Modification of the solubility product of a mineral in the database

[quitte]

I am trying to model the kinetics of a dissolution experiment (mixture of olivine and pyroxene), and consider the formation of various secondary minerals including antigorite.
At the time being I let antigorite precipitate at equilibrium.

EQUILIBRIUM_PHASES
Antigorite 0.0 0.0 

I however observe large discrepancies between the model results and my experimental data.
I now would like to let antigorite form only for a SI value higher than 1, corresponding in my case to a Si concentration in the solution of 3e-5 mol.L-1. I do not know how to modify the solubility product of antigorite in the database. Could you please give me some advices?
Alternatively, how could I modify the main script to have antigorite precipitate only when [Si]>3e-5 in the solution and keeping then the Si concentration in solution constant while olivine and pyroxene dissolution goes on?
Thank you very much for your help.

[dlparkhurst]

The simplest way is just to set the target SI to 1.0:

Code: [Select]

EQUILIBRIUM_PHASES
Antigorite 1.0 0.0 

Alternatively, you can use PHASES to redefine the log K to be 1 unit larger. I would do this in your input file, rather than the database. Here, I simply change the log K at 25 C; if you include an analytical expression, then you would need to add 1.0 to the first parameter of the analytical expression.

Code: [Select]

PHASES
        Mg48Si34O85(OH)62 +96.0000 H+  =  + 34.0000 SiO2 + 48.0000 Mg++ + 79.0000 H2O
        #log_k           477.1943
        log_k           478.1943
END


[quitte]

Thank you very much indeed for your very quick answer.
I was looking for a too complicated solution...