Input equation for Pyroxene and Hornblende in PhreeqC

[evansmanu]

Dear all,

Please I am using the Phreeqc.dat database and have also looked in the other PhreeqC databases, but I do not find the above mineral phases to include in my model. Please can someone be of help?

Regards

[dlparkhurst]

There are instances of pyroxene and hornblende minerals in the databases distributed with PHREEQC. If they are not the ones you want, then you will need to find other sources of thermodynamic data or make assumptions about log K vs stoichiometry.

Note that the reactions for silicate minerals in llnl.dat need to be converted to reactions involving H4SiO4 for most other databases. llnl.dat uses SiO2 for the master species, so you need to add the appropriate amount of the following reaction to the llnl.dat reaction

2H2O + SiO2 = H4SiO4 (log K assumed to be 0)

to obtain a balanced chemical for phreeqc.dat and other databases.

If you are doing inverse modeling, the log K does not matter, only the balanced chemical reaction.

[evansmanu]

Thanks very much David

[evansmanu]

Dear David,

Many thanks for your support. Please I would like to be educated on the issue of using the results of the Foward model to verify the inverse model results. I want to understand what I should look out for in the forward model results to verify if my inverse model result is in line or not. Here I have provided the model results of the phase mole transfer for the inverse model as well as the Forward model.

INVERSE MODEL RESULTs

Phase mole transfers:                 Minimum        Maximum
    Plagioclase      3.931e-04      3.688e-04      4.175e-04   Na0.62Ca0.38Al1.38Si2.62O8
  Chlorite(14A)     -2.081e-04     -2.242e-04     -1.919e-04   Mg5Al2Si3O10(OH)8
           CaX2      3.501e-04      2.988e-04      4.015e-04   CaX2
            NaX     -7.003e-04     -8.030e-04     -5.976e-04   NaX
        Calcite     -1.445e-03     -1.566e-03     -1.324e-03   CaCO3
       Dolomite      1.172e-03      1.083e-03      1.260e-03   CaMg(CO3)2
      Muscovite      1.688e-05      1.455e-05      1.921e-05   KAl3Si3O10(OH)2
         Halite      9.629e-04      9.022e-04      1.024e-03   NaCl
         Gypsum     -9.807e-05     -1.313e-04     -6.480e-05   CaSO4:2H2O
      Kaolinite     -8.816e-05     -1.049e-04     -7.146e-05   Al2Si2O5(OH)4

FORWARD MODELING RESULTS

Phase Assemblage

Phase               SI  log IAP  log K(T, P)   Initial       Final       Delta

Calcite           0.00    -8.48     -8.48    0.000e+00   5.247e-05   5.247e-05
Chlorite(14A)    -4.09    64.29     68.38    0.000e+00           0   0.000e+00
Dolomite          0.00   -17.09    -17.09    1.000e-01   9.981e-02  -1.862e-04
Gypsum           -3.85    -8.43     -4.58    0.000e+00           0   0.000e+00
Halite           -3.79    -2.22      1.57    1.000e-01           0  -1.000e-01
Kaolinite       -10.47    -3.04      7.43    0.000e+00           0   0.000e+00
Muscovite        -0.00    -0.00      0.00    1.000e-01   1.000e-01   2.109e-08
Plagioclase      -0.00    -0.00      0.00    1.000e-01   1.000e-01  -7.065e-08

Please your help is needed for me to understand this important part of my study.
 

[dlparkhurst]

I don't think you can verify an inverse model with forward modeling.

The inverse model should produce a reaction(s) that reproduce the final solution, given the uncertainty constraints. It is possible to force a forward model to have the same result, but you probably have not added much additional information.

You should consider whether the reactions are consistent with saturation indices. For example, it looks like gypsum is undersaturated, so precipitation of gypsum is unlikely. If this is a reducing environment, sulfate reduction is possible; otherwise, you must consider whether the difference in sulfate is significant, or if the waters are on the same flow path.

The other question is whether the reaction is consistent with the mineralogy. Do the minerals proposed to be dissolving show dissolution textures and the minerals proposed to be precipitating show consistent morphology?

[evansmanu]

Dear David,

I am grateful for your explanation. from the two solutions, which are the initial solution (rainwater) and final end member solution It is evident that sulphate reduction does exist in the environment. There is generally a low sulphate concentration of groundwater in the area. Here are the input data for your kind consideration. Find attached the complete phreeqc model

 I have also understood from your explanation that I have to check the SI produced from the Forward model and compare with the phase mole transfers if indeed they are in agreement with dissolution (SI<0) or precipitation (SI>0)

SOLUTION 15 Rain_Water
    temp      25
    pH        4.68
    pe        4
    redox     pe
    units     mg/l
    density   1
    C(4)      6.66
    Ca        0.81 charge
    Cl        4.46
    K         0.58
    Mg        0.25
    Na        0.37
    S(6)      11.1
    -water    1 # kg
SOLUTION 12 Group 1 Marc_Cluster
    temp      28.07
    pH        5.85
    pe        4
    redox     pe
    units     mg/l
    density   1
    Al        0.019499621
    C(4)      61.45
    Ca        10.86
    Na     12.01
    Cl        9.83 charge
    K         1.24
    Mg        3.44
    S(6)      1.68
    Si        16.83
    -water    1 # kg