> -first question: it's possible to calculate a saturation index as regards to a solid solution? For example, if I've a water composition with Fe, Mg, SiO2, can I calculate the S.I. for: SOLID_SOLUTIONS 1 olivina Mg(X)Fe(1-X)SiO4 -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 composition. > -second question: it's possible to use the solid solution assemblage in the REACTION function formula with other pure phase mineral. For example (hypothetical input file): REACTION 1 Enstatite 1 Hedenbergite 1 SOLID_SOLUTIONS 1 olivina Mg(X)Fe(1-X)SiO4 -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). REACTION 1 Enstatite 1 Hedenbergite 1 Forsterite 0.9 Fayalite 0.1 1 moles in 25 steps David David Parkhurst (dlpark@xxxxxxxx) U.S. Geological Survey Box 25046, MS 413 Denver Federal Center Denver, CO 80225 Project web page: http://wwwbrr.cr.usgs.gov/projects/GWC_coupled
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