Electrical conductivity measurements on synthetic olivines and on olivine,enstatite and diopside from Dreiser Weiher,Eifel (Germany) under defined thermodynamic activities as a function of temperature and pressure

Results of electrical conductivity measurments on synthetic olivines of the system Mg₂SiO₄Fe₂SiO₄ and on minerals of Dreiser Weiher peridotite nodule Ib-8 (Eifel, Germany) are discussed in relation to the measuring procedure and to the variation of thermodynamic parameters.The measurements were performed in solid state high-pressure vessels between 340 and 1100°C and at a pressure of 10 kbar. It is shown that for ternary olivines and for pyroxenes, the control of two further variables, like the chemical activities of the components, is needed besides temperature and pressure control. The experimental set-up for the control of chemical activities and oxygen partial pressure is shown. From the slopes of the lines of log σ against 1/T the activation energies were calculated. Though in most cases the same oxygen fugacity ?_O2 is applied, the results reveal different values for synthetic and natural samples since the chemical activities of SiO₂ are different.