Open AccessQuantitative Olivine‐Defect Chemical Model: Insights On Electrical Conduction, Diffusion, and the Role of Fe Content
Author(s) -
Hirsch L. M.,
Shankland T. J.
Publication year - 1993
Publication title -
geophysical journal international
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.302
H-Index - 168
eISSN - 1365-246X
pISSN - 0956-540X
DOI - 10.1111/j.1365-246x.1993.tb01463.x
Subject(s) - olivine , mineral redox buffer , polaron , diffusion , vacancy defect , fugacity , wüstite , thermal conduction , crystallographic defect , materials science , analytical chemistry (journal) , mineralogy , thermodynamics , oxygen , electron , chemistry , oxide , crystallography , metallurgy , physics , organic chemistry , chromatography , quantum mechanics , composite material
Summary By simultaneously solving the governing equations for defect formation and the relevant conservation laws, a general calculation of equilibrium concentrations of point defects in olivine as functions of temperature, oxygen fugacity ( f O 2 ), and Fe content is made. We present calculations both for olivine buffered by either pyroxene or magnesiowüstite and for unbuffered olivine having fixed (Fe + Mg)/Si of 2 and Fe/(Fe + Mg) up to 0.1. the aim of this work is to better understand the mechanisms that control the transport processes of electrical conduction and diffusion. the following controlling mechanisms are consistent with calculated defect populations and experimental transport property data. Electrical conduction is dominated at low f O 2 by electrons and at high f O 2 by polaron hopping involving Fe 3+ on Mg sites (Fe Mg · ) and Mg vacancies. Mg diffuses by a vacancy mechanism whereas O and Si more likely diffuse via interstitial mechanisms. We define the intrinsic‐extrinsic division as the concentration of Fe at which the concentrations of other defects are perturbed; this division depends strongly on f O 2 and temperature. Thus, even minute amounts of Fe (i.e. Fe/(Fe + Mg) = 1 ppm) may affect material properties at temperatures below 1000 K and f O 2 above 10 −17 MPa, whereas at temperatures above 2000 K and f O 2 below 10 −9 MPa, material properties may be unaffected up to Fe/(Fe + Mg) = 0.01. Although the energy parameters in the current model are probably not unique, calculated defect populations over wide ranges of temperature, f O 2 , buffer condition, and Fe content agree well with available experimental data.