Open AccessEffect of temperature, pressure and iron content on the electrical conductivity of olivine and its high‐pressure polymorphs
Author(s) -
Yoshino Takashi,
Shimojuku Akira,
Shan Shuanming,
Guo Xinzhuan,
Yamazaki Daisuke,
Ito Eiji,
Higo Yuji,
Funakoshi Kenichi
Publication year - 2012
Publication title -
journal of geophysical research: solid earth
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.67
H-Index - 298
eISSN - 2156-2202
pISSN - 0148-0227
DOI - 10.1029/2011jb008774
Subject(s) - activation energy , electrical resistivity and conductivity , olivine , materials science , mineral redox buffer , polaron , analytical chemistry (journal) , fugacity , atmospheric temperature range , mineralogy , thermodynamics , chemistry , oxygen , physics , organic chemistry , chromatography , quantum mechanics , electrical engineering , engineering , electron
The electrical conductivity of olivine and its high‐pressure polymorphs with various iron contents [ X Fe = Fe/(Fe + Mg) = 0.1, 0.2, 0.3, 0.5, 0.7 and 1.0] was measured over a wide range of pressure ( P ) and temperature ( T ) conditions covering the stability field of olivine, wadsleyite and ringwoodite in a Kawai‐type multianvil apparatus. The pressure was determined using in situ X‐ray diffraction of MgO as a pressure marker in SPring 8. Molybdenum electrodes were used so that oxygen fugacity is similar to that for the iron‐wüstite buffer. The transition from low‐pressure phase to high‐pressure phase led to an increase of conductivity. In the stability field of each phase, the electrical conductivity slightly increased with increasing pressure at a constant temperature, suggesting a negative activation volume. The conductivity increased with increasing total iron content for each phase. All electrical conductivity data fit the formula for electrical conductivity σ = σ 0 X Fe exp{−[Δ E 0 − αX Fe 1/3 + P (Δ V 0 − βX Fe )]/ kT }, where σ 0 is the pre‐exponential term, Δ E 0 and Δ V 0 are the activation energy and the activation volume at very low total iron concentration, respectively, and k is the Boltzmann constant. The activation energy decreased with increasing total Fe content in olivine and ringwoodite. Dependence of the activation energy on the total Fe content suggests that the dominant mechanism of charge transport is Fe 2+ ‐Fe 3+ hopping (small polaron). The activation volume for small polaron conduction in olivine and its high‐pressure polymorphs tends to decrease with total Fe content. For olivine with low Fe content, the activation volume for small polaron conduction still is negative and very small. Assuming constant Fe content ( X Fe = 0.1) and oxygen buffer condition, the conductivity will increase with depth mainly due to the increase of the temperature along the mantle adiabat.