If water evaporates and solutes are concentrated so much that a mineral precipitates, the concentrations
become fixed if the concentration ratios in solution are equal to the stoichiometric ratios in the mineral.
However, if the ratios are different, the solute with the higher concentration increases while
all the other constituents of the mineral decrease in
concentration as soon as the mineral starts to precipitate.
The reason is that subtraction of the precipitated amount works out differently for the solutes. For example, if
gypsum precipitates:
Ksp = [Ca+2] [SO4-2] = n [(Ca+2)init - y] [(SO4-2)init - y]
where n is the concentration factor (-);
y is the amount of precipitate (mol/L);
(i)init indicates the initial concentration (mol/L);
Ksp is the solubility product of gypsum, leaving out water for simplicity.
Also, activity [i] is assumed equal to concentration (i) in mol/L.
Subtracting y from a small initial concentration of Ca+2 will give a smaller number than
subtracting y from a higher initial concentration of SO4-2. Since the product
of the two must remain equal to the solubility product, SO4-2 will
increase, and Ca+2 will decrease as n and y increase.
Running the PHREEQC file evap.phr shows the effects.
In the first simulation, the initial Ca+2- and
SO4-2-concentrations are both 3.5 mmol/L, and they remain exactly equal when the concentrations increase as water evaporates (the concentrations of SO4-2 and Ca+2 overlap in the figure).
The concentrations become fixed at 10-1.8 M when the solubility of gypsum is reached.
In the second simulation, the initial SO4-2
concentration is increased to 7 mmol/L, and the concentrations diverge when gypsum precipitates.
The evaporation of water is simulated with keyword REACTION.
Common precipitates are calcite (CaCO3), gypsum (CaSO4.2H2O), and Mg-silicate. The chemistry of salt lakes is often rather simple due to the diversion effect and dominated by only a few cations and anions, resulting in {(Ca, Mg)-Cl}, {(Na, Mg)-(Cl, SO4)} or {Na-(Cl, SO4, CO3)} water types.