Precipitating Certain Minerals

[Dwalker]

I'm trying to figure out how to only allow certain minerals to precipitate and exclude other minerals that would also precipitate. Currently, I am trying to accomplish this goal.
 Add the following components and concentrations:
 [Ca2+]= 1x10-3 M
 [PO43-]= 1x10-5 M
 [Fe3+]= 1x10-5 M
 Run system fixed at pH 4 and 8
– Allow only the following solids to precipitate
• Strengite: FePO4* 2H20
• Hydroxyapatite: Ca5(PO4)3(OH)

I have tried using the Equilibrium and forcing them to both precipitate and I also tried to precipitate the mineral I know would occur at the certain pH assigned. Goethite always precipitates though and causes Strengite to have a -0.04 SI. The only idea I have is changing the ionic strength but I do not know where I could change its value at.

[dlparkhurst]

Only minerals defined in EQUILIBRIUM_PHASES can precipitate. If you do not include a mineral, then it cannot react.

In addition, you can require minerals only to precipitate or only to dissolve as part of the EQUILIBRIUM_PHASES definitions.