HR: 08:45h
AN: GP21C-04    [Abstracts]
TI: The Verwey Transition Under Nonhydrostatic Stress
AU: * Coe, R S
EM: rcoe@es.ucsc.edu
AF: University of California, Earth Sciences Department and Institute of Geophysics and Planetary Physics, Santa Cruz, CA 95064 United States
AB: Because the Verwey transition in magnetite is a coherent, reversible transformation that is characterized by a change in shape as well as volume, its thermodynamic phase boundary will be sensitive to nonhydrostatic stress. Synchrotron X-ray diffraction experiments by Wright [2002] imply that, as the low-temperature structure forms, the lattice extends 0.31 percent along a direction that departs by 3.7 degrees from the [111] cube diagonal and lies in the plane containing the monoclinic c-axis and bisecting the a- and b-axes. The lattice also contracts by about 0.15 percent perpendicular to that direction in the same plane and by about 0.12 percent perpendicular to that plane. Because these principal transformation strains involve both extension and contraction, the volumetric strain is considerably smaller than any of the principal strains in absolute value. Thus nonhydrostatic stress can have a larger effect on the Verwey transition temperature per GPa than hydrostatic pressure. Using Shepherd et al.'s [1991] entropy of transition of -5.91 J/mol K, thermodynamic theory predicts changes in the Vervey temperature of -23.3, 9.1, and 11.1 K/GPa for compression along each of the principal directions, respectively. This compares with -3.2 K/GPa for hydrostatic pressure. Internal nonhydrostatic stresses due to impurities and dislocations, as well as those generated by differential thermal contraction in igneous rocks, can be large, so their effect could be significant. Whether nonhydrostatic stress contributes significantly to the scatter in observed Verwey temperatures remains to be seen. One would think that as a crystal of cubic magnetite cools under nonhydrostatic stress, it would be transform to the most favorable twin orientation of the low-temperature monoclinic phase (the twin with the highest calculated Verwey temperature), unless physical constraints prevent it from doing so. In such case nonhydrostatic stress could only raise the Verwey temperature.
DE: 3939 Physical thermodynamics
DE: 1540 Rock and mineral magnetism
SC: Geomagnetism and Paleomagnetism [GP]
MN: 2004 AGU Fall Meeting