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Re: solid solution

> -first question: it's possible to calculate a saturation index as regards
a solid solution?
For example, if I've a water composition with Fe, Mg, SiO2, can I calculate
the S.I. for:

-comp Forsterite 0.9
-comp Fayalite 0.1

> or only for the pure phase?

If you are not using additional activity coefficients for the solid
components, the activity of the solid component is assumed to be equal to
the mole fraction. Equations for solid solutions are given on page 20 of
the PHREEQC manual.

If the composition of the solid is known the saturation state of the water
can be calculated by including the mole fraction of the solid in the
equation for SI:

SI = (IAP(Forsterite)/.9)/K(Forsterite)

If you have information on non-ideality, there is an extra activity
coefficient factor included with the mole fraction.

If the volume of solution is small relative to the moles of solid solution,
the mole fraction of the solid will not change much and you can simply
adjust the K and assume it is a fixed composition solid.

If the volume of solution is large or there are many pore volumes of water
moving through the solid, then you can not assume the solid is fixed

> -second question: it's possible to use the solid solution assemblage in
REACTION function formula with other pure phase mineral. For example
(hypothetical input file):

Enstatite 1
Hedenbergite 1
-comp Forsterite 0.9
-comp Fayalite 0.1
1 moles in 25 steps

No. Only pure phases or specified chemical formulas can be included in the
REACTION input. You can include Forsterite and Fayalite in specified
proportions relative to Enstatite and Hedenbergite (for example).

Enstatite 1
Hedenbergite 1
Forsterite 0.9
Fayalite 0.1
1 moles in 25 steps


David Parkhurst (dlpark@xxxxxxxx)
U.S. Geological Survey
Box 25046, MS 413
Denver Federal Center
Denver, CO 80225

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