4:30 PM - 4:45 PM
[PPS22-21] Symmetry of majorite garnet in shocked chondrites revisited: A TEM study
Keywords:majorite, shock metamorphism, high-pressure phase transition, TEM
Majorite is a garnet-structured mineral with a composition on the join (Mg,Fe)SiO3-(Mg,Fe)3Al2Si3O12, which contains Si in the octahedral sites. Al-free majorite samples synthesized by Kawai-type multianvil apparatus are known to have a tetragonal symmetry (space group I41/a) [1]. However, all of natural Al-free majorite samples found in shocked chondrites have been reported to be cubic (Ia-3d) [2]. Single-crystal X-ray study of a synthetic MgSiO3 majorite clarified the tetragonal distortion of majorite is caused by Mg-Si ordering in the octahedral sites [3]. Subsequent microstructural observations of synthetic Al-free majorite by TEM concluded that the tetragonal phase is formed from the cubic phase through the cation ordering even upon rapid quenching (103 ℃/sec) and the symmetry reduction phase derives modulated and twinning structures [4]. On the contrary, cubic (Mg,Fe)SiO3 majorite in shocked chondrites is inferred to have been preserved due to higher cooling rate than that in high-pressure synthesis where the cation ordering is inhibited.
Due to very small tetragonal distortion from the cubic phase (c/a = 0.99) and very week extra reflections for the tetragonal phase, it is difficult to judge the symmetry of small amount of natural majorite samples by powder X-ray diffractometry. For the symmetry analyses of submicron-sized grains, single-crystal electron diffraction is a suitable method, since the intensities of weak reflections to diagnose the tetragonal symmetry are enhanced by the effect of dynamical diffraction. In this study, we revisit symmetries of majorite grains in shocked ordinary chondrites (Tenham; L6, Y-75100; H6) and also synthetic (Mg,Fe)SiO3 majorite by TEM. The intensity of {101} reflections, which appears only for the I41/a tetragonal phase, is under investigation by selected are electron diffraction.
References:
[1] e.g. Kato, T., and Kumazawa, M. (1985) Garnet phase of MgSiO3 filling the pyroxene-ilmenite gap at very high temperature. Nature, 316, 803-805.
[2] e.g. Smith, J.V., and Mason, B. (1970) Pyroxene-garnet transformation in Coorara meteorite. Science, 168, 832-833; Jeanloz, R. (1981) Majorite: Vibrational and compressional properties of a high-pressure phase. J. Geophys. Res., 86, 6171-6179.
[3] Angel, R.J., Finger, L.W., Hazen, R.M., Kanzaki, M., Weidner, D.J., Liebermann, R.C., Veblen, D. R. (1989) Structure and twinning of single-crystal MgSiO3 garnet synthesized at 17 GPa and 1800 ℃. Amer. Mineral., 74, 509-512.
[4] e.g. Heinemann, S., Sharp, T.G., Seifert, F., and Rubie, D.C. (1997) The cubic-tetragonal phase transition in the system majorite (Mg4Si4O12) - pyrope (Mg3Al2Si3O12), and garnet symmetry in the Earth's transition zone. Phys. Chem. Minerals, 24, 206-221.; Tomioka, N., Fujino, K., Ito, E., Katsura, T., Sharp, T., and Kato, T. (2002) Microstructures and structural phase transition in (Mg,Fe)SiO3 majorite. Eur. J. Mineral., 14, 7-14.
Due to very small tetragonal distortion from the cubic phase (c/a = 0.99) and very week extra reflections for the tetragonal phase, it is difficult to judge the symmetry of small amount of natural majorite samples by powder X-ray diffractometry. For the symmetry analyses of submicron-sized grains, single-crystal electron diffraction is a suitable method, since the intensities of weak reflections to diagnose the tetragonal symmetry are enhanced by the effect of dynamical diffraction. In this study, we revisit symmetries of majorite grains in shocked ordinary chondrites (Tenham; L6, Y-75100; H6) and also synthetic (Mg,Fe)SiO3 majorite by TEM. The intensity of {101} reflections, which appears only for the I41/a tetragonal phase, is under investigation by selected are electron diffraction.
References:
[1] e.g. Kato, T., and Kumazawa, M. (1985) Garnet phase of MgSiO3 filling the pyroxene-ilmenite gap at very high temperature. Nature, 316, 803-805.
[2] e.g. Smith, J.V., and Mason, B. (1970) Pyroxene-garnet transformation in Coorara meteorite. Science, 168, 832-833; Jeanloz, R. (1981) Majorite: Vibrational and compressional properties of a high-pressure phase. J. Geophys. Res., 86, 6171-6179.
[3] Angel, R.J., Finger, L.W., Hazen, R.M., Kanzaki, M., Weidner, D.J., Liebermann, R.C., Veblen, D. R. (1989) Structure and twinning of single-crystal MgSiO3 garnet synthesized at 17 GPa and 1800 ℃. Amer. Mineral., 74, 509-512.
[4] e.g. Heinemann, S., Sharp, T.G., Seifert, F., and Rubie, D.C. (1997) The cubic-tetragonal phase transition in the system majorite (Mg4Si4O12) - pyrope (Mg3Al2Si3O12), and garnet symmetry in the Earth's transition zone. Phys. Chem. Minerals, 24, 206-221.; Tomioka, N., Fujino, K., Ito, E., Katsura, T., Sharp, T., and Kato, T. (2002) Microstructures and structural phase transition in (Mg,Fe)SiO3 majorite. Eur. J. Mineral., 14, 7-14.