Solid-solutions

[bappy]

Hi,

 

I've seen older posts enquiring about GWB handling solid-solutions. I've taken onboard warnings outlined in Bethke's books about lack of thermodynamic data and potential errors, but I would like to replicate some calculations I've seen published where solid-solutions are assumed to be ideal mixtures. The calculations are done in EQ3/6 and solid solutions are represented in the database like this, with two end members specified elsewhere:

 

CLINOPYROXENE ca(fe,mg)si2o6

date last revised = 13......

keys = ss ideal active

2 end members

1.0000 DIOPSIDE 1.0000 HEDENBERGITE

type = 1

0 model parameters

1 site parameters

1.000 0.000 0.000 0.000 0.000 0.000

 

Is it possible to do a similar thing in GWB?

 

thanks for your help.

[Brian Farrell]

Hi,

 

There is no built-in solid solution model in the GWB, but you can easily edit the thermo datasets to add new minerals of a discrete composition. For example, Diopside and Hedenbergite are already in the default thermo dataset, thermo.dat (entries shown below).

 

Diopside type= pyroxene

formula= CaMgSi2O6

mole vol.= 66.090 cc mole wt.= 216.5524 g

5 species in reaction

2.000 H2O 1.000 Ca++ 1.000 Mg++

2.000 SiO2(aq) -4.000 H+

23.1479 20.9695 18.3141 15.8741

13.5109 11.6602 10.0791 8.4303

 

Hedenbergite type= pyroxene

formula= CaFe(SiO3)2

mole vol.= 68.300 cc mole wt.= 248.0944 g

5 species in reaction

-4.000 H+ 1.000 Fe++ 2.000 SiO2(aq)

1.000 Ca++ 2.000 H2O

21.4548 19.4380 16.9637 14.6739

12.4416 10.6793 9.1582 7.5506

 

If you wanted to consider a mineral of intermediate composition like Clinopyroxene, it would be pretty simple to add as long as you know the stability. I've provided a template below. You would need to provide the molecular weight, the molecular volume (optional), and the equilibrium constants for the reaction. By following this example you can add a range of minerals covering a spectrum of compositions (Fe.6Mg.4, Fe.4Mg.6, …).

 

Clinopyroxene type= pyroxene

formula= CaFe.5Mg.5(SiO3)2

mole vol.= ??? mole wt.= ???

6 species in reaction

-4.000 H+ 1.000 Fe++ 2.000 SiO2(aq)

0.500 Ca++ 0.500 Mg++ 2.000 H2O

??? ??? ??? ???

??? ??? ??? ???

 

Hope this helps,

 

Brian Farrell

Aqueous Solutions LLC

[Brian Farrell]

With the GWB 2022 release, binary solid solutions can now be modeled. Either a Guggenheim or ideal activity model can be used, and either a continuous or discrete implementation can be chosen. Solid solutions can be defined in the thermodynamic dataset or configured by the user at run time. For more information, please see section 2.5, Solid solutions, in the GWB Essentials Guide, as well as relevant sections in the Using Rxn, Using Act2, and Using SpecE8 chapters of that same guide. Please see as well section 3.4.7, Solid solutions, in the Thermo datasets chapter of the GWB Reference Manual.

Regards,

Brian Farrell
Aqueous Solutions

[Brian Farrell]

In GWB 2023, TEdit can be used to import any EQ3/6 dataset (including any solid solution definitions) to GWB format. And conversely, GWB datasets using the b-dot and h-m-w models can be exported to EQ3/6 format. For more information, please see 9.3.2 Importing EQ3/6 datasets and 9.3.3 Exporting EQ3/6 datasets in the GWB Essentials Guide.