P-T projections

Here we explain how to configure THERMOCALC for calculating P-T projections. If you want to know more about THERMOCALC’s other functions and its history, see the overview. If you want to start using the program for the first time, see Get started with THERMOCALC. Documentation and tutorials can be found on the THERMOCALC help page.

On this page:

Accessing phase diagram calculation mode

In the prefs file tc-prefs.txt, set

calcmode 1

to make THERMOCALC run in phase diagram calculation mode.

In the scriptfile, tc-<project>.txt, set

pseudosection no

What does it do?

PT projections are grids of stable univariant and invariant reactions, without a mass balance constraint. Combined with compatibility diagrams, they provide a powerful insight into the basic phase relations of a system. Since they are independent of bulk composition, and therefore are not specific to an individual rock, there is no need to re-calculate a P-T projection if a suitable one has already been published.

Specialist scripts

Scripts are used to specify which calculations THERMOCALC should do, via the scriptfile tc-<project>.txt. In addition to the pseudosection no script, you will need to provide the following scripts for THERMOCALC 3.50 (Dec 2020 release onwards):

  • axfile <system>: use axfile tc-system.txt.
  • with <list of phases from axfile> : phases to use in this calculation; or with someof <list of phases from axfile>: choose interactively from this list during the calculation.
  • diagramPT <minP maxP minT maxT>: region in P (kbar) – T (ºC) space the diagram will cover.
  • mineral composition scripts.

A sample scriptfile for P-T projection calculations can be downloaded from the documentation and tutorials section below. In addition to the mandatory scripts listed above, this shows how additional scripting can be done, for example to control the pressure and temperature of calculations, and the output generated.

Mineral composition scripts

In phase diagram calculation mode, THERMOCALC calculates mineral/fluid compositions based on the user-specified conditions of P and/or T. However, it needs starting guesses for the phase compositions, in order to solve the non-linear equations of thermodynamic equilibrium. These are presented in the form:

A block of xyzguess scripts, specifying starting guesses for the compositional and order-disorder variables of biotite (bi).

This is one of the more difficult parts of the THERMOCALC user experience, but it’s crucial to be aware of it, since, if the starting guesses are not good enough, THERMOCALC may either fail to find any equilibrium when one exists, or may find one that is not the most stable one present.

To help users with this problem:

  • thermodynamic input files can be downloaded with suggested sets of starting guesses – but do check that they make sense for the phase compositions you’re interested in.
  • the script printguessform yes directs THERMOCALC to print the phase composition output from each calculation, in the input form shown above, in the output file tc-log.txt. These values can then be copy-pasted back into the scriptfile to start the next calculation.
  • the document compositional_variables.pdf, below, contains more information about compositional input and output in THERMOCALC.


THERMOCALC produces a number of output files containing information about the calculated mineral/fluid assemblages. Some key output files are as follows (not all files are available in older versions of the program):

tc-<project>-o.txtrecord of on-screen output and user interactive commands
tc-<project>-dr.txtoutput for plotting in DRAWPD*
tc-<project>-ic.txtcompositions in terms of oxides, site fractions,
many thermodynamic properties for phases and the system
(note that entropy and volume are considerably less
well known than relative Gibbs energies)
tc-<project>-it.txtrecord of the HPx-eos used in the calculation
tc-log.txtxyzguess scripts to use in successive calculations

*All versions produce output for plotting via Roger Powell’s program DRAWPD. The use of DRAWPD can be seen in the tutorials.

Documentation and tutorials

Sample scriptfile

The sample scriptfile tc-gridproject.txt contains scripts suitable for THERMOCALC 3.50, Dec 2020 onwards:


Simon Schorn has written a beginner’s guide to calculating P-T projections with THERMOCALC 3.50 (Dec 2020 onwards) – the first in a three-part series that links P-T projections, compatibility diagrams and pseudosections:

To follow this tutorial, you should download your own copy of THERMOCALC 3.50 (tc350beta or tc350) from the THERMOCALC download page, and a copy of DRAWPD version 1.16 or above from the DRAWPD download page.


A useful guide to computing geological phase diagrams is Roger Powell’s compilation of course notes, accompanied by some guidance on using THERMOCALC:

Perhaps the most important piece of THERMOCALC documentation available, the following notes explain compositional and order variables within phases, how to set the bulk composition and interpret the modal proportions of minerals and fluids correctly, setting phases “in excess”, solvus calculations and more. (current Dec 2020):

A handy guide to troubleshooting phase diagram calculations in THERMOCALC – for complete beginners; includes THERMOCALC error exit codes (current Dec 2020):

Key scripts available in tc350: