HR: 0800h
AN: T21C-0497    [Abstracts]
TI: Experimental deformation of Al2SiO5 polymorphs
AU: * Goergen, E T
EM: goer0074@umn.edu
AF: University of Minnesota, Department of Geology and Geophysics, Minneapolis, MN 55455 United States
AU: Whitney, D L
EM: dwhitney@umn.edu
AF: University of Minnesota, Department of Geology and Geophysics, Minneapolis, MN 55455 United States
AU: Zimmerman, M E
EM: zimme030@umn.edu
AF: University of Minnesota, Department of Geology and Geophysics, Minneapolis, MN 55455 United States
AB: The 3 Al2SiO5 polymorphs (andalusite, kyanite, sillimanite) are common in metapelitic rocks and impure quartzite, and are very useful for determining metamorphic conditions. Owing to their sluggish reaction kinetics, 2-3 polymorphs may coexist metastably, allowing interpretation of part of the pressure-temperature path if the sequence of crystallization can be determined. To investigate the role of deformation on polymorphic transformation, textures, and crystallization sequence; determine the mechanical properties and relative strengths of Al2SiO5 phases; and develop the Al2SiO5 minerals as strain gauges to compare with quartz fabric interpretations, we experimentally deformed fine-grained aggregates of each polymorph in gas medium deformation apparatus in two different deformation modes: shear and torsion. In both experiments, we simultaneously deformed a stack of hot-pressed discs of And, Ky, and Sil. Because these phases are very strong, we used alumina pistons in the deformation column and conducted the experiments at high T, in the stability field of sillimanite. Shear experiment conditions: 1000° C, 300 MPa confining pressure, 4hrs, 150% shear strain. Torsion conditions: 1250° C, 300 MPa, constant shear strain rate of 2 x 10E-4 s-1 to 400% shear strain, shear stress varied from max = 200 MPa for the first 200% shear strain to an average of 100 MPa during the last 200%. Powder XRD and EBSD analyses show no polymorphic transformation in the shear experiment, and initial EBSD results for the torsion experiment suggest that most of the grains did not transform, although we have not evaluated whether transformation occurred at the submicron scale. In the shear experiment, Ky developed a strong shape and weak crystallographic preferred orientation (CPO), with strongest alignment along (100), 15 degrees oblique to the shear plane. Andalusite and Sil developed a strong CPO, with (001) slightly oblique (5 degrees) to the shear plane; slip was along the dominant glide systems for both. In torsion, Ky and And experienced grain size reduction compared to the starting material, but Sil experienced grain growth. We could not obtain good diffraction patterns from Ky in the torsion experiment, but And shows a strong CPO (consistent with the glide (110)[001]) and Sil a variable CPO depending on proximity of grains to the contact with the Ky disc. Our preliminary results suggest that Sil is the most resistant to deformation even at high T within the Sil stability field. In contrast, Ky and And deform more easily under the conditions of our experiments. Further experiments on the individual phases will be conducted to determine deformation mechanisms, and results compared with qz experiments at the same conditions.
DE: 3612 Reactions and phase equilibria (1012, 8412)
DE: 3902 Creep and deformation
DE: 8412 Reactions and phase equilibria (1012, 3612)
SC: Tectonophysics [T]
MN: Fall Meeting 2005