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[OLD] Activity of solids in Act2


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From: Ian Hutcheon

Subject: Activity of solids in act2

I am trying to figure out how to assign more than one activity for a solid species in ACT2. For the system:

show system

Diagram speciation of Paragonite on Na+/H+ vs SiO2(aq)

Temperature is 25 C, Pressure is 1.013 bars

Thermo dataset: .gtdatathermo.dat

Working directory: c:program filesgwb


Basis is:

Paragonite (swapped for Al+++) Activity = 10^-2.57 (main species)

H2O Activity = 10^0 (solvent)

Na+/H+ (swapped for Na+) Y-axis variable

SiO2(aq) X-axis variable

The components that appear on a Paragonite, Na+/H+ vs SiO2(aq) diagram are as follows.






Albite low


Using the Aagaard and Helgeson approach to defining activities of these components in smectite requires assigning an activity for each of the components Paragonite and Pyrophyllite. However, we seem to only be able to define one activity?


From: Craig Bethke

Subject: Re: Activity of solids in act2

I think the following should work for you. The reactions for Paragonite and Pyrophyllite (like all the minerals) are written in the database as destruction reactions for a single stoichiometric unit of mineral. If you change the log K for these two reactions to log K + log a, where a is the desired mineral activity, therefore, this activity will be carried implicitly throughout your calculations. The easiest way to change the log K values is to use the “alter� command -- there's no need in this case to edit the thermo database.

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