2:45 PM - 3:00 PM
[SCG45-05] Spatially-resolved cell parameter measurement for fine lamella texture within Tenkawa iridescent garnet
Keywords:iridescent garnet, electron microscopy, electron diffraction, cell parameters, 4D-STEM, CBED
There are garnet grains that exhibit iridescence on their crystal faces, known as iridescent garnet (or rainbow garnet). Iridescent garnet basically belongs to the grandite series, the solid solution of grossular (Ca3Al3+2Si3O12)–andradite (Ca3Fe3+2Si3O12) system. It has been explained that the iridescence originates from optical interference with very fine layers, ~100–300 nm thick, named as fine lamella [e.g., 1,2]. The fine lamellae consist of two components caused by the partitioning of trivalent Al and Fe in the octahedral site.
Nakamura et al. [3] conducted X-ray diffraction (XRD) experiments on a sector of the iridescent garnet from Tenkawa village, Nara prefecture, Japan, with the support of transmission electron microscopy (TEM) and deduced that the Al-rich parts of the fine lamellae lost the cubic symmetry (space group of Ia-3d) with the ordered cation arrangement (Al3+ / Fe3+) at octahedral sites. In addition, authors performed high-spatial resolution diffractometric analyses utilizing an electron nanoprobe (i.e., 4D-STEM[4,5]) across the fine lamellae within a Tenkawa garnet grain, and we proposed the symmetry of the Al-rich parts as rhombohedral system [6]. In this study, we further report additional 4D-STEM results of the low-symmetry Al-rich region, specifically regarding the cell-parameter estimation of the low-symmetry phase.
First, we constructed 2D lattice strain map from the [110] 4D-STEM dataset, following procedures outlined in previous studies [e.g., 7]. These maps show that the εyy map reveals approximately 0.5% compression of the Al-rich zone along the y direction, perpendicular to the fine lamella plane (1-10). As the εxx value shows minimal variation, the axial length of the rhombohedral cell, arh, is presumed to be nearly identical to the surrounding cubic cell. Consequently, the compression in the y direction is attributed to the rhombohedral distortion, reflecting a slight decrease in the interaxial angle αrh from αcub (= 90°). Considering the geometry of the lattice distortion under the assumption of arh ≃ acub, the interaxial angle αrh was successfully estimated from the εyy value.
Next, we also obtained maps of the slight deviation of the [110] crystallographic axis from the same 4D-STEM dataset using transmission Kikuchi diffraction signals within diffuse scattering patterns. The [110] orientation in the Al-rich zone is slightly deviated from surrounding region. The deviation angle was quantified by positional shift of the Kikuchi patterns. Considering the geometry of the lattice distortion, the interaxial angle αrh was successfully estimated from the deviation angle of [110].
Finally, we obtained energy-filtered convergent beam electron diffraction (CBED) patterns along the [111] zone axis. In the high-order Laue zone (HOLZ) line pattern of the Al-rich zone, the 3-fold axis is absent, which is consistent with the symmetry lowering from cubic system. We performed dynamical electron diffraction calculations to simulate the HOLZ line patterns and optimize the cell parameters. Under the assumption that cell parameters of the Fe-rich zone are same with previous studies of andradite, the most appropriate value of the Al-rich zone was successfully determined.
The above estimates of the arh and αrh are consistent with each other, supporting the validity of symmetry lowering to rhombohedral structure. In the presentation, specific measurement results will be presented and discussed.
References:
[1] Hirai, H., & Nakazawa, H. (1982) Phys. Chem. Miner., 8, 25.
[2] Shimobayashi, N., et al. (2005) GAAJ Research Lab. Report, Web page content.
[3] Nakamura, Y., et al. (2017) J. Mineral. Petrol. Sci., 112, 97.
[4] Ophus, C. (2019) Microsc. Microanal. 25, 563.
[5] Igami, Y. & Miyake, A., (2023) JpGU meeting, abstract
[6] Igami, Y. & Miyake, A., (2023) Annual Meeting of JAMS, abstract
[7] Pekin, T.C., et al. (2017) Ultramicroscopy, 176, 170.
Nakamura et al. [3] conducted X-ray diffraction (XRD) experiments on a sector of the iridescent garnet from Tenkawa village, Nara prefecture, Japan, with the support of transmission electron microscopy (TEM) and deduced that the Al-rich parts of the fine lamellae lost the cubic symmetry (space group of Ia-3d) with the ordered cation arrangement (Al3+ / Fe3+) at octahedral sites. In addition, authors performed high-spatial resolution diffractometric analyses utilizing an electron nanoprobe (i.e., 4D-STEM[4,5]) across the fine lamellae within a Tenkawa garnet grain, and we proposed the symmetry of the Al-rich parts as rhombohedral system [6]. In this study, we further report additional 4D-STEM results of the low-symmetry Al-rich region, specifically regarding the cell-parameter estimation of the low-symmetry phase.
First, we constructed 2D lattice strain map from the [110] 4D-STEM dataset, following procedures outlined in previous studies [e.g., 7]. These maps show that the εyy map reveals approximately 0.5% compression of the Al-rich zone along the y direction, perpendicular to the fine lamella plane (1-10). As the εxx value shows minimal variation, the axial length of the rhombohedral cell, arh, is presumed to be nearly identical to the surrounding cubic cell. Consequently, the compression in the y direction is attributed to the rhombohedral distortion, reflecting a slight decrease in the interaxial angle αrh from αcub (= 90°). Considering the geometry of the lattice distortion under the assumption of arh ≃ acub, the interaxial angle αrh was successfully estimated from the εyy value.
Next, we also obtained maps of the slight deviation of the [110] crystallographic axis from the same 4D-STEM dataset using transmission Kikuchi diffraction signals within diffuse scattering patterns. The [110] orientation in the Al-rich zone is slightly deviated from surrounding region. The deviation angle was quantified by positional shift of the Kikuchi patterns. Considering the geometry of the lattice distortion, the interaxial angle αrh was successfully estimated from the deviation angle of [110].
Finally, we obtained energy-filtered convergent beam electron diffraction (CBED) patterns along the [111] zone axis. In the high-order Laue zone (HOLZ) line pattern of the Al-rich zone, the 3-fold axis is absent, which is consistent with the symmetry lowering from cubic system. We performed dynamical electron diffraction calculations to simulate the HOLZ line patterns and optimize the cell parameters. Under the assumption that cell parameters of the Fe-rich zone are same with previous studies of andradite, the most appropriate value of the Al-rich zone was successfully determined.
The above estimates of the arh and αrh are consistent with each other, supporting the validity of symmetry lowering to rhombohedral structure. In the presentation, specific measurement results will be presented and discussed.
References:
[1] Hirai, H., & Nakazawa, H. (1982) Phys. Chem. Miner., 8, 25.
[2] Shimobayashi, N., et al. (2005) GAAJ Research Lab. Report, Web page content.
[3] Nakamura, Y., et al. (2017) J. Mineral. Petrol. Sci., 112, 97.
[4] Ophus, C. (2019) Microsc. Microanal. 25, 563.
[5] Igami, Y. & Miyake, A., (2023) JpGU meeting, abstract
[6] Igami, Y. & Miyake, A., (2023) Annual Meeting of JAMS, abstract
[7] Pekin, T.C., et al. (2017) Ultramicroscopy, 176, 170.