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How can I incorporate solid solutions into GWB Standard version 7.0? For example, I want to look into the effect of Mg++ on the stability of greenalite versus minnesotaite, both of which incorporate Mg in minor amounts.

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How can I incorporate solid solutions into GWB Standard version 7.0? For example, I want to look into the effect of Mg++ on the stability of greenalite versus minnesotaite, both of which incorporate Mg in minor amounts.

Hello:

In GWB models, only minerals of fixed composition are considered.

You can refer to page 34-35 in Craig Bethke's Geochemical and Biogeochemical Modeling textbook for a discussion on solid solutions. In short:

  • There is a dearth of thermodynamic data necessary to calculate activities, even for simple binary solid solutions
  • There are a number of conceptual and theoretical problems with incorporating solid solution theory into reaction models

You could assume ideal behavior for mineral solid solutions so that activities can be calculated based on mole fraction, but this assumption often leads to greater error than simply ignoring the solid solution altogether.

I hope that helps,

Tom Meuzelaar

RockWare, Inc.

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Hi,

as Tom has said, GWB apparently has no provision for solid-solutions. In general, geochemical modeling codes that are based on the law of mass action (LMA) method have difficulties and limitations when dealing with solid-solutions. Some LMA based programs can deal with binary non-ideal solid-solutions, and many can deal with multi-component ideal solid-solutions. There are some workarounds, for example P. Lichtner and coworkers use a so-called disrete composition approach, i.e. they slice each solid-solution into many phases of fixed composition, generate their thermodynamic properties, and then use the LMA code to find those compositions that are stable.

If you want to calculate water-mineral equilibria that involve non-ideal solid-solutions of higher order (e.g., ternary feldspars, micas, chlorites etc.), you should use a modeling code based on the Gibbs energy minimization (GEM) approach. This method of solving the equilbrium problem has no problems whatsoever with any highly nonideal phases, including solid-solutions, fluids and gases, melts etc. A code that I can recommend is GEM-Selektor, which can be downloaded for free for academic (research and teaching) puropose:

http://gems.web.psi.ch/

This code has already provision for many generic non-ideal phase models (solid-solutions, liquid solutions, aqueous models like extended Debye-Huckel, Pitzer, SIT, four different gas equations of state), but you can also directly write your own activity models in the database (no compiling, no links to dll required) using a simple scripting language. For any minerals, aqueous species and fluids, the code has built-in routines for consistent pressure and temperature corrections (not limited to saturation curve). I hope this information is of use.

Best regards,

Thomas

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