p-V-T EQUATION OF STATE OF PHENGITE 3T FROM SYNCHROTRON RADIATION POWDER DIFFRACTION

A. Pavese1, G. Ferraris2, V. Pischedda2 and M. Mezouar3

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:

  1. the different EOSs used yield thermal expansion coefficients, ranging from 28 to 31 10-6 K-1, in good agreement with earlier determinations;
  2. the bulk modulus values are scattered from 51 to 58 Gpa, and show that phengite 3T is stiffer than muscovite;
  3. an isochor with slope about 0.02 kbar/K was calculated using the V-EOS and the thermoelastic data derived by the same EOS and compared with the thermobaric conditions of 30 kbar and 1000 K which have been experienced by the natural sample according to Compagnoni et al. (1995). By taking into account HRTEM results (Ferraris and Wessiken, 1998), showing that a-quartz and talc are exsoluted under p/T decreasing conditions from an originally Mg/Si-richer phengite, it is concluded that our isochor apparently does not fit petrologic data because the sample of phengite 3T used in this experiment is poorer in Mg/Si than the original one, a fact that decreases the bulk modulus as shown by the point (2) above.

 

  1. Compagnoni R., Hirajima T. & Chopin C. (1995). in Ultra high pressure metamorphism, Coleman R.G. & Wang X. Eds., Cambridge Univ. Press., Cambridge, 206-243.
  2. Ferraris C. & Wessiken R. (1998). Terra Abs. 1 suppl. to Terra nova, 10, 15.
  3. Pavese A., Ferraris G., Pischedda V. & Ibberson R. (1998). Eur. J. Mineral., submitted..
  4. Pavese A., Ferraris G., Prencipe M. & Ibberson, R. (1997). Eur. J. Mineral., 9, 1183-1190.
  5. Sassi F.P., Guidotti C., Rieder M. & De Pieri R. (1994). Eur. J. Mineral., 6, 151-160.