KFMASH THERMOCALC Comparison #1

KFMASH THERMOCALC Comparison #2

White, Powell & Holland (JMG, 2007) THERMOCALC Comparison

Stixrude & Lithgow-Bertelloni (JGR, 05) Comparison

Kelsey et al. (JMG, 2004) THERMOCALC Comparison

Fe-Si alloys and liquid, Lacaze & Sundman (Metal. Trans., 1990), Perple_

Xvs Thermo-Calc

Perple_**X**
and THERMOCALC should yield identical results for calculations from the same
thermodynamic data and models, thus comparisons provide a simple means of
testing whether solution models and data from THERMOCALC are implemented
correctly in Perple_**X**. Conversely, once such
comparisons verify the integrity of the Perple_**X**
implementation, Perple_**X** can be used to
check that phase relations computed in THERMOCALC are correct in terms of
absolute stability. Unfortunately direct Perple_**X**
and THERMOCALC comparisons are difficult to make because of the
expertise required to assure that the models used as input to
both programs are identical. Consequently it is often difficult to assess the
origin of discrepancies between the two programs. Over the last year this
difficulty has become a concern for some users because Perple_**X**
does not implement the
equipartition constraint used in many of the THERMOCALC solution models.
This page shows the results of three comparisons made by Mark Caddick (ETH) to
assess the importance of equipartion. The comparisons show remarkable agreement,
suggesting that equipartition does not have
a large influence on computed phase relations. The consistency is surprising given that
the equipartition effect should be maximized for pelitic bulk compositions such
as those used in the comparisons. The Perple_**X ** calculations were made with thermodynamic data from hp02ver.dat while those for THERMOCALC were made with a more recent revision of the Holland & Powell (1998)
data base in which the aluminosilicate polymorph properties have been adjusted to relocate
the polymorph triple point. This adjustment accounts for most of the
discrepancies between the calculations, and these could
be eliminated by using the more recently revised version of the THERMOCALC data
base in Perple_**X** (or vice versa, dependent
on your beliefs).

The THERMOCALC and Perple_**X** input files for
this comparison are here.
The Perple_**X** calculation requires < 2
minutes of computer time (Perple_**X**
'07).

The additional field
at high temperature and low pressure in the Perple_**X** result
is due to the breakdown of muscovite (which is assumed stable for
the THERMOCALC calculation). The Perple_**X**
calculation requires < 2 minutes of computer time (Perple_**X**
'07).

Comparison of the THERMOCALC calculation shown in Fig 6 of White at al
(JMG, 2007) with a Perple_**X** calculation for
the same bulk composition. The solution models for all phases except feldspar
should be identical. The Perple_**X**
calculation was made with the Furman & Lindsley (1988) ternary feldspar model,
the corresponding THERMOCALC feldspar models were not used because the models
cannot predict the stable feldspar structural state (i.e., the structural state must be specified by the user).
As there are no discrepancies in the feldspar-absent phase field boundaries, it
appears probable the discrepancies between the two calculations are due entirely
to the choice of the feldspar model. The prominent discrepancies (marked in red)
are the absence of the ksp+pl+opx+cd+melt field in the Perple_**X**
calculation (where the feldspar is predicted to be homogeneous) and the
existence of a water under-saturated phase field at low temperature and high
pressure (this phase field is suppressed if the water-content is increased by <
1 mole, the absence of the phase field in the THERMOCALC calculation is most
likely due to the feldpar model, but also simply may be an oversight).

The
input for the Perple_**X**
calculations is
here.

This is
EXAMPLE #23 from Perple_**X**
examples, refer to the example for the input files.

The Perple_**X** input files for
this comparison are here.
**NOTE:** the input files have been modified to
use the Bio(TCC) model rather than the Bio(HP) model, this modification also
requires appropriate make definitions in the thermodynamic data file
kel04ver.dat, these definitions have been added to the Perple_**X**
666 version of kel04ver.dat, but they are not present in the Perple_**X** '07
version of kel04ver.dat.

The Perple_**X** calculation requires < 4
minutes of computer time (Perple_**X** '07).

This example is the isobaric phase diagram for the Fe-Si alloy and liquid
system as a function of temperature and composition (Si mole fraction) at a
pressure of 1 bar. The diagram on the right (Fig 6 of Lacaze & Sundman 1990)
was computed with the commercial metallurgical Thermo-Calc program (e.g.,
Sundman 1991), the diagram on the right was computed with Perple_**X**
(input files for the calculation are here).
The comparison reveals discrepancies on the order of a few degrees in some of
the peri/eutectoidal equilibrium, presumably these discrepancies reflect minor
differences between the data entered in Thermo-Calc and given by Lacaze &
Sundman (1990). The graphics algorithm used in Perple_**X**
(usually) draw the one-phase field of a stoichiometric compound if the field
terminates at a singular point, these fields (drawn in red) were added manually
to the computer generated plot. This graphics issue can be avoided by using
"unconstrained minimization" rather than "gridded
minimization" for the phase diagram calculation in VERTEX. Note that the
FeSi2 and Fe3Si7 fields are labeled incorrectly in the Thermo-Calc plot.