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Stability diagram of aluminium silicate minerals!


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Hi,


A quick scan through the default thermo dataset, thermo.tdat, shows a number of minerals described as smectites. For example, Smectite-high-Fe-Mg (or low-Fe-Mg, Reykjanes variants); Beidellit-Ca (or H, K, Mg, Na); and Nontronit-Ca (or K, Mg, Na). One of those, Beidelite-K, would actually appear under the conditions you specified except that you suppressed it. Others, like Nontronit-K, are not even considered because the Basis does not include all of the thermodynamic components necessary to form it. By adding Fe++ under the "in the presence of section" you'd be able to at least consider Nontronite-K in your diagram. Check the Results pane to see what species or minerals are suppressed and which are considered in your diagram.


Hope this helps,


Brian Farrell

Aqueous Solutions
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Hi Brain,

Thanks for the response. I'm interested to particularly include smectite-high-Fe-Mg in my stability diagram plot using the file that I sent you. I tried to add Fe++, Ca++, Mg ++ and Na+ "in the presence of section" in order to see smectite-high-Fe-Mg or smectite-low-Fe-Mg but it din't help. I tried to play with the activity of these cations to see if it helps but it didn't work. Do you have any comment on this? Thank you.

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Looking at the reaction in the thermo dataset for Smectite-low-Fe-Mg, I see that it's composed of both ferrous and ferric iron. In order to consider it, then, your system needs to include some measure of oxidation state. That way, the Fe++ added can be distributed between ferrous and ferric iron. You can add O2(aq) for example, and set an activity, or you can swap in O2(g) and set its fugacity, or swap in the e- and set an Eh. Alternatively, you could swap in an activity ratio (Fe+++/Fe++).

 

You'll also need to set the pH under "in the presence of". To see why, use Rxn to write a reaction between Smectite-high-Fe-Mg and Kaolinite, the species you swapped into the Basis. With K+/H+ swapped for H+ (also part of your Basis), you'll see that the reaction still includes H+.

 

Kaolinite + 2.84 H2O + 3.6 SiO2(aq) + .32 K+ + 1.12 Fe++ + .16 Na+ + .04 Ca++ + 1.84 Mg++ + .08 O2(aq) = 1.6 Smectite-high-Fe-Mg + 6.48 H+, or

Kaolinite + 2.84 H2O + 3.6 SiO2(aq) + .32 K+/H+ + 1.12 Fe++ + .16 Na+ + .04 Ca++ + 1.84 Mg++ + .08 O2(aq) = 1.6 Smectite-high-Fe-Mg + 6.16 H+

 

Many other minerals can be considered without specifying pH. K-feldspar is an example. If you look at the reaction between Kaolinite and K-feldspar, you'll see that the K+ and H+ are balanced, so that when the activity ratio is swapped in the reaction doesn't include any more H+.

 

Kaolinite + 4 SiO2(aq) + 2 K+ = H2O + 2 K-feldspar + 2 H+, or

Kaolinite + 4 SiO2(aq) + 2 K+/H+ = H2O + 2 K-feldspar

 

Hope this helps,

 

Brian

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  • 2 years later...

Hi,

 

You'll first want to verify that the thermo dataset you're using contains all of the species that you're interested in. A quick look in TEdit at thermo.tdat, the default dataset, shows that ClO4- is indeed included (you can quickly filter the species list to show only those containing Cl). After you make your diagram with O2(g) (or some other measure of redox state) on one of the axes, you'll need to go to Plot -> Diagram and uncheck "water limits" to draw Cl- species above the upper stability limit for water. You may need to increase the range of your diagram to find the O2(g) fugacity in which ClO4- predominates.

 

Hope this helps,

Brian

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