Activities of minerals in act2 and tact

[thomasw]

Dear all,

in some calculations of activity-activity and activity-temperature diagrams it is pretty useful to set the activity of minerals involved into the set of reactions to a value that is different from 1.0. For example, consider that you work on fluid-rock interaction and want to look at the controls on mineral stability in an hydrothermally altered rock. Now you have analyzed for example the chlorite in this rock by electron microprobe and used some composition-activity model (in the simplest case just configurational energy arising from ideal mixing) to calculate the activity of one end-member that you consider in the activity-activity diagram (e.g., clinochlore in the chlorite). Then in principle it is possible to calculate and plot an activity-activity diagram for exactly this activity (usually a value smaller than unity). I have done this many times with the help of SUPCRT92, a spreadsheet and paper and pencil. Unfortunately, I have not found if and how this could be done in act2 and tact. In principle, this would be a rather simple and straightforward extension of the set of additional constraints that can be set at the bottom of the Basis pane of act2 (the section labelled with: in the presence of ...). Any suggestions?

Best regards,

Thomas

[Tom Meuzelaar]

Hi Thomas:

There's actually a simple work-around that allows you to do this- change the log K value of a mineral mass action equation using the Config - Alter Log K's option:

If a mineral dissolves according to reaction:

MIN <-> A + B

Then the mass action equation is:

log K = log aA + log aB - log aMIN

Generally, we take aMIN = 1, so log aMIN = 0 and the last term vanishes.

However, if you want to set log aMIN to non-unity, use the alter command to change the value carried by Act2 for the log solubility product <log K> to a value of <log K + log aMIN>.

Now all your calculations will reflect the desired mineral activity.

I hope that helps,

Tom Meuzelaar

RockWare, Inc.

[thomasw]

Hello Tom,

thanks for the suggestion of that workaround, which makes it at least in principle possible to calculate activity diagrams for mineral activities that are different from unity. But you would certainly agree that having to calculate the log k increments manually from the activity of the mineral every time time, especially when for example you want to contour the mineral activity (e.g. for Au activity in Au-Ag alloy to look at the effect of electrum composition and similar), this is pretty inconvenient and time-consuming. Speaking frankly, my intention in purchasing Gwb was actually a substantial speedup effect in terms of producing different sorts of activity diagrams compared to calculations using SUPCRT92, spreadsheet and paper and pencil. Now it looks like quite a number of advanced calculations that I would like to do actually require much more manual preparation work than I thought. I do not see a theoretical reason why for example one cannot implement that the activity of minerals involved in reaction could be set directly to a value different from unity. It is by default possible for solvent H2O, so it should be possible for minerals involved in reactions as well. I strongly encourage the Gwb development team to consider this option as a feature request for the future. This would also partly address the general issues that some people raised about dealing with solid-solutions in Gwb.

Best regards,

Thomas

[Tom Meuzelaar]

Thank you for the suggestion Thomas.

We will weigh this in with the numerous other feature requests we get for the next development cycle.

Regards,

Tom