Advanced features: Entropy and Volume grids

Entropy is a measure of the heat content of a thermodynamic system and is the appropriate proxy for heat content in a variable pressure field. Both entropy and volume are "natural" variables for the visualization of magmatic phase equilibria because a magma body evolves as a consequence of heat flow out of the body (the outward flow of entropy) and deformation on the walls of the magma chamber, which controls its volume.

PhasePlot permits the calculation of phase equilibria on entropy-pressure grids, temperature-volume grids, and entropy-volume grids. The calculations are performed by minimizing at each grid point the system enthalpy, Helmholtz free energy, or internal energy, respectively. By default, temperature-pressure grids are calculated by minimization of the Gibbs free energy at each grid point.

To plot entropy on the ordinate or volume on the abscissa:

  1. Choose one of the following. Either selection repopulates the axial limits and intervals with suggested values that correspond roughly to derived limits for volume and/or entropy corresponding to default pressure and/or temperature limits.
    • Entropy (J/K-kg) from the Ordinate popup
    • Volume (cc/kg) from the Abscissa popup
  2. Click to populate the grid.

Common problems and suggestions

There are a number of numerical problems that may arise when attempting to calculate phase equilibria on non-default grids. The user should exercise some patience in coaxing these calculations along, and especially in choosing appropriate numerical values for grid limits. In general, calculations require more computing resources on non-default grids, and individual cells in the grid may need to be recomputed after adding appropriate phase suppression constraints or altering numerical convergence constraints, in order to help the program recognize and achieve an equilibrium assemblage.

Keep the following in mind:

Please send feedback if persistent problems occur. Use the "Send Feedback ..." option on the PhasePlot menu. Every effort will be made to try and correct issues related to entropy/volume grids in future releases of PhasePlot.