Jump to content
Geochemist's Workbench Support Forum

Adding new mineral into thermodynamic database


Neda
 Share

Recommended Posts

Hi,

 

I'd like to integrate the a new mineral (Ankerite) into the thermodynamic database. My question is how and from which formula, equilibrium constants corresponding to the reaction could be generated at principle temperatures? I have seen in thermoddem database (http://thermoddem.brgm.fr/data/mineraux.php), these log K values are provided for many minerals but not for Ankerite. I used Van t'Hoff equation to check if I can generate the Log K values, but it seems there is another equation for equilibrium constants.

Could anyone please advise on how a new mineral can be integrated into the database only by knowing its thermodynamic properties such as Molar volume, Standard Gibbs energy and Standard enthalpy?

 

Thank you.

Neda

Link to comment
Share on other sites

Hi Neda,

 

You’ll want to write a balanced reaction for Ankerite in terms of species in the thermo dataset that you’re using. If you look at any dataset, you’ll see that the program writes all reactions in terms of a set of basis species or redox species (which are just basis species in a different oxidation state). If you use TEdit to modify a dataset, though, you can add a reaction in any form (i.e., one that’s written in terms of an aqueous species) and the program will automatically rewrite the reaction in terms of the appropriate basis or redox species and recalculate the log K.

 

In thermo.tdat, for example, HCO3- is the basis species for inorganic carbon while CO3-- and CO2(aq) are aqueous species. If you found a reaction in the literature for Ankerite written in terms of CO3--, you could add it to your thermo dataset using TEdit and the program would rebalance it in terms of HCO3- and recalculate the log K for you.

 

If you don’t have the data in terms of a log K and reaction already, though, you might as well write the reaction yourself in terms of the basis or redox species. You can calculate the standard state free energy of reaction from the standard state free energies of formation of species in the reaction, then convert that to a log K (most likely at 25 C). Then you can use the Van t’Hoff equation or some other method to calculate the log K at different temperatures if necessary. For more information, try taking a look at Chapter 9, The equilibrium constant, in Greg Anderson’s Thermodynamics of Natural Systems or a similar reference.

 

Hope this helps,

 

Brian Farrell

Aqueous Solutions

Link to comment
Share on other sites

Hi Brian,

Thanks for your reply. As you said I found a balanced reaction for Ankerite and I included it into the database using basis species. Please find the attached file which includes Ankerite for more details. The problem is that the program doesn't recalculate the log K automatically according to my reaction. I have noticed it only re-balance the equation and it gives warning if Mole Weight is entered incorrectly. I am not sure if I missing any step and would appreciate if you could take a look at the attached file. Thanks again.

 

Regards,

Neda

thermo-Ankerite.tdatpost-12219-0-48387300-1441119511_thumb.png

Link to comment
Share on other sites

Hi Neda,

 

You're on the right track, but you need to supply the log K (at one or more of the principal temperatures) for the reaction however you've added it. Take calcite, as an example. You could add the reaction directly in terms of the basis species of your thermo dataset (HCO3-, Ca++, H+ in thermo.tdat). The reaction would be Calcite + H+ = Ca++ + HCO3- with a log K at 25 C of about 1.71. Imagine if instead your found the reaction Calcite = Ca++ + CO3-- in the literature, with a log K of -8.63. Both are equivalent, but because CO3-- is an aqueous species (with reaction CO3-- + H+ = HCO3- and log K of 10.34), TEdit will automatically rebalance the reaction and recalculate the log K to be in the form I first described.

 

Assuming that you're calculating the log K and not finding this information in the literature, then you should just calculate it for the reaction exactly as you've added it.

 

Hope this helps,

Brian

Link to comment
Share on other sites

Thanks Brian. I calculated and added the log K values for the principle temperatures for Ankerite. Values seems fine to me. I still don't know how TEdit automatically recalculates the log K since I don't see any option for it but I assume this is done internally in the program. Thanks so much again for your help.

 

Best,

Neda

Link to comment
Share on other sites

Join the conversation

You can post now and register later. If you have an account, sign in now to post with your account.

Guest
Reply to this topic...

×   Pasted as rich text.   Paste as plain text instead

  Only 75 emoji are allowed.

×   Your link has been automatically embedded.   Display as a link instead

×   Your previous content has been restored.   Clear editor

×   You cannot paste images directly. Upload or insert images from URL.

Loading...
 Share

×
×
  • Create New...