Acknowledgements: This work was supported by grants from the Austrian Science Fund (FWF), project number P33904, which is gratefully acknowledged. We thank G. Tippelt for performing the X-ray experiments and E. Forsthofer for maintaining the Materials Studio software at the Salzburg University. We also thank two anonymous reviewers for their detailed and constructive comments.Funder: Paris Lodron University of SalzburgThe cations of an ordered omphacite from the Tauern window were gradually disordered in piston cylinder experiments at temperatures between 850 and 1150 °C. The samples were examined by X-ray powder diffraction and then investigated using low-temperature calorimetry and IR spectroscopy. The low-temperature heat capacity data were used to obtain the vibrational entropies, and the line broadening of the IR spectra served as a tool to investigate the disordering enthalpy. These data were then used to calculate the configurational entropy as a function of temperature. The vibrational entropy does not change during the cation ordering phase transition from space group C2/c to P2/n at 865 °C but increases with a further temperature increase due to the reduction of short-range order
