Structural, vibrational and thermophysical properties of pyrophyllite by semi-empirical density functional modelling

Pyrophyllite has a significant role in both geophysics as a hydrous phase, which can recycle water into the Earth's mantle, and many industrial applications, such as petroleum and civil engineering. However, very few works have been proposed to fully characterize the thermodynamic properties of this mineral, especially at atomic scale. In the present work, we report structural, vibrational, thermochemical and thermophysical properties of pyrophyllite, calculated at the density functional theory level with the hybrid B3LYP functional, all-electron Gaussian-type orbitals and taking into account a correction to include dispersive forces. V(P, T) data at 300 K fit with isothermal third-order Birch-Murnaghan equations of state and yield K-T0 = 46.57 GPa, K' = 10.51 and V-0 = 213.67 angstrom(3), where K-T0 is the thermal bulk modulus at 0 GPa, K' is the first derivative and V-0 is the volume at zero pressure, in very good agreement with recent experimental results obtained by in situ single-crystal synchrotron XRD. The compressional behaviour is highly anisotropic, with axial compressibility in ratio beta(a):beta(b):beta(c) = 1. 218:1.000:4.188. Pyrophyllite bulk modulus, thermal expansion coefficients and heat capacity at different P-T conditions are provided. The results of this kind of analysis can be useful in both geophysical and technological applications of the mineral and expand the high-temperature and high-pressure knowledge of this phase at physical conditions that are still difficult to obtain by experimental means. The simulated vibrational spectrum can also be used as a guideline by other authors in their experimental investigation of pyrophyllite.