Theriak-Domino: Pixelmaps to Phase Formulae


Domino, in pix X Y mode, gives you all the information you need to calculate phase compositions at each grid point. This is a brief explanation how to do it.

Easy solution models

These have only one deterministic solid solution site (the alkali site in binary or ternary feldspars), or almost one site (the 8-coordinated X site in andradite-poor garnets). For feldspars, for example, Domino reports the albite, anorthite, and K-feldspar mole fractions in the solid solution directly. For example take the reported results for x_an2_[abh1] and x_an2_[san2] (PLc03 solution model, mafic rock thermodynamic data file). These are read as the mole fraction of anorthite in plagioclase, and the mole fraction of anorthite in K-feldspar, respectively. For help reading Domino output names in pix X Y mode, see here. Here is a spreadsheet example showing two feldspars reported by Domino in pix X Y mode.

Hard solution models

These have elements that exchange over multiple sites, and include the micas, pyroxenes, and amphiboles. The bad news is that phase compositions for these have to be calculated, they can’t generally be read directly from Domino results. The good news is that the calculations are straight-forward. Basically, each phase is expressed in the results as mole fractions of the solution model end members. For example x_mrb_[parg1] is read as the mole fraction of the magnesioriebeckite end member (mrb) in the CAMPG16 solution model, represented by _[parg1] (pargasite), which is the dominant end member at some grid points. See here for more help reading these codes. If you look at the solution models in the thermodynamic data file you used, you will see the end members listed (see image above for the CAMPG16 solution model, which includes parg1 and mrb). In the end member list, each has its defined composition to its right.

Here’s how to calculate a phase composition from the end member abundances reported by Domino. Select one element, multiply the molar abundance of each end member by the number of times that element appears in the end member formula, and sum all the products. Repeat for the other elements. It’s as simple as that. Here’s a spreadsheet example that shows how it works for two coexisting amphiboles reported by Domino.

If reality is more complicated that this, please let me know.