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 (Grenob le, France) on the ID30 beamline to study the equation of state of 3T pheng ite (Dora-Maira massif, Italian western Alps) by a large volume cell up to P = 50 kbar and T = 1000 Ii. Several equations of state (EOS) models (the V inet EOS, the Birch-Murnaghan EOS and its variants, a VT-polynomial expansi on) have been used to interpolate the experimental data and discussed in th e light of the results achieved. The thermoelastic properties of,3T phengit e (bulk modulus, its derivatives versus pressure or temperature, bulk therm al 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 s upports either the occurrence at the peak conditions (about 30 kbar and 100 0 K) of an originally Mg/Si-richer and stiffer phengite or a non-isochoric P-T retrograde path.