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Minerals with high log K


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Hi

Looks like as if GWB, or at least Act2, skips minerals having large log K values, as is the case of enargite

 

Enargite type=

formula= Cu3AsS4

mole vol.= 88.350 cc mole wt.= 393.7996 g

6 species in reaction

3.000 Cu+ 1.000 As(OH)4- 4.000 SO4--

-5.000 H2O -7.500 O2(aq) 6.000 H+

465.4360 414.1113 362.0154 312.9855

263.9463 221.6313 186.0364 156.4474

 

These data should be OK since are similar to those of the SOLTHERM data file. Do you any way of avoiding this problem?

Thanks

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Hi,

 

I moved your post from the archive of old posts to the main GWB page (for new posts). I'm not exactly sure what you're asking. It doesn't look like Enargite is included in any of the default databases distributed with GWB, but it would be easy enough to add it according to the procedure in the Appendix to the GWB Reference Manual. As for whether Act2 (and the other GWB programs) account for Enargite, all minerals, species, and gases which can be created from the current basis set will be considered. In an Act2 diagram, however, only the most stable species and minerals will end up being plotted.

 

Hope this helps,

 

Brian Farrell

Aqueous Solutions LLC

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  • 1 month later...

Hi Brian

Let me clarify the problem. Enargite is NOT included in the original databases but I have calculated the constants from other sources. However, log K values are really high and in the GWB databases we cannot lower logK by division since the program constrains that the minerals always have a coefficient in the reaction of 1. Also, it looks like that in the GWB databases unknown log K values are set to 500; I wonder if there is any instruction that simply skips minerals having high log K - at least in version 8.0. In fact, when having a look to the log of the diagrams in Act2, enargite simply does not exist as loaded mineral despite it is in the database as has been checked in Config/Show.

Any idea?

Thanks

 

Fernando

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Hi Fernando,

 

I was able to make an Act2 diagram with Enargite using the data you supplied. Perhaps you did not add it to the dataset correctly, or maybe you did not include in your Act2 basis all of the components necessary to form Enargite? If you look at the thermodynamic information you provided above, you'll see the 6 species needed to form it (Cu+, As(OH)4-, SO4--, H2O, O2(aq), and H+). All of those must be present to form Enargite. For example, if I diagram Cu+ on Eh-pH (or O2(aq)-pH) axes, I would additionally need to add As(OH)4- and SO4-- under the "in the presence of" field for Enargite to be considered.

 

If you're still having trouble, you can post your thermo dataset and Act2 script so that I can take a look? You can also email them to me at support@gwb.com.

 

Regards,

Brian

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Hello all,

 

Just one more thing to add here. 500.0000 is not a cutoff value for GWB in deciding whether or not to consider a mineral. Entering exactly 500.0000 for one of the principal temperatures indicates to the program that data does not exist for that particular temperature. 500.0001, on the other hand, will be interpreted as a valid equilibrium constant.

 

Hope this helps,

Brian

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