Chemical Weathering in changing rock-water ratio

[Arindam Basu]

Hello All,
I am new to PHREEQC. I am trying to assess that during chemical weathering of a specific rock how the concentration of various ions in aqueous solution vary with changing rock : water ratio. Also I am trying to assess the appearance and disappearance of certain secondary minerals with changing rock : water ratio. I am trying to do this evaluation using PHREEQC by taking river water solution and reacting it with a bulk rock composition. I am attaching the PHREEQC file with this message. Please take a look at it and let me know if this is the correct approach to do this task. I have incorporated some of the discussions in this forum to develop this model.
Some of the problems that I am facing include : 1. Some of the primary minerals such as k-feldspar , diopside are appearing as secondary minerals in the final output 2. Some secondary minerals like goethite are not appearing in the final output.

[dlparkhurst]

I don't know whether you approach is correct. You are assuming all minerals of your rock dissolve at the same rate. Maybe that makes sense for a laterite soil. For other environments, you could dissolve the minerals at different rates.

(1) If you do not want primary minerals to form, then do not include them in EQUILIBRIUM_PHASES.

(2) Thermodynamics says that hematite is less soluble than goethite, so goethite will not form if you include hematite in EQUILIBRIUM_PHASES.

[Arindam Basu]

Thank you for your valuable response. It will help me a lot in preparing the model.