5:15 PM - 7:15 PM
[SMP29-P01] Crystal symmetry of birefringence garnet by Transmission Electron Microscopy
Keywords:garnet, TEM, Symmetry
Garnets generally have space group I 41/a -3 2/d (Ia-3d) and optical isotropy. However, some natural garnets have been reported to exhibit birefringence. In particular, grossular (Ca3Al2Si3O12) - andradite (Ca3Fe2Si3O12) solid solutions have been reported to have space group I 2/a 1 2/d (unconventional setting for Fddd) (e.g., Xu et al., 2023) or I-1 (e.g., Nakamura et al., 2017) by single-crystal X-ray diffraction (XRD) experiments. In addition, it has been reported that a garnet possibly have space group I 1 -3 2/d (R-3c) by powder X-ray diffraction (PXRD) (Tančić et al., 2019). There are still some questions about the reported symmetries, and the relationship between composition and symmetry.
In this study, we analyzed the garnet from Kobushi mine, which has both areas showing birefringence and no birefringence, by polarized optical microscope, scanning electron microscope (SEM), and transmission electron microscope (TEM) to investigate the relationship between composition and symmetry.
The result of optical microscope shows that the garnet has isotropic areas and birefringence lamellae. In addition, the rim of the garnet has higher birefringence and is straight extinction. SEM-EDS revealed that the isotropic area has no Al, while the birefringence area has some Al content. The Al/(Al+Fe) ratio of the birefringence area is 0~0.3,0.5~0.8.
By TEM, the selected area electron diffraction (SAED) pattern from the isotropic area shows weak a 002 reflection that violates the extinction rules for d-glide and 41-screw axes that Ia-3d garnet has. One explanation for the appearance of the 002 reflection is the symmetry reduction from Ia-3d to cubic I 21/a -3 1 (Ia-3). On the other hand, this 002 reflection can also be explained by the Ia-3d structure, if we consider multiple scattering via high order Laue zone (HOLZ) reflection. To demonstrate this, we calculated the SAED pattern of the pure andradite crystal structure by dynamical diffraction simulation using ReciPro (Seto and Ohtsuka, 2022) software. This simulation indicated that the 002 reflection can appear by accidentally excited HOLZ reflections even when the structure has Ia-3d symmetry. Therefore, it is not clear whether the symmetry reduction to Ia-3 occurs or not.
Each area with Al/(Al+Fe) ratio =about 1/4, about 1/2, and about 2/3 was selected for TEM work. Bright-Field TEM images show fine textures with diffraction contrast from each specimen. SAED patterns and Nano Beam Electron Diffraction (NBED) patterns were taken away from the fine structure. All patterns show strong 110 reflections. Some patterns additionally show weak 002 reflections.
We determined suitable space group for the straight extinction and the experimentally obtained diffraction patterns, from the centrosymmetric t-subgroup of Ia-3d. If the garnet has space group I 41/a, optical axis of the garnet is one of these 41-screw axes [100],[010], or [001]. However, if optical axis is [001], the garnet should not show any interference color because the thin section is perpendicular to [001]. Otherwise, if the optical axis is [100] or [010], the garnet should show oblique extinction because (110) growth face is not parallel to [100] and [010]. Therefore, the space group I 41/a is not suitable for straight extinction. Assuming that the 002 reflection is caused by some symmetry reduction, the space group of the garnet is restricted to I 2/a 1 2/d (Fddd), I 1 -3 (R-3), I 2/a (C2/c), I 1 1 2/d (C2/c), or I-1.
Otherwise, in the case that the 002 reflections are not due to any symmetrical reduction and are caused just by multiple scattering via HOLZ reflections, the space group of the studied garnet can be further restricted to be I 1 -3 2/d (R-3c) or I 1 1 2/d (C 2/c). Both of these space groups are consistent with the observation results under cross-polarized microscope, which shows straight extinction.
The present results show that our sample show birefringence and symmetrical reduction with strong 110 reflections when Al enters andradite even at 1at%. However, there are no significant differences in the characteristics of the diffraction patterns were found between the regions with Al/(Al+Fe) ratio = about 1/4, about 1/2, and about 2/3.
In this study, we analyzed the garnet from Kobushi mine, which has both areas showing birefringence and no birefringence, by polarized optical microscope, scanning electron microscope (SEM), and transmission electron microscope (TEM) to investigate the relationship between composition and symmetry.
The result of optical microscope shows that the garnet has isotropic areas and birefringence lamellae. In addition, the rim of the garnet has higher birefringence and is straight extinction. SEM-EDS revealed that the isotropic area has no Al, while the birefringence area has some Al content. The Al/(Al+Fe) ratio of the birefringence area is 0~0.3,0.5~0.8.
By TEM, the selected area electron diffraction (SAED) pattern from the isotropic area shows weak a 002 reflection that violates the extinction rules for d-glide and 41-screw axes that Ia-3d garnet has. One explanation for the appearance of the 002 reflection is the symmetry reduction from Ia-3d to cubic I 21/a -3 1 (Ia-3). On the other hand, this 002 reflection can also be explained by the Ia-3d structure, if we consider multiple scattering via high order Laue zone (HOLZ) reflection. To demonstrate this, we calculated the SAED pattern of the pure andradite crystal structure by dynamical diffraction simulation using ReciPro (Seto and Ohtsuka, 2022) software. This simulation indicated that the 002 reflection can appear by accidentally excited HOLZ reflections even when the structure has Ia-3d symmetry. Therefore, it is not clear whether the symmetry reduction to Ia-3 occurs or not.
Each area with Al/(Al+Fe) ratio =about 1/4, about 1/2, and about 2/3 was selected for TEM work. Bright-Field TEM images show fine textures with diffraction contrast from each specimen. SAED patterns and Nano Beam Electron Diffraction (NBED) patterns were taken away from the fine structure. All patterns show strong 110 reflections. Some patterns additionally show weak 002 reflections.
We determined suitable space group for the straight extinction and the experimentally obtained diffraction patterns, from the centrosymmetric t-subgroup of Ia-3d. If the garnet has space group I 41/a, optical axis of the garnet is one of these 41-screw axes [100],[010], or [001]. However, if optical axis is [001], the garnet should not show any interference color because the thin section is perpendicular to [001]. Otherwise, if the optical axis is [100] or [010], the garnet should show oblique extinction because (110) growth face is not parallel to [100] and [010]. Therefore, the space group I 41/a is not suitable for straight extinction. Assuming that the 002 reflection is caused by some symmetry reduction, the space group of the garnet is restricted to I 2/a 1 2/d (Fddd), I 1 -3 (R-3), I 2/a (C2/c), I 1 1 2/d (C2/c), or I-1.
Otherwise, in the case that the 002 reflections are not due to any symmetrical reduction and are caused just by multiple scattering via HOLZ reflections, the space group of the studied garnet can be further restricted to be I 1 -3 2/d (R-3c) or I 1 1 2/d (C 2/c). Both of these space groups are consistent with the observation results under cross-polarized microscope, which shows straight extinction.
The present results show that our sample show birefringence and symmetrical reduction with strong 110 reflections when Al enters andradite even at 1at%. However, there are no significant differences in the characteristics of the diffraction patterns were found between the regions with Al/(Al+Fe) ratio = about 1/4, about 1/2, and about 2/3.