Demonstration of the electrical conductivity jump produced by the olivine-spinel transition

The electrical conductivity of both the olivine and spinel polymorphs of Fe2SiO4 has been determined at 300 to 1200°K and at 31 to 62 kb by means of a tetrahedral anvil-type high-pressure apparatus. It is established that the olivine-spinel transition is responsible for the discontinuous change in the electrical conductivity of Fe2SiO4, the conductivity jump at the time of the formation of spinel from olivine amounting to about two orders of magnitude at 900°K. It is suggested that this discontinuous increase in the electrical conductivity of Fe2SiO4 arises chiefly from a decrease in the activation energy which may be directly related to the smaller molar volume of the spinel polymorph. The electrical conductivity distribution within the earth's upper mantle as related to the experimental results is discussed.