HR: 1330h
AN: V42A-0319    [PDF]
TI: A Model for Shear Mechanism in the Pyroxene-Ilmenite Transition in MgSiO$_{3}$
AU: * Tomioka, N
EM: nao@kobe-u.ac.jp
AF: Department of Earth and Planetary Sciences, Faculty of Science, Kobe University, Rokkodai 1-1, Nada, Kobe, 657-8501 Japan
AB: Phase transition mechanisms of silicate minerals under high-pressure play significant roles to the rheological properties and transformation kinetics in the EarthOs mantle. To understand the dynamics of the EarthOs interior, previously, many researchers have studied the transformation mechanisms of olivine, which is the most abundant mineral in the upper mantle, to wadsleyite or ringwoodite structures. Poirier (1981) first proposed a shear-promoted mechanism for the transition from olivine to ringwoodite in Mg$_{2}$ SiO$_{4}$ . This diffusionless mechanism forms coherent lamellar intergrowths of product high-pressure phases in the host minerals. Pyroxene is the second major constituent of the upper mantle. However, there is no direct experimental work for the transition mechanism of MgSiO$_{3}$-rich pyroxene under high pressure. In a natural shocked chondritic meteorite, (Mg,Fe)SiO$_{3}$ ilmenite (akimotoite) (Tomioka and Fujino 1998) was found to be intergrown in the host (Mg,Fe)SiO$_{3}$ clinopyroxene and they have topotaxial relationships similar to that formed by the shear mechanism. In this study a model of shear mechanism for the MgSiO$_{3}$ clinoenstatite to akimotoite transition based on their natural occurrences and the topological study on both the structures is proposed. The shear mechanism in the high pressure clinoenstatite (s.g.C2/c)-akimotoite (R-3) transition can be expressed by the sweeping of partial dislocations associating cation shuffling without long-range atomic diffusion. The shortest translation vector [001] of clinoenstatite on (100) plane would dissociate into 1/3[001]+1/6[011]+1/3[001] +1/6[0-11] and the first two partial dislocations bring about the hcp oxygen sublattice for the akimotoite structure. This transition mechanism possibly occurs under high shear stress or under high overstepping pressure at relatively low temperature like that was suggested for the olivine-ringwoodite transition.
DE: 1213 Earth's interior--dynamics (8115, 8120)
DE: 3662 Meteorites
DE: 3924 High-pressure behavior
DE: 8124 Earth's interior--composition and state (old 8105)
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2003 Fall Meeting