Electrical properties of forsterite,Mg2SiO4

In this paper, we describe a model for the electrical properties of forsterite and report the results of dc conductivity measurements made on single crystals of forsterite and analyzed in terms of the model. It is proposed that the thermal band gap is $E_g=6.4\mathrm{}$ eV and that ${\mathrm{Fe}}^{3+}$ is the major electrically active impurity which introduces an acceptor level 3.0 eV above the valence band. It is found that electronic transport is isotropic, extrinsic and due to holes in the valence band, and that it predominates in the $b$ direction at all temperatures of measurement and in the $a$ and $c$ directions above ∼1700 K. At lower temperatures, extrinsic ionic transport, apparently due to magnesium ions and vacancies, predominates in the $a$ and $c$ directions. We find that the ions and vacancies are bound at lower temperatures, possibly to ${\mathrm{Fe}}^{3+}$ or ${\mathrm{Al}}^{3+}$ ions, with a binding energy of $E_b=1.9\mathrm{} \mathrm{to} 2.3$ eV, and that the ionic transport energy is $E_t\mathrm{}\left(a\right)=1.0\mathrm{}$ and $E_t\mathrm{}\left(c\right)=1.3\mathrm{}$ eV. We find no evidence for intrinsic transport of either kind up to the limit of the measurements, 1800 K. This last result may have serious consequences for the interpretation of laboratory data on the pressure and temperature dependence of transport processes in olivine as they apply to its behavior in the earth's mantle.