HR: 0800h
AN: MR11A-0909    [Abstracts]
TI: High Pressure - Temperature Effects on Cation Ordering in Magnesioferrite, MgFe$_{2}$O$_{4}$, Using in situ Synchrotron X-ray Powder Diffraction up to 1430 K and 6 GPa
AU: * Antao, S M
EM: sytle.antao@stonybrook.edu
AF: State University of New York, Mineral Physics Institute & Department of Geosciences, Stony Brook, NY 11794-2100 United States
AU: Hassan, I
EM: ishmael.hassan@uwimona.edu.jm
AF: University of the West Indies, Department of Chemistry, Kingston, 7 Jamaica
AU: Crichton, W A
EM: crichton@esrf.fr
AF: European Synchrotron Radiation Facility, BP 220, Grenoble, F 38043 France
AU: Parise, J B
EM: john.parise@stonybrook.edu
AF: State University of New York, Mineral Physics Institute & Department of Geosciences, Stony Brook, NY 11794-2100 United States
AB: The structural behaviour of magnesioferrite, MgFe$_{2}$O$_{4}$, was determined from in situ synchrotron X-ray powder-diffraction data at room pressure and temperatures up to 1255 K on heating and cooling ($\lambda$ = 0.92225(4) $\AA$), and also at 6, 5, and 3 GPa and temperatures up to 1430 K ($\lambda$ = 0.3738(4) $\AA$). Cation order was analyzed in terms of the inversion parameter, {\it x}, {$^{iv}$ [Mg$_{1-x}$Fe$_{x}$]$^{vi}$ [Mg$_{x/2}$Fe$_{1-x/2}$]$_{2}$O$_{4}$}. At room pressure, {\it x} shows no change on heating until the temperature is high enough to cause exchange of Mg$^{2+}$ and Fe$^{3+}$ cations between the octahedral (vi) and tetrahedral (iv) sites. At 854 K, the sample achieves the maximum ordered state on heating ({\it x}$_{max.}$ = 0.867(4)) and begins to move towards equilibrium. Above 854 K, the cations continuously disorder along the equilibrium pathway to the maximum temperature studied ({\it T}$_{max.}$ = 1255 K, {\it x} = 0.769(3)) and reverse along the equilibrium pathway on cooling. At the blocking temperature, {\it T}$_{B}$, the maximum equilibrium order is frozen in, and maintained to room temperature, where {\it x}$_{max.}$ = 0.895(4). With increasing pressure at a constant temperature, magnesioferrite becomes more inverse. Therefore, pressure induces cation ordering in magnesioferrite. Higher pressures require higher temperatures for cations to disorder and reach equilibrium (e.g., \sim 1010 K at 6 GPa, \sim 995 K at 5 GPa, \sim 970 K at 3 GPa, \sim 854 K at room pressure). O'Neill and Navrotsky (1983) and Landau models were used to describe the equilibrium non-convergent ordering process in MgFe$_{2}$O$_{4}$, and they both fit the data well within the measured experimental range.
DE: 3924 High-pressure behavior
DE: 3954 X ray, neutron, and electron spectroscopy and diffraction
DE: 3999 General or miscellaneous
SC: Mineral and Rock Physics [MR]
MN: 2004 AGU Fall Meeting