PHREEQCi

["Andrew Mitchell" <acm95@xxxxxxxxxx>]

Dear Dr. Parkhurst,

I am using PHREEQCi (1.3) to help elucidate solute provenance and potential
chemical reaction processes from chemical analysis of glacial meltwaters
collected at the snouts of various glaciers.

Having used PHREEQCi for a while for inverse modelling calculations, I have
been trying to add my own phases, i.e. (i) different mineral stoichiometry,
and (ii) different phase reaction steps. Although I am probably incorrect, I
have wanted to do this because I wanted to simulate particular reactions,
not just to get an idea of the final mole transfers from particular
minerals. For example the weathering of Anorthite to Kaolinite, NOT to the
end member H4SiO4.

e.g. CaAl2Si2O8 + 2H2CO3 + H2O = Ca+2 + 2CO3-2 + 2H+ + Al2Si2O5(OH)4

not as defined in MINTEC database : CaAl2Si2O8 + 8H+ = Ca+2 + 2Al+3 +
2H4SiO4

I define the reaction within the PHASE block, and error messages are
returned

ERROR: Elements in species have not been tabulated, Al2Si2O5(OH)4.
ERROR: Reaction for species has not been defined, Al2Si2O5(OH)4.

Therefore I thought I had to define the Al2Si2O5(OH)4 species in
SOLUTION_SPECIES, as:

SOLUTION_SPECIES 1 # Do not change this number it is used by the program!
	Al2Si2O5(OH)4 = Al2Si2O5(OH)4
		log_k	0
		gamma	0 0.1
	Al2Si2O5(OH)4 + 6H+ = H2O + 2Al+3 + 2H4SiO4
		log_k	0
		delta_h	0 kJ/mol

since looking at the examples in the manual you seem to define the species,
and the associated reactions. However

ERROR: Could not reduce equation to secondary master species, Al2Si2O5(OH)4.
ERROR: Non-master species in secondary reaction, Al2Si2O5(OH)4.

was returned.  I therefore removed the Al2Si2O5(OH)4 = Al2Si2O5(OH)4
equation and was returned with the previous errors of elements not tabulated
etc. I'm obviously doing something totally wrong, I do have a bit of trouble
trying to figure out the manual, and I am definately not a natural! Could
you help on this one? How do I generally go about sucefully defining my own
phases and the reaction processes?

Many thanks,

Andrew

Andrew Mitchell
Centre for Glaciology / Centre for Research into Environment and Health,
Institute of Geography and Earth Sciences,
University of Wales,
Aberystwyth.
Ceredigion. SY23 3DB

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