HR: 08:05h
AN: T41B-01 INVITED     [PDF]
TI: Synthetic Grain Boundaries in Rock Forming Minerals
AU: * Heinemann, S
EM: stefan.heinemann@gfz-potsdam.de
AF: GFZ Potsdam, Telegrafenberg, Potsdam, D-14469 Germany
AU: Wirth, R
EM: wirth@gfz-potsdam.de
AF: GFZ Potsdam, Telegrafenberg, Potsdam, D-14469 Germany
AU: Dresen, G
EM: dre@gfz-potsdam.de
AF: GFZ Potsdam, Telegrafenberg, Potsdam, D-14469 Germany
AB: A grain boundary may be defined as the zone separating two crystals differing in crystallographic orientation, composition, or dimension of the crystal lattice. In polycrystalline, multiphase materials, grain boundaries or phase boundaries are present in many different configurations forming three-dimensional networks very much like the networks of liquid films that A constitute foams. Physical properties of rocks, as for example, strength, electrical conductivity and diffusivity are largely controlled by grain boundary properties. Most of our present knowledge on grain boundaries stems from studies of metals and alloys. Likewise, the structure and properties of grain or phase boundaries in ceramics consisting of complex ionic and covalent compounds are well investigated. However, relatively little is known about the structure and physical properties of grain boundaries in rocks. For example, grain boundary diffusivity and mobility depend on orientation, and they are different for low and high angle grain boundaries. We successfully synthesized bicrystals of rock-forming minerals with defined grain boundaries to investigate their physical properties. We used the direct bonding technique to avoid plastic deformation of the grain boundary region. We synthesized quartz, periclase (MgO), forsterite and feldspar bicrystals. In addition, we successfully produced a forsterite bicrystal directly from a melt using the Czochralski method. For each direct bonded bicrystal two oriented mineral single crystals were joined at room temperature and annealed at 400$^\circ$C for one week. All bicrystals were cut in two parts and one part was annealed further at 0.9 $T_m$ for 48h. Specimens were prepared for investigation in the transmission electron microscope (TEM) using the focused ion beam (FIB) technique. High-resolution TEM studies reveal similar grain boundary structures produced at 400$^\circ$C and at 0.9 $T_m$ between undisturbed crystals. This suggests that bonding of bicrystals was effective at or below 400$^\circ$C. Common structural elements of the synthetic grain boundaries encompass dislocations, steps and disconnections. }
DE: 3902 Creep and deformation
DE: 3904 Defects
DE: 3947 Surfaces and interfaces
DE: 5112 Microstructure
DE: 8030 Microstructures
SC: Tectonophysics [T]
MN: 2003 Fall Meeting