < 0 and moles > 0, then all of them are dissolved simultaneously. I don't know of any guarantee that it won't make some aragonite in an intermediate calculation during the second step and then just keep repeating step 2 of the process, but I've never had any problem so far. David David Parkhurst (dlpark@xxxxxxxx) U.S. Geological Survey Box 25046, MS 413 Denver Federal Center Denver, CO 80225 Annett Sullivan <4as@xxxxxxxx> To: dlpark@xxxxxxxx cc: In-Reply-To: <220.127.116.11.2.20020102141318.00a5b3c0@xxxxxxxxxxxxxxxxxxxxx> 01/02/02 01:35 Subject: PHREEQC question PM 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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