why halite or mirabilite do not precipitate after total evaporation?

[P.Merdy]

Hello, we are trying to simulate gradual precipitation of soil minerals under evaporation. We started with a solution containing no mineral but dissolved species including Na+, Cl-, carbonates and S. We evaporated 55, 55moles of water in 10 steps. At the final step, the phase assemblage moles indicates 0 mole of water but no minerals including Na, Cl or S have precipitated. Here is the script: 

SOLUTION_MASTER_SPECIES
Si SiO2 0 SiO2 28.0843


SOLUTION_SPECIES
SiO2 + 2H2O = H3SiO4- + H+
      -log_k  -9.83; -delta_h 6.12 kcal
      -analytic   -302.3724   -0.050698   15669.69    108.18466   -1119669.0
      -Vm  7.94  1.0881  5.3224  -2.8240  1.4767 # supcrt + H2O in a1

SiO2 + 2H2O  = H2SiO4-2 + 2 H+
      -log_k  -23.0;  -delta_h 17.6 kcal
      -analytic   -294.0184   -0.072650   11204.49    108.18466   -1119669.0

H3SiO4- = HSiO3- + H2O
     -log_k -50

SOLUTION 1
   pH 9.64
   pe 0.84
   Units mmol/L     
     C(4) 67.6
     F 0.42
     Cl 66.6
     Br 0.11
     S(6) 0.48
     Ca 0.81
     Mg 0.16
     Na 140.6
     K 24.2
     Si 107.9
     Al 0.154
     Fe 0.166
     -water 1 # kg
   
EQUILIBRIUM_PHASES
     CO2(g) -3.5

SAVE Solution 1
Use Solution 1
   
EQUILIBRIUM_PHASES
       #Quartz 1 0
       #Illite 1 0
       SiO2(am) 1 0
       Calcite 1 0
       Dolomite 1 0
       Nahcolite 1 0
       Stevensite 1 0
       Fe-Kaolinite 1 0
      Fe-Illite 1 0   
      Goethite 1 0
   Halite 1 0
   Gypsum 1 0
   Alum-K 1 0
   Mirabilite 1 0
   Epsomite 1 0
   Sylvite 1 0
   water

PHASES 
 water
  H2O = H2O
  log_K 0

Stevensite
Ca0.15Na0.33Mg2.485Fe0.2Si4O10 (OH) 2 + 6H + = \
0.15Ca + 2 + 0.33Na + + 2.485Mg + 2 + 0.2Fe + 2 + 4SiO2 + 4H2O
log_k 25.45

Fe-Kaolinite
Al1.9Fe0.1Si2O5(OH)4 + 6H+ = 1.9Al+3 + 0.1Fe+3 + 2SiO2 + 5H2O
    log_k 6.471 

Fe-Illite
(K0.18)(Si3.61 Al0.39)O10 (Al0.62 Fe1.45)(OH)2  + 7.56 H+ = \
1.45Fe+3 + 0.18K+ + 1.01 Al+3 + 3.61000 SiO2 + 4.78 H2O
    log_k 9.0260

REACTION
water 1
-55.55 moles in 10 steps

END


Thank you in advance for your help.
Patricia

[dlparkhurst]

Your definitions set a target saturation index of 1, which means a mineral will not precipitate until it is 10 times supersaturated. So, I think you intended target saturation indices of 0, which would be equilibrium.

If you want to simulate to nearly total removal of water, you probably need to use pitzer.dat, which limits the set of minerals (no aluminum, minimal silica). PHREEQC will have trouble at some point when the ionic strength is too high and the mass of water is too small. Attached is a version of your file modified for pitzer.dat. It starts with 10 kg water, which helps get to higher concentrations. Some minerals were added to attain more or less the stable phase assemblage, although I don't know how close that is to reality at these high ionic strengths and using pure thermodynamics relative to a natural soil environment.

Halite does indeed form, and depending on your selection of minerals, mirabilite could also be stable at the highest concentrations.

Code: [Select]

SOLUTION 1
   pH 9.64
   pe 0.84
   Units mmol/L     
     C(4) 67.6
#     F 0.42
     Cl 66.6
     Br 0.11
     S(6) 0.48
     Ca 0.81
     Mg 0.16
     Na 140.6
     K 24.2
     Si 107.9
#     Al 0.154
#     Fe 0.166
     -water 10 # kg
   
EQUILIBRIUM_PHASES
   CO2(g) -3.5
   Chalcedony 0 0
   Calcite 0 0
   #Dolomite 0 0
   Nahcolite 0 0
   Halite 0 0
   Gypsum 0 0
   Mirabilite 0 0
   Epsomite 0 0
   Sylvite 0 0
   Trona  0 0
   Pirssonite 0 0
   Talc 0 0
   Glaserite  0 0       
INCREMENTAL_REACTIONS
REACTION
H2O 1
-555 moles in 10 steps
END


[P.Merdy]

Thank you so much, David. Your explanations and corrections were very helpful!