[OLD] Act2 mosaic plot

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From: Finch, Robert J.

Subject: Act2 mosaic plot

Following the example in the GWB users' manual (v. 3.0, p.44), I diagramed As(OH)4- (with activity of As(OH)4- fixed) and SO4-- speciated over X-Y. The resulting diagram is as it is shown in the manual. But when I diagram SO4-- over the same ranges and speciate As(OH)4- over X-Y (specifying the same activities for SO4-- and As(OH)4- as for the first diagram), the stability fields shown for the solids are different. Specifically, the orpiment field is much reduced compared with the first diagram. My intuition tells me that the stability fields for the solids should be the same in both diagrams. Am I missing something?

 

From: Craig Bethke

Subject: Re: Act2 mosaic plot

Perhaps this is why you seldom hear the expression "geochemist's intuition"! Seriously, although what you say seems at first glance like it might make sense, you have constructed two very different diagrams: one describes the reaction of As in a system containing S at known chemical potential, and the other reaction of S in a system containing As. To see why the diagrams differ, consider the reaction between Orpiment (As2S3) and Realgar (AsS), which appears on both plots. The reactions are:

Plot 1: As2S3 + H2O <=> 2 AsS + H2S(aq) + 1/2 O2(aq)

Plot 2: As2S3 + As(OH)3 <=> 3 AsS + 3/2 H2O + 3/4 O2(aq)

As you can see, one reaction is balanced on As and the other on S. They have differing reaction coefficients and log K's, and hence appear in on the diagram in different positions. Remember that Act2 diagrams need never be a mystery: just set "reactions = on" and all of the reactions considered in constructing the plot are written into Act2_output.txt, which you can view with any editor or browser.