Abstract.Vibrational modelling is at the present time the only known way to predict the heat capacities of the Earth's mantle minerals at high-pressure and high-temperature. To test the validity of this method for 2-type transitions, we have applied it to the c~-fl quartz transition (T o = 846 +_ 1 K). Raman spectra of quartz were recorded up to 900 K. Measured frequency shifts of the c~-quartz Raman modes were then used in conjunction with available high-pressure Raman data to calculate intrinsic mode anharmonicity, through the parameter a i = (c~LnvJ c~T)v. Vibrational modelling of the heat capacity at constant volume, Cv, and at constant pressure, Cp, including anharmonic corrections deduced from the a~ parameters, are compared to experimental data. Taking into account the soft-mode associated to the ~-fl quartz transition, the model reproduces the excess of Cp related to the transition. Then, this study confirms that detecting a soft-mode from vibrational data allows one to predict 2-type transitions. However, when modelling the thermodynamic properties, the contribution of a soft-mode cannot be established from spectroscopic data. Therefore, one needs first to determine this contribution in order to predict the heat capacities of Earth's mantle minerals displaying 2-type transitions. In c~-quartz, this contribution has been determined as 0.007% of the total number of the optic modes in the model of the density of states.

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