Elastic properties and stability of coexisting 3T and 2M(1) phengite polytypes

The elastic properties of coexisting natural 3T and 2M (1) phengite samples (Cima Pal, Sesia Zone; Val Savenca; Western Alps, Italy) with similar chemical compositions have been studied by room temperature-high pressure powder diffraction, using synchrotron radiation on the ID9A beam-line at ESRF (Grenoble, France). The P-V curves have been modelled by the Birch-Murnaghan model; a third-order expansion fitted to the experimental data yields for 3T and 2M (1) K (0)=60.4(+/- 0.7) GPa, K'=5.79(+/- 0.11) at V (0)=703.8851 angstrom(3), and K (0)=57.3(+/- 1.0) GPa, K'=6.97(+/- 0.24) at V (0)=938.8815 angstrom(3), respectively. The relative stability of 3T vs. 2M (1) has been explored as a function of pressure and temperature in terms of configuration and deformation contributions to the Gibbs energy, using the elastic properties determined here and other thermodynamic parameters from earlier investigations. The results presented agree with the hypothesis of stability of the 3T polytype in the high pressure regime.