Effects of composition and high pressures on the electrical conductivity of Fe-rich (Mg,Fe)2SiO4 olivines and spinels |
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Authors: | A Lacam |
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Institution: | 1. Groupe des Laboratoires de Bellevue, C.N.R.S., E.R. 211 1, Place Aristide-Briand, F-92190, Meudon-Bellevue, France
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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: - 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.
- 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.
- 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. |
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