HR: 17:00h
AN: MR14A-05    [Abstracts]
TI: Spin Transition in (Mg,Fe)O at High Pressure
AU: * Fei, Y
EM: fei@gl.ciw.edu
AF: Carnegie Institution of Washington, Geophysical Laboratory 5251 Broad Branch Rd. NW, Washington, DC 20015 United States
AU: Zhang, L
EM: l.zhang@mars.swjtu.edu.cn
AF: Southwest Jiaotong University, Institute of Physics, Chengdu, 610031 China
AU: Corgne, A
EM: a.corgne@gl.ciw.edu
AF: Carnegie Institution of Washington, Geophysical Laboratory 5251 Broad Branch Rd. NW, Washington, DC 20015 United States
AU: Watson, H C
EM: h.watson@gl.ciw.edu
AF: Carnegie Institution of Washington, Geophysical Laboratory 5251 Broad Branch Rd. NW, Washington, DC 20015 United States
AU: Shen, G
EM: gshen@hpcat.aps.anl.gov
AF: HPCAT, Advanced Photon Source, Argonne national Laboratory 9700 South Cass Ave, Argonne, IL 60439 United States
AU: Prakapenka, V
EM: prakapenka@cars.uchicago.edu
AF: GSECARS, The University of Chicago, Argonne national Laboratory 9700 South Cass Ave, Argonne, IL 60439 United States
AB: Ferropericlase (Mg,Fe)O is likely the second most abundant phase in the Earth's lower mantle. Understanding its high-pressure behavior is crucial for modeling the chemistry and physics of the lower mantle. Previous experiments have showed that ferropericlase undergoes either a spin transition or dissociation into end-member oxides at pressures above 60 GPa. In this study, we report new compression data on three compositions, (Mg_0.8,Fe_0.2)O, (Mg_0.6,Fe_0.4)O, and (Mg_0.4,Fe_0.6)O, up to 140 GPa. The experiments were performed at the Advanced Photon Source, using monochromatic X-radiation and a CCD area detector. The powdered samples were sandwiched between NaCl powders that serve as pressure media and internal pressure standard. The samples were annealed at high pressures by laser heating. High-quality diffraction data were collected up to 140 GPa. For the (Mg_0.6,Fe_0.4)O composition, we observed an abrupt decrease of volume at around 52 GPa. At pressures greater than 60 GPa, the compression curve of (Mg_0.6,Fe_0.4)O is almost identical to that of MgO. We interpret that the abrupt volume change is associated with the transition of high-spin to low-spin in Fe(II). To verify that the observed volume change is caused by the spin transition rather than dissociation, we decompressed the sample from 140 GPa to ambient condition and observed reversible volume change across the transition. In order to further understand the effect of iron content on the transition, we carried out additional experiments on (Mg_0.8,Fe_0.2)O and (Mg_0.4,Fe_0.6)O and observed the abrupt volume changes at 30 GPa and 74 GPa, respectively. In all three experiments, the compression curves of the low-spin phases are identical to that of MgO, regardless the iron contents, indicating that the ionic radius of Fe(II) at low-spin state is comparable to that of Mg(II). As a consequence, the effect of iron on the volume of (Mg,Fe)O solid solution is negligible when Fe(II) is at low-spin state.
DE: 3620 Mineral and crystal chemistry (1042)
DE: 3909 Elasticity and anelasticity
DE: 3919 Equations of state
DE: 3924 High-pressure behavior
DE: 3954 X-ray, neutron, and electron spectroscopy and diffraction
SC: Mineral and Rock Physics [MR]
MN: Fall Meeting 2005