[OLD] Al+++ vs. pH using thermo.dat and thermo.com.v8.r6+.da

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From: Alexander Neaman

Subject: Al+++ vs. pH using thermo.dat and thermo.com.v8.r6+.dat

I am plotting log a Al+++ vs. pH at 25oC using Act2, version 4.0 of GWb. When thermo.dat database is used, gibbsite appears to be as a solid phase in equilibrium and (Al(OH)4)- as a species at alkaline pH. When thermo.com.v8.r6+.dat is used, however, diaspore appears to be as a solid phase in equilibrium and (AlO2)- as a species at alkaline pH. (Al(OH)4)- is not present in the thermo.com.v8.r6+.dat database and (AlO2)- is not present in the thermo.dat database. I thought that gibbsite was the stable phase at 25oC. Does anyone know why gibbsite is not stable phase using the thermo.com.v8.r6+.dat database?

From: Tom Meuzelaar

Subject: Re: Al+++ vs. pH using thermo.dat and thermo.com.v8.r6+.dat

Act2 plots different assemblages for the two databases because each database lists a different log K value at 25C for the same mineral formation reaction:

Thermo.dat, log K at 25C:

Gibbsite- 7.9603

Diaspore - 8.7517

Thermo.com.., log K at 25C:

Gibbsite- 7.7560

Diaspore- 7.1603

Act2 will plot the lowest free energy assemblage. You can remove a mineral from consideration by using the 'Config- suppress' command. Note that when using thermo.com, you would need to suppress both diaspore and boehmite to get Act2 to plot Gibbsite. The reasons for the variations in the log K values among the databases I'll leave to someone better versed in the topic.

From: Craig Bethke

Subject: Re: Al+++ vs. pH using thermo.dat and thermo.com.v8.r6+.dat

There's quite a bit of information about the various thermodynamic databases, as well as deficiencies of the aluminum data in thermo.com.v8.r6+.dat in Alex Smirnov's archive of queries to

the Users' Group: http://gwb.geo.stonybrook.edu/.

From: Jeffrey R. S. Brownson

Just commenting that the Thermo.dat values are closest to the Robie et al.

(1979) data and Apps et al. (1989) sets. I've found these original data to be the most internally consistent in the Al mineral assemblages for my own calculations.