p-V-T EQUATION OF STATE OF PHENGITE 3T FROM SYNCHROTRON RADIATION POWDER DIFFRACTION
1Dpt. Sci. Terra, Univ.
Milano, Italy
2Dpt. Sci. Mineral. Petrol., Univ. Torino, Italy - ferraris@dsmp.unito.it
3ESRF, Grenoble, France
Keywords: Powder patterns, non
ambient-conditions, phengite 3T, equation of state
Micas are sheet silicates formed by stacking one octahedral (O) and two tetrahedral (T) sheets, so as to obtain TOT layers. Order/disorder phenomena and occurrence of one polytype rather than others have been suggested to be related to the environmental conditions (Sassi et al., 1994). Previous works by the authors' group (Pavese et al., 1997, 1998) have shown ordering phenomena and discussed energetics connected with 3T and 2M1 polytypes.
The study of the p-V-T equations of state (EOS) of minerals is a very key to interpret petrologic data, to model the interior of the Earth and to provide insights on the thermodinamics and stability of all phases that experience non-ambient thermobaric conditions. X-ray powder diffraction measurements have been carried out at ESRF (Grenoble, France) on the ID30 beamline to study the EOS of a phengite 3T sample from a well known ultra-high pressure outcrop of Dora-Maira massif (Italian western Alps; Compagnoni et al., 1995). High pressure and high temperature were achieved by means of a Paris-Edinburgh-like cell and were gauged by NaCl as internal calibrant and CrAl-thermocouple, respectively. Measurements were performed along seven isotherms, from 300 up to 1000 K, over 0-50 kbar p-range, by 100 K and 3-5 kbar steps (88 patterns in total).
Several models of EOS (the Vinet EOS, the Birch-Murnaghan EOF and its variants, VT-polynomial expansion) have been used to interpolate the experimental data, for determining the thermoelastic properties of phengite 3T (bulk modulus, its derivatives versus pressure or temperature, bulk thermal expansion). Main results are: