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Novel phase transition in orthoenstatite

Jackson, Jennifer M. and Sinogeikin, Stanislav V. and Carpenter, Michael A. and Bass, Jay D. (2004) Novel phase transition in orthoenstatite. American Mineralogist, 89 (1). pp. 239-244. ISSN 0003-004X. http://resolver.caltech.edu/CaltechAUTHORS:20130422-104015762

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Abstract

Single-crystal Brillouin scattering measurements on natural orthoenstatite [OEN] to 1350 °C at 1 atm show significant softening of the elastic moduli C_33 and C_55 ahead of a phase transition. To our knowledge, these are the first observations of acoustic mode-softening in orthoenstatite at high temperature and room pressure and could have important implications for Earth's mantle. The phase transition is rapid and shows some hysteresis in the observed transition temperature, T_tr. Experiments performed on increasing and decreasing temperature bracket the transition temperature between 1090(10) °C ≤ T_tr ≤ 1175(10) °C, and pronounced acoustic mode-softening is evident at temperatures above 900 °C. Backscattering measurements to T = 1350 °C show no evidence for additional transitions. OEN was recovered at room temperature. Our results are interpreted in terms of elastic softening ahead of a displacive phase transition. Before the displacive transition can occur, however, the elastic softening appears to trigger the observed reconstructive transition to the more-stable protoenstatite (or high clinoenstatite) structure. We suggest that the displacive phase transition would lead to a previously unreported pyroxene structure with Cmca symmetry.


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Additional Information:© 2004 Mineralogical Society of America. Manuscript received May 16, 2003; manuscript accepted August 19, 2003; manuscript handled by Simon Kohn. We thank George Rossman (California Institute of Technology) for providing the orthoenstatite samples (GRR 649, AMNH 100634), Ian Steele (University of Chicago) for assistance with the microprobe analyses, and Scott Wilson (UIUC) for the XRD analyses. We are also grateful to Charlie Prewitt, Bruno Reynard, Philippe Gillet, Ross Angel, Wolfgang Sturhahn, Holger Hellwig, Donald Lindsley, and Dmitry Lakshtanov for helpful discussions. Richard Harrison and Ross Angel are thanked for their helpful reviews.
Record Number:CaltechAUTHORS:20130422-104015762
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Deposited By: Jason Perez
Deposited On:23 Apr 2013 21:17
Last Modified:23 Apr 2013 21:17

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