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dandylyon

delta34S and solubility distribution in logfO2-pH space

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I am a new user of GWB and have 2 questions about plotting in logfO2-pH space using ACT2 (or any of the programs)

1)I would like to add d34S isopleths to my logfO2-pH diagrams. Is this possible, and if so how would I go about it?

2)Is it possible to plot solubility contours of a species in these diagrams?

Thanks, this is a great resource!

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Hi,

Act2 does not have any capabilities for calculations involving isotopes. If you know the values for these isopleths you could always add scatter data to your plot, however.

As for your solubility contours, you cannot make these types of diagrams directly in Act2, but they are easy to assemble in a program like Powerpoint by overlaying several diagrams. You'll need to create several plots (each with a different activity for your main species, perhaps), copy them as enhanced metafiles, then paste into Powerpoint. You can then ungroup each image, and copy and paste into a single plot.

Hope this helps,

Brian Farrell

Aqueous Solutions

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Dear all,

I typically deal with solubility contours exactly the way Brian described it, and it works well because act2 produces vector graphics output. Having the possibility to use isotopic reactions in activity diagrams would be very helpful I must say, but I understand that implementing this feature just for few potential users that would need is might not be feasible.

The most troublesome issue with act2 at the moment IMHO is that one cannot set activities for any solid phases, they are just assumed to be 1. If one wants to display for example the solubility of solid gold, where the activity of Au(s) is non-unity (beacause its not pure Au, but an alloy with Ag), then this simply is not possible in act2 (at least not in a reasonable and time-efficient way). OK one can theoretically calculate a RTLn(a) term for the Au and then lump that into the reaction equilibrium constant and then modify the database. This, however, becomes very time-consuming and cumbersome. I notice that one can change the activity of H2O, so theoretically it should be possible to do the same thing for any other reactant or product that is involved.

Best regards,

Thomas

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Hi,

The method Thomas mentioned will work for any GWB program. Act2, however, does have some ability to handle nonunity mineral activities much more easily. I've attached two plots which demonstrate this. The first plot is an example taken from the GWB Essentials Guide and the second is the same example with Quartz activity set to 0.4. Lowering the activity of Quartz, the Kaolinite field decreases (hard to tell - take a look at the printed output) and eventually Gibbsite replaces Kaolinite. Unfortunately at this time you cannot save nonunity mineral activities in your scripts, but you can easily edit your input files before running.

Hope this helps,

Brian

post-10181-041935600 1321977684_thumb.jpg

post-10181-048151100 1321977700_thumb.jpg

Solubility.ac2

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Dear Brian,

thanks for pointing this out. Unfortunately the script that you attached to your posting does not contain any command that would set the activity of quartz to a non-unity value. Could you please add the example that contains the syntax of that command that one would need to enter into the act2 script? Does this approach also work for the diagram species? If I calculate the solubility of gold as function of pH and log(fO2), then I set up Au+ or Au+++ as my diagram species, and then swap in Au(s) for the selected aqueous species. Then I would need to enter a command in the act2 script that would set up the non-unity activity of Au(s) for example. The same principle can then be used for modeling activity-activity diagrams that involve non-unity activity of several minerals that are involved into the reactions. Ffor examples one can calculate stability of K-feldspar andf muscovity in an aK+/aH+ vs. aSiO2 diagram and set up activities of the minerals calculated from an external solid-solution model.

Cheers,

Thomas

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Hi Thomas,

You have to set the activity of a mineral like Quartz using the GUI, or with the command "activity Quartz = 0.4," for example. As I mentioned above, it is not possible at this time to save a script with a nonunity mineral activity, so I can't post the exact script. Give that a shot in your examples. You might have an issue with your Gold example, however, depending on how you want to set up your problem. I believe for a solubility diagram, the main species must match one of the axes (you can't have Au(s) and Au+ both as the Gold component) and you can't make mineral activity one of the axes. Let me know how that works for you.

Hope to help,

Brian

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DEar BRian,

I have tried that command out, but unfortunately I receive an error message:

Act2> activity Gold = 0.4

-- Error: Gold needs to be swapped into the basis

-- Error: Can't set activity of Gold

I attach for you the act2 script that I am using. The stup is such that Au+ is the diagram species, and pH and O2(g) are the species on axes. I can then set up activities of other aqueous species (in the presence of), in my example that of Cl-, H2O and H2S(aq). However, I CANNOT set up the activity of Gold (solid phase), neither through the GUI nor on the command pane (I receive that error above).

I simply do not understand why this is not possible. Gold(s) is a participant of the solubility reactions in my system (and reaction system of equations) in exactly the same way as for example H2O (water solvent) or H2S(aq), but yet the program does not allow me to set up its activtiy. Hence I am left with having to modify the equilibrium constants for every single change in solid phase activity, a very time-consuming and annoying way to accomplish this.

There are many more similar cases, for example the solubility of any Fe-bearing aluminosilicate minerals (annite in biotite etc.) at given activity of Fe2+ in the aqueous solution, and non-unity activity of the solid phase (which would we the endmember activity of annite in biotite solid solution). However, the program does not let me do this in a straightforward and interactive way (recalculating equilibrium constants for every single case is really not the way forward, I can as well then calculate the diagrams by hand).

I must say that it would be really desirable if the option I outline above would be supported.

Best regards,

Thomas

practical2a.ac2

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Hi Thomas,

In the Al solubility example, you must have the mineral swapped into the Basis before its activity can be set. As for what you are attempting to do with your Gold example, I believe this is not possible at the moment, at least without altering log K values. This limited ability to specify mineral activity might be useful to some users, but it is not a true solid solution model in any way. We will consider your ideas along with those of other users as we begin the next stages of development. Sorry for any confusion this may have caused.

Sincerely,

Brian

P.S. Are you using the alter command to change your log K values, or are you manually changing the thermo datasets and reloading them every time? The alter command provides a quick way to search through minerals, gases, and species and alter log Ks in your run without permanently changing the dataset (though you can save your altered settings for future runs). Hope this helps.

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