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Definition of "Free" in React + other Suggestions for Thermodynamic Modeling


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Hello, 

I am unsure what the meaning of "free volume %" versus "volume %" in the React software is. I am trying to figure out if it is a percentage of the total mineral mass (as I tried to input it as in my screenshot) or a percentage of the solvent mass / volume. The values I input for each mineral come from "The Crust of Venus: Theoretical Models of Chemical and Mineral Composition" by Barsukov et. al. (1982) which is shown in the first column of the second screenshot. We are trying to run React on a mixture of minerals in equilibrium with water to see what precipitates. In the past we have successfully done this with a starting liquid, but now we get errors when minerals get in the mix. I believe our first issue comes from our use of volume %, but any suggestions would be appreciated. A screenshot of the error message is included as well.

Thank you,

Basis6-29.PNG

Screen Shot 2022-07-01 at 2.49.08 PM.png

Run6-29.PNG

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Hello Vincent,

In the software, you can specify components as free or bulk quantity for most entries. For example, you can constrain the sodium component in your basis pane by setting a bulk quantity, which would include the total amount of sodium present in all sodium bearing species. Alternatively, you can also set a quantity for the free sodium ion, which does not include the mass of sodium in Na-complexes, and have the software calculate bulk composition. For more details and examples, please refer to section 7.2 Equilibrium models in the GWB Essentials User Guide. A mineral volume will always be treated as a free quantity, meaning that the value you set in the basis will only reflect the volume of the mineral and should typically be the case for minerals. 

I think it's unlikely that all the minerals observed in the composition are in equilibrium with respect to a fluid at the same time. You said that you were able to set up the problem to converge with just the fluid phase previously, so I would suggest you use that as a starting point. If you think a mineral is in equilibrium with the fluid, you can try swapping in the mineral and then run the calculation. You can try doing that for multiple minerals until you have something that seems reasonable. 

You can also double check that other values such as gas partial pressures and the eh are reasonable. It seems like the mole fraction of CO2 is about 0.96 with a total pressure of 96.1 atm but 40 bars was entered. 

Hope this helps,
Jia Wang
Aqueous Solutions LLC

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Hello Vincent,

Just a couple more thoughts to add to my previous response. The vol % is the volume of minerals relative to the volume of your bulk system (fluid volume + mineral volume + inert volume). So for example, if your bulk system has a total volume of 1000 cm3, then 10 volume % of a albite would occupy 10% of the total volume (100 cm3). 

For equilibrium calculation, the mass or volume of mineral does not matter, so you should focus on getting the initial assemblage right. Once you have the mineral assemblage correct, you can then worry about adding back in the exact values. 

Additionally, you might also want to consider the redox conditions in your system. Is N2(g) in equilibrium with NH3(aq)? If they are not, then they should be decoupled and each redox state constrained separately in your model. 

Best regards,
Jia

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