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Mineral alteration (e.g. Ferrihydrite to Goethite)

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I try to model the alteration of Ferrihydrite to Goethite (including it's effect on sorption). I have suppressed all Fe minerals beside Fe(OH)3(ppd) and Goethite. Since Goethite has a much lower solubility, GWB uses Goethite as stable mineral phase and instantantiosly forms Goethite from all the Fe(OH)3(ppd) I give into the system. My only idea is to remove all the Fe(OH)3(ppd) first from the system and then add the same molar amount of Goethite. But this is somehow inelegant. I also found some kinetic data on the Ferrihydrite-to-Goethite alteration, that I would like to use (http://pubs.acs.org/doi/pdf/10.1021/ie010931z).

My question is:

Is there a way to handle two mineral phases with the same basis species / redox species (in this case Fe+++) in coexistence?


Thank you in advance!


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Hi Frank,


I think your best bet is to set up a kinetic model. The example in section 26.4, Ostwald’s step rule, in the Geochemical and Biogeochemical Reaction Modeling text is very similar to what you’re trying to do. As you’re doing, suppress the other Fe minerals. Then set the initial fluid in equilibrium with Fe(OH)3(ppd) and set a kinetic rate law for both minerals. You’ll either want to set a small initial mass for the Goethite on the Reactants pane or account for its nucleation in some way.


Hope this helps,


Brian Farrell

Aqueous Solutions LLC

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Hi Brian,

works great, thank you!


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