Effects of cation disordering in a natural MgAl2O4 spinel observed by rectangular parallelepiped ultrasonic resonance and Raman measurements |
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| Authors: | Hyunchae Cynn Orson L Anderson Malcolm Nicol |
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| Institution: | (1) Department of Earth and Space Sciences, University of California, 90024-1564 Los Angeles, CA, USA;(2) Present address: Department of Geology, University of California, 95616 Davis, CA, USA;(3) Center for Chemistry and Physics of the Earth and Planets, Institute of Geophysics and Planetary Physics, University of California, 90024-1564 Los Angeles, CA, USA;(4) Department of Chemistry and Biochemistry, University of California, 90024-1569 Los Angeles, CA, USA |
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| Abstract: | At moderate temperatures, the elastic properties of natural MgAl2O4 spinel differ in several significant ways from properties of synthetic spinels. Below 1000 K, the ultrasonic resonant frequencies of an ordered natural spinel change significantly after heat treatment; at higher temperatures, both types of spinels have similar resonant responses. The temperature derivatives of the elastic constants of an ordered spinel also differ from those of disordered spinels at moderate temperatures; again, at higher temperatures, both types of spinels have similar behaviors. The Raman spectra also differ below 1000 K for ordered natural and disordered spinels and are similar at higher temperatures and after cooling to ambient temperature. We associate these changes in ultrasonic resonance and Raman spectra of spinel with cation disordering at high temperature which may be quenched by cooling. We deduce estimates of the inversion parameter from the relative intensities of the two A1g Raman modes in very good agreement with estimates made from other measurements. We find thatC
11 andC
12 decrease by 4 and 8%, respectively, with 20% inversion in spinel;C
44 is less sensitive to cation order. These results imply that previous measurements of the adiabatic elastic constants of spinels at ambient conditions have been affected by the state of cation disorder of the specimen. |
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| Keywords: | Spinel cation disordering Raman ultrasonic resonance inversion elasticity |
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