> I am using PHREEQCi (1.3) to help elucidate solute provenance and potential chemical reaction processes from chemical analysis of glacial meltwaters collected at the snouts of various glaciers. > Having used PHREEQCi for a while for inverse modelling calculations, I have been trying to add my own phases, i.e. (i) different mineral stoichiometry, and (ii) different phase reaction steps. Although I am probably incorrect, I have wanted to do this because I wanted to simulate particular reactions, not just to get an idea of the final mole transfers from particular minerals. For example the weathering of Anorthite to Kaolinite, NOT to the end member H4SiO4. > e.g. CaAl2Si2O8 + 2H2CO3 + H2O = Ca+2 + 2CO3-2 + 2H+ + Al2Si2O5(OH)4 > not as defined in MINTEC database : CaAl2Si2O8 + 8H+ = Ca+2 + 2Al+3 + 2H4SiO4 In this case, you do not need to redefine the PHASES data for either inverse or forward modeling. In inverse modeling, you need only include Anorthite and Kaolinite (without quartz or chalcedony) in the INVERSE_MODELING -phases, and the models will have to form kaolinite. For forward modeling, the equilibrium is independent of the precise reaction that is used to define Anorthite (provided the log K's are internally consistent). You are getting an error because PHASES requires dissociation reactions that have the mineral as the first entity on the left-hand-side of the equation, and aqueous species for the rest of the reaction. So you can't use kaolinite, but you can use any aqueous Al and Si species, provided you have a log K for the anorthite reaction that you specify. However, you shouldn't have to redefine the phases unless you want to change the stoichiometry of the mineral or change the log K for the reaction. For inverse modeling, only the stoichiometry of the mineral is important, not the equation or the log K. In the equation you were trying to redefine, the H4SiO4 is dissolved silica. There is a mole-balance equation for silica, so if you add Si from dissolution of anorthite, more than is in the final solution, then the excess silica must go into another silica phase. In the extreme, if you constrain kaolinite as the only silicate that can precipitate, then the silica has to go into kaolinite. David David Parkhurst (dlpark@xxxxxxxx) U.S. Geological Survey Box 25046, MS 413 Denver Federal Center Denver, CO 80225
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