HR: 0800h
AN: MR31B-0366    [Abstracts]
TI: Magnetic and electronic thermodynamics of fayalite
AU: Aronson, M
EM: maronson@bnl.gov
AF: State University of New York, Department of Physics, Stony Brook, NY 11794, United States
AU: * Stixrude, L
EM: stixrude@umich.edu
AF: University College London, Department of Earth Sciences, London, WC1E 6BT, United Kingdom
AU: Davis, M K
EM: mkdavis@umich.edu
AF: University of Michigan, Department of Geological Sciences, Ann Arbor, MI 48109, United States
AU: Gannon, W
EM: wgannon@umich.edu
AF: University of Michigan, Department of Physics, Ann Arbor, MI 48109, United States
AU: Ahilan, K
EM: kahilan@umich.edu
AF: University of Michigan, Department of Physics, Ann Arbor, MI 48109, United States
AB: Unique among the major elements, iron produces magnetic and electronic contributions to the thermodynamic properties or minerals, which influence phase stability and physical properties, including elasticity. To better understand these contributions, we have focused on a material that is widely studied, that may exhibit many features in common with high pressure iron-bearing phases, and which illustrates the richness and geophysical significance of magnetic and electronic excitations. We have used inelastic neutron-scattering measurements to study the magnetic excitations in the antiferromagnetic and paramagnetic phases of polycrystalline fayalite, Fe2SiO4. Sharp, nondispersing excitations are found in the ordered state, at 3.3, 5.4, 5.9, and 11.4 meV, and are interpreted as arising from the spin-orbit manifold of the high-spin Fe²⁺ ions. These excitations are increasingly damped with increasing temperature, merging into a quasielastic continuum near the 65 K Neel temperature, although their energy does not vary with temperature. We have calculated the contribution of the heat capacity arising from these magnetic excitations and found that it compares favorably with the magnetic heat capacity deduced experimentally. Our analysis indicates that the M1 and M2 sites behave distinctly. The M1 site behaves quasi-locally and appears in the heat capacity as a Schottky anomaly that explains the shoulder in the heat capacity curve near 20 K, while the M2 site contributes predominantly to the critical lambda anomaly. The behavior of fayalite illuminates the nature of magnetic states in several related minerals, including others that also show shoulders and lambda anomalies in the heat capacity (tephroite), those that show only lambda anomalies (cobalt olivine and liebenbergite), and those that show only non-lambda anomalies (bronzite, anthophyllite, and almandine). We find no evidence to support the recent claim that some transition metal silicate and germanate olivines exhibit strong geometric frustration.
DE: 3600 MINERALOGY AND PETROLOGY
DE: 3900 MINERAL PHYSICS
DE: 3909 Elasticity and anelasticity
DE: 3924 High-pressure behavior
DE: 8124 Earth's interior: composition and state (1212, 7207, 7208, 8105)
SC: Mineral and Rock Physics [MR]
MN: 2007 Fall Meeting