HR: 0830h
AN: V31D-0956    [PDF]
TI: Electron Channelling Spectroscopy of Iron in Majorite and Silicate Perovskite
AU: * Miyajima, N
EM: enkichi@issp.u-tokyo.ac.jp
AF: Institute for Solid State Physics, University of Tokyo, 5-1-5 Kashiwanoha, Kashiwa, Chiba, 277-8581 Japan
AU: * Miyajima, N
EM: enkichi@issp.u-tokyo.ac.jp
AF: Bayerisches Geoinsitut, Bayerisches Geoinstitut, Universitat Bayreuth, Bayreuth, D-95440 Germany
AU: Langenhorst, F
EM: Falko.Langenhorst@uni-bayreuth.de
AF: Bayerisches Geoinsitut, Bayerisches Geoinstitut, Universitat Bayreuth, Bayreuth, D-95440 Germany
AU: Frost, D J
EM: Dan.Frost@uni-bayreuth.de
AF: Bayerisches Geoinsitut, Bayerisches Geoinstitut, Universitat Bayreuth, Bayreuth, D-95440 Germany
AU: Yagi, T
EM: yagi@issp.u-tokyo.ac.jp
AF: Institute for Solid State Physics, University of Tokyo, 5-1-5 Kashiwanoha, Kashiwa, Chiba, 277-8581 Japan
AB: Results from high temperature and pressure experiments indicate that MgSiO$_{3}$ silicate perovskite is likely the most important carrier mineral phase for trivalent cations such as aluminium and ferric iron in the Earth$^{'}$s lower mantle. Due to their substitution into A$^{2+}$B$^{4+}$O$_{3}$ perovskite, a certain amount of oxygen vacancies might be stable in the crystal structure. The incorporation mechanisms are expected to play a major role in determining the physical and chemical properties of perovskite. The crystal structure has two potential lattice sites for trivalent cations. To clarify the crystal chemistry of Al$^{3+}$ and Fe$^{3+}$-bearing perovskite, it is necessary to determine the site-specific affinities. Majorite and silicate perovskite assemblages synthesized from natural pyroxene in the multi-anvil press and diamond-anvil cell were studied by energy dispersive X-ray spectroscopy (EDXS) and electron energy-loss spectroscopy (EELS) in a transmission electron microscope (TEM) to quantify the composition and oxidation state of iron. In these assemblages of the (Mg,Fe)SiO$_{3}$-Al$_{2}$O$_{3}$ system, ferric iron has a much stronger affinity to silicate perovskite than to majorite at the same pressure, temperature and oxygen fugacity. To determine the site occupancy of iron in majorite and silicate perovskite, we have also performed both spectroscopic techniques (EDXS and EELS) under channelling conditions. Electron channelling spectroscopy enables us to distinguish the octahedral sites of both phases from other cation sites by variations in the intensity ratios of the EDXS and EELS spectra acquired systematically at tilting conditions about the channelling-sensitive crystal plane and zone axis. In majorite almost all of ferric iron is partitioned into the octahedral site, whereas in silicate perovskite both ferrous and ferric iron substitute preferentially into the pseudo-dodecahedral site of the perovskite structure. The latter is charge-balanced by aluminium in the octahedral site.
DE: 3600 MINERALOGY AND PETROLOGY (replaces
DE: 5100 PHYSICAL PROPERTIES OF ROCKS
DE: 7200 SEISMOLOGY
DE: 8100 TECTONOPHYSICS
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2003 Fall Meeting