Synchrotron powder diffraction study of phengite 3T from the Dora Maira massif: p-V-T equation of state and petrological consequences

X-ray powder diffraction measurements have been carried out at ESRF (Grenoble, France) on the ID30 beamline to study the equation of state of 3T phengite (Dora-Maira massif, Italian western Alps) by a large volume cell up to P = 50 kbar and T = 1000 K. Several equations of state (EOS) models (the Vinet EOS, the Birch-Murnaghan EOS and its variants, a VT-polynomial expansion) have been used to interpolate the experimental data and discussed in the light of the results achieved. The thermoelastic properties of 3T phengite (bulk modulus, its derivatives versus pressure or temperature, bulk thermal expansion) have been obtained and an isochoric curve with slope P/T = 0.02 kbar/K has been calculated by means of the Vinet EOS. This slope value supports either the occurrence at the peak conditions (about 30 kbar and 1000 K) of an originally Mg/Si-richer and stiffer phengite or a non-isochoric P-T retrograde path.