Kinetics and Equilibrium > Equilibrium

Solid solutions

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**hmal**:

Dear all,

Apologies if I post in the wrong topic/folder. I am wondering if anyone know how PHREEQC allows solid solutions form/precipitate given that their compositions keep changing (during the alteration/modelling) and no defined logKs (equilibrium constants) for them in the databases?

Thank you very much!

Hmal

**dlparkhurst**:

Solid solutions contain two or more pure phases. The definitions of the thermodynamic data for the pure phases are in the PHASES data block.

At equilibrium, the following holds for each component (for an ideal solid solution):

K = IAP/X, where X is the mole fraction.

Rewriting,

X = IAP/K = SI (for the pure phase).

A solid solution will precipitate from a nonequilibrium solution when the sum of the SIs for the components is greater than 1.0. As motivation, that means the sum of the mole fractions would be greater than 1.0. The solid solution will form and adjust such that the sum of the mole fractions is 1.0.

**hmal**:

Thank you very much David. If solid solution is enable in PHREEQC this means the individual components will not precipitate as they will participate in the solid solution? Take the aragonite and strontianite solid solution for example. When we define the solid solution in the model, the aragonite and strontianite will never precipitate?

Thank you very much again!

Hmal

**dlparkhurst**:

Solid solution calculations in PHREEQC could have numerical issues, but in theory the solid solution will always be more stable than a pure phase.

**hmal**:

Thank you very much!

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