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Fe S O minerals and aq S species redox/pH

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I am running GWB 9 on Windows 7. I would like to make fO2 v pH diagrams for the Fe-S-O minerals (pyrite, pyrrhotite, magnetite and hematite), for the aqueous S species, and for K minerals (K-feldspar, musc and kaolinite). I am completely new to GWB, but have been using Act2 and managed to get the axes fine, but the minerals and species aren't showing up. So, in general I think I may not be using the basis field properly, I'm not sure whether to use an ion/aq species or a mineral?I tried looking at the examples and manual but still couldn't figure it out.(If I get this figured out in Act2, it's basically the same input in Tact, just changing one axes, correct?)


And if I indicate Fe++ will it also know to use minerals with Fe+++ in oxidizing conditions? I was concerned because I'm using the default thermo.dat database and since the Fe-S-O and S(aq) system includes redox couples, I wasn't sure if it would be better to use a different database, or if I should disable the redox couples?


Last question, (promise!) is GWB able to take into consideration different salinities if I include aqueous NaCl in the 'in the presence of' field? Or because I am specifying the activity of the basis species this won't work?


Sorry for all the questions, I can provide more info if necessary!

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Hi Steph,


Not sure what you'll get out of plotting potassic-aluminosilicates on a fO2-pH diagram, but the first two plots you've described should be fine. Could you attach some screenshots, or maybe the .ac2 files (File - Save as...) so that I can take a look? If you tell me what you expect, or what you're trying to do, hopefully I can help you out.


You can specify the main diagram species, as well as the additional basis entries (the "in the presence of" field) using either minerals, gases, or aqueous species. Try modifying the Uranium fO2-pH diagram in the User's Guides (Figure 5.1) by adjusting the activity of U++++. For the K minerals, you might take a look at Figure 5.3 as a starting point. Once you get the hang of Act2 you'll have no trouble with Tact.


As long as you include the O2 component in your system in one way or another (as an axis, or in the presence of) then the basis entry Fe++ will consider the appropriate forms of iron. In an fO2-pH diagram with Fe++ selected as the main species, you'll see species like Fe+++ and Hematite under oxidizing conditions, and species like Fe++, Magnetite, FeCO3, etc. under more reducing conditions. Without adding O2, you'll only consider ferrous iron. If you're interested in a fO2-pH diagram for S, then you'll need to leave the SO4--/H2S pair coupled.


As for salinity, try taking a look at the first example shown in Figure 5.2. Just change the activity of Cl- in the "in the presence of" field to see how salinity affects your diagram.


Try taking a look at the pre-calculated diagrams on our website, as well as our slideshows and

. You can click on an Act2 icon on our website to bring up the input files which were used to create that plot.


Hope this helps you get started.




Brian Farrell

Aqueous Solutions LLC


P.S. I moved your post from the archive of old topics to the GWB forum front page.

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