HR: 0800h
AN: GP41A-0862    [Abstracts]
TI: Nano-Exsolutions in Quenched Synthetic Ti-Bearing Wuestite
AU: Engelmann, R
EM: ralf.engelmann@min.uni-heidelberg.de
AF: Mineralogisches Institut Universität Heidelberg, Im Neuenheimer Feld 236, Heidelberg, 69120 Germany
AU: * Sauerzapf, U
EM: ursula.sauerzapf@min.uni-heidelberg.de
AF: Mineralogisches Institut Universität Heidelberg, Im Neuenheimer Feld 236, Heidelberg, 69120 Germany
AU: Wirth, R
EM: wirth@gfz-potsdam.de
AF: GeoForschungsZentrum Potsdam, Telegrafenberg, Potsdam, 14473 Germany
AU: Lattard, D
EM: dlattard@min.uni-heidelberg.de
AF: Mineralogisches Institut Universität Heidelberg, Im Neuenheimer Feld 236, Heidelberg, 69120 Germany
AB: In the frame of a study of the crystal chemical and magnetic properties of Fe-Ti oxides in the system Fe-Ti-O, we have synthesized assemblages of wuestite (Wus, Fe_1-y-2zTi_zO) and titanomagnetite (TM, Fe_3-xTi_xO₄, magnetite-ulvoespinel solid solution) under subsolidus conditions, at 1bar, 1300° C, under a range of oxygen fugacities. Careful SEM examination of the quenched synthesis products revealed fine exsolution features in wuestite. Though the samples were rapidly drop-quenched into water, these features originated during quenching (Simons & Woermann 1978, Senderov et al. 1993). Temperature-dependent magnetic susceptibility curves (χ-T curves) suggest that the exsolutions consist of TM with two discrete compositions (x around 0.1 and around 0.5). Thin foils (thickness < 150 nm) of the samples have been prepared with the Focussed Ion Beam (FIB) technique (Wirth 2004) and examined with the transmission electron microscope (TEM). Electron diffraction confirmed that the exsolved phase is TM. The (111) planes of the exsolved TM particles are parallel to the (111) planes of host wuestite. Bright field and dark field images reveal nanocrystallites with a particle size < 50 nm. Element mapping shows that these particles are significantly richer in Ti compared to the host wuestite. Diffraction contrasts indicate, that the TM nanocrystallites are chemically inhomogeneous or/and composed of domains with different orientations.
Based on our current results, we propose the following preliminary explanation. At the very beginning of the quenching process, i.e. at still high temperatures, a first TM generation exsolves from wuestite and segregates to nanocrystallites in the (111) wuestite planes. As the temperature further drops below the consolute temperature of the titanomagnetite series (T < 600° C ?), spinodal decomposition may result in very fine intergrowths of Ti-rich and Fe-rich TMs within the nanocrystallites. Senderov, E., Dogan, A.U., Navrotsky, A. (1993) Am. Mineral. 78, 565-573.
Simons, B., Woermann, E. (1978) Contrib. Mineral. Petrol. 66, 81-89.
Wirth, R. (2004) Eur. J. Mineral. 16, 863-876.

DE: 1540 Rock and mineral magnetism
DE: 3620 Mineral and crystal chemistry (1042)
DE: 3625 Petrography, microstructures, and textures
DE: 3630 Experimental mineralogy and petrology
SC: Geomagnetism and Paleomagnetism [GP]
MN: Fall Meeting 2005