Effects of composition and high pressures on the electrical conductivity of Fe-rich (Mg, Fe)₂SiO₄ olivines and spinels

Abstract

Synthetic olivines, with composition Fa₅₀, Fa₇₅ and Fa₁₀₀, have been transformed into spinel in a laser-heated diamond-cell at pressures from 70 to 200 kbar and at a luminance temperature of about 1,200° C. The electrical conductivity σ was measured, at room temperature and up to 200 kbar, on olivine (Lacam 1982; 1983) and spinel (present study). The data obtained permit the following conclusions:

• a) Sample nature effect: under the same conditions (composition, pressure), the σ of spinel is more than three orders of magnitude of the σ of olivine.

• b) Composition effect: there are more than three orders of magnitude between the values of σ for spinels derived from initial compositions of Fa₅₀ and Fa₁₀₀, respectively.

• c) Pressure effect: The P-effect on σ is greater for olivines than for spinels.

Besides, as in the case of olivine, in spinel the σ obeys an empirical Boltzmann relation:

$$\log {\text{ }}\sigma = n \times x + S \times P + const$$

where the first and second term are the composition and pressure contributions, respectively; x the ratio Fa/Fo in mole percent.

In spinel, the activation volume, in direct connection with S, was found to be in the order of 0.3 cm³/mol, about one half of that for olivine.