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Saturation index for sulfides

Silvain Rafini

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Hi Brian
I'm currently making may first steps using GWB. I'm trying to calculate mineral saturation indexes for some sulfide minerals (pyrite, sphalerite, chalcopyrite, galene) that do not appear in the default list as proposed by GSS. Equilibrium constants for the dissolution reactions of these mineral are in the thermo.tdat file (more or less the LLNL database), then I can not figure why GSS do not propose it by default. Of course, my dataset includes Zn and SO4 concentrations.
Thanks for helping.
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Hi Silvain,

The reaction for Pyrite in the LLNL dataset, thermo.tdat, is shown below:

Pyrite  + H2O  = Fe++  + 1.75 HS-  + .25 SO4--  + .25 H+  

You’ll notice that the reaction is written in terms of the basis species SO4-- and the redox species HS- because the oxidation state of sulfur in pyrite is between that of SO4-- and HS-. When the reaction gets loaded into an app like Rxn, SpecE8, or GSS, though, it’s rebalanced to be in terms of the basis species only:

Pyrite  + H2O  + 3.5 O2(aq)  = Fe++  + 2 SO4--  + 2 H+

This is because each of the apps assume that redox coupling reactions are enabled by default. 

In any case, to be able to calculate a saturation index for a mineral like pyrite, you need a way to constrain every species in the reaction. So in addition to adding Fe++ and SO4-- to your spreadsheet, you need to add O2(aq) (or Eh or pe), as well as pH. In this case, the concentration you set for the SO4-- component should represent all the sulfur in the system. The program uses the oxidation state you supply to find the equilibrium distribution of mass between sulfate and sulfide species.

You can alternatively decouple HS- from SO4-- to set up a disequilibrium model. This is done from the Config > Redox Couples dialog in Rxn and SpecE8, and from Data > Redox Couples in GSS. Alternatively, simply adding the redox species HS- to your spreadsheet triggers HS- to be decoupled from SO4--. In this case, you don’t need to supply a measure of oxidation state, but you do need to provide separate measurements for the SO4-- and HS- entries.

Hope this helps,

Brian Farrell
Aqueous Solutions LLC

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