Saturation ratio when mixing

[Sinaatalebi]

Hi all
I'm simulation the mixture of two brines. I'm investigating the potential formation of gypsum and anhydrate.
In low temperatures, the output results show that both the saturation indexes are positive, but by using Equilibrium phases, it is indicated that no anhydrate forms.
I think it is correct because anhydrate forms in high temperatures, and gypsum in low temperatures.
My question is that why anhydrate SI is still positive in low temperatures while no precipitation is observed?
Thank you in advance.

[dlparkhurst]

Attach an input file for a simple example that demonstrates the issue.

[Sinaatalebi]

SOLUTION 1   
temp     22
pH       6.19
units    ppm
Na       18300
Mg       2210
S(6)     4290
Cl       31501
-water   0.05

SOLUTION 2   
temp     22
pH       6
units    ppm
Na       59491
Ca       21895
Mg       2439
S(6)     350
Cl       137335
-water   0.05

END

MIX
1 0.5
2 0.5
END

[dlparkhurst]

Code: [Select]

SOLUTION 1   
temp     22
pH       6.19
units    ppm
Na       18300
Mg       2210
S(6)     4290
Cl       31501
-water   0.05
SOLUTION 2   
temp     22
pH       6
units    ppm
Na       59491
Ca       21895
Mg       2439
S(6)     350
Cl       137335
-water   0.05
END
EQUILIBRIUM_PHASES 1
    Anhydrite 0 0
    Gypsum    0 0
MIX
1 0.5
2 0.5
END


[Sinaatalebi]

Dear dlparkhurst
I know about the Equilibirum_phases.
My question was something else.
Without using equilibrium_phases both saturation index for gypsum and anhydrite are positive, but no anhydrite will precipitate in ambient temperature.
I wanted to know that why the index is positive for anhydrite and how it can be explained

[dlparkhurst]

Both minerals are supersaturated, but gypsum is the stable phase under these conditions. If anhydrite precipitated to equilibrium, gypsum would still be supersaturated, in which case, thermodynamic drive is to dissolve anhydrite and precipitate gypsum.