Phys. Chem. Glasses., 2004, 45 (3), 197–214 The pressure dependence of the glass transition tempera- ture (T g ) has been estimated from the pressure effect on the activation energy of electric conductivity in alkaline (albite, haplogranite), alkaline earth (anorthite) sili- cate and SiO 2 glasses. The dc conductivity (s) has been determined from the electrical impedance spectroscopy in the frequency range 10 5 –10 -2 Hz. The impedance measurements have been performed in a controlled at- mospheric furnace and in three types of high pressure apparatuses: piston cylinder, belt and multi-anvil presses in the pressure range 0·3–6 GPa. Below T g , the activa- tion energy of s is less than that at T>T g . The inflection point on the dependency of ln(s) versus 1/T defines T g . The T g in anorthite glass varies with pressure as T g =848°C+5·3°/GPa P (P is in GPa), in albite as T g =688°C–9·4°/GPa P, in haplogranitic glass as T g =777°C–45°/GPa P and in SiO 2 glass as T g =1050°C+50°/GPa P. The measured at T g dielectric re- laxation times are several orders of magnitude smaller than the structural relaxation times and become slower with the increasing pressure. In Na bearing glasses, T g estimated from the electrical conductivity is a ‘sodium ion mobility’ T g Na , corresponding to the temperature range of the overlapped a- and b-relaxation processes and is, therefore, shifted to lower temperatures in comparison with calorimetric and dilatometric T g . The activation energy of the dielectric relaxation in anorthite increases with pressure having an activation volume of +10·5±5 cm³/mol, and in albite glass the activation volume is nega- tive -6·5±2 cm³/mol. Pressure dependence of viscosity and T g The glass transition temperature T g of silicates is the isoviscous temperature at which melts are believed to possess a viscosity ~10 12–13 Pas, and a relaxation time for shear stress of about 100 s. (1) Knowledge of the glass transition temperature at high pressures provides indirect information about the pressure dependent rhe- ology and the effect of glass densification on relaxa- tion processes. This is of special interest in geosciences and in silicate melt physics, where the viscosity meas- urements at high pressures and temperatures are tech- nically difficult to carry out, while in situ rheological measurements require x-ray radiography and synchro- tron radiation techniques. (2) The pressure dependence of silicate melt viscosities at high pressures plays a key role in the prediction of partial melting in mid-oceanic regions and ascent of magma to the earth's surface. (3) For decades, earth sci- entists have sought a general equation to model the viscosity of magmatic silicate melts as a function of temperature, pressure and composition (see, for exam- ple, Refs 4–8). In early viscosity models (4,5) the Arrhenian temperature dependence of viscosity logh T =A+(DE a /RT) (1) was suggested, where DE a is the activation energy of viscous flow, A the pre-exponential factor, R the univer- sal gas constant. Equation (1) contains two parameters both of which depend on melt composition. In Ref. 5 the number of fitting parameters in Equation (1) was reduced to DE a (A was assumed equal to -3·5 for all substances), making the use of this viscosity model highly impractical. Recent experimental results demon- strate that Equation (1) is not adequate for most silicate melts over a wide temperature range. (6,7) In recent years some new models of viscosity have been applied to sili- Pressure dependence of T g in silicate glasses from electrical impedance measurements N. S. Bagdassarov, 1 J. Maumus 2 Institut für Meteorologie und Geophysik, Universität Frankfurt, Feldbergstraße 47, D-60323 Frankfurt/Main, Germany B. Poe 3 Bayerisches Geoinstitut, Universität Bayreuth, Universitätsstraße 30, D-95447 Bayreuth, Germany A. B . Slutskiy & V. K. Bulatov Vernadsky Institute of Geochemistry and Analytical Chemistry, Russian Academy of Sciences, Kosyguine str. 19, GSP-1, 117975 Moscow, Russia Manuscript received 10 March 2003 Revision received 3 October 2003 Accepted 6 October 2003 1 Corresponding author. Email: nickbagd@geophysik.uni-frankfurt.de 2 Present address: Université Blaise Pascal, 5 rue Kessler, 63038 Clermont- Ferrand cedex, France 3 Present address: Dipartomento di Scienza della Terra, Università degli Studi “G. Annunzio” Chieti, Italy A. B. Slutskiy died on 18 March 2004 Bagdassarov et al.pmd 10/06/04, 17:11 197