Molar volumes for solid phase components (end-member compositions) are well known from X-ray diffraction studies.
The coefficients for thermal expansion are also known for many phases. Unknown thermal expansion coefficients were estimated using the observation that for phases with similar structures (e.g. phyllosilicate, amphiboles etc.) the thermal expansion coefficients are similar. The molar volumes of solids at elevated pressure and temperature are evaluated with the compressibility coefficient α and the thermal expansion coefficient β.
For some of the phases considered, compressibility coefficients were also unknown. For the estimation of these values an approach was chosen very similar to that used by Powell & Holland (1985) (for details see Gottschalk, 1997).
The integration leads with the assumption that α
and β are not a function of pressure
and temperature to:
