Vibrational entropy of disordering in omphacite

Artur Benisek; Edgar Dachs; Michael A Carpenter; Bastian Joachim-Mrosko; Noreen M Vielreicher; Manfred Wildner

doi:10.1007/s00269-023-01260-7

Vibrational entropy of disordering in omphacite

Phys Chem Miner. 2023;50(4):36. doi: 10.1007/s00269-023-01260-7. Epub 2023 Nov 27.

Authors

Artur Benisek¹, Edgar Dachs¹, Michael A Carpenter², Bastian Joachim-Mrosko³, Noreen M Vielreicher¹, Manfred Wildner⁴

Affiliations

¹ Chemistry and Physics of Materials, University of Salzburg, Jakob-Haringer-Str. 2a, 5020 Salzburg, Austria.
² Department of Earth Sciences, University of Cambridge, Downing Street, Cambridge, CB2 3EQ UK.
³ Institute of Mineralogy and Petrography, University of Innsbruck, Innrain 52, 6020 Innsbruck, Austria.
⁴ Institute of Mineralogy and Crystallography, University of Vienna, Josef-Holaubek-Platz 2, 1090 Vienna, Austria.

Abstract

The 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.

Supplementary information: The online version contains supplementary material available at 10.1007/s00269-023-01260-7.

Keywords: Autocorrelation; Calorimetry; Density functional theory; Disorder; Enthalpy; Entropy; IR spectroscopy; Line broadening; Omphacite; Order; Pyroxene.