9:45 AM - 10:00 AM
[PPS08-04] Analysis of Crystal Orientation of Plaquette Magnetite in Ryugu Particles
Keywords:Ryugu, Magnetite, Carboneceous chondrite, crystallography
Introduction The sample particles brought back from the C-type asteroid Ryugu by JAXA's Hayabusa2 were found to resemble CI chondrites based on initial analysis. Ryugu particles mainly consist of sheet silicate minerals, with magnetite, pyrrhotite, and carbonates present in the matrix. Initial analysis using 3D XnCT revealed that magnetite exists in various shapes (plaquette, spherical, framboidal, rod-like, equant, etc.) distinct from other minerals. The diversity in the morphology of magnetite particles is believed to result from highly supersaturated aqueous solutions generated because of the dissolution of amorphous silicate minerals by melting ice, leading to a decrease in the degree of supersaturation. It is considered that insights into their detailed formation and evolution can be gained by the classification of the various morphologies using the shape and crystallographic orientations of magnetite. While the 3D shapes of magnetite have been revealed using XnCT, their crystallographic orientations remain unknown. Therefore, in this study, a technique combining XnCT with TEM and SAED was developed to describe the 3D crystal shapes, including crystallographic orientations, and applied to magnetite crystals in Ryugu samples. Herein, we report on the magnetite particles with plaquette morphology.
Samples and Methods We analyzed an analysis of plaquette magnetite particles in particles A0063-FC010 and A0067-FIB006. Ono et al. (2024) described the method for combining CT images, TEM images, and SAED information in detail. One plaquette magnetite particle was examined in A0063-FC010 (Mt_1), while two plaquette magnetite particles were present in A0067-FIB006 (Mt_2 and Mt_3). In Mt_3, SAED was acquired for each of the two plates, while in Mt_1 and Mt_2, SAED was obtained at one location each. For each plaquette magnetite particle, we accurately established the relationship with the 3D crystal shape between crystal orientation from CT images and SAED patterns.
Results and Discussion In Mt_1 of A0063-FC010, it was observed that the platelets constituting the plaquette are stacked in nearly the [101] direction. The platelets were also found to be stacked in approximately the [101] direction in Mt_2 of A0067-FIB006. In Mt_3, SAED patterns were obtained for two adjacent platelets, both of which were stacked in mostly the [001] direction. However, the crystal orientations perpendicular to the stacking direction of each platelet did not match; one platelet's [100] and the other's [110] were nearly aligned in the same direction (i.e., the two platelets were rotated relative to the stacking direction by approximately 45°). Similar results have been reported in previous studies (Chen et al., 2016), where SEM/EBSD mapping results showed groups of platelets with nearly identical crystal orientations stacking along the [001] direction while rotating about the [001] axis. Mt_2 and Mt_3 are proximally located, suggesting that the plaquette with [001] and [101] stacking is formed at the same location. Alternatively, it is possible that a single plaquette contains a mixture of [001] and [101] stacking. Furthermore, since there is internal vacancy (or full of material resembling the matrix in the vacancy) within the platelets in plaquette magnetites observed in this study, the previous study was not reported the presence of such vacancy within the plaquette interior.
Summary In this study, we studied the crystal orientations of plaquette magnetite from two Ryugu particles using 3D CT images and SAED patterns. This revealed that plaquette magnetite not only stacks in the [001] direction, as indicated by previous study, but also in the [101] direction. If Ryugu and CI chondrite plaquettes include the same structural features, it is conceivable that plaquettes stack in either the [001] or [101] direction, with groups of plates sharing the same crystal orientation rotating about the stacking direction. Such complex structures are considered to result from the rapid growth of plaquettes under relatively high saturation conditions. Going forward, we try to obtain SAED patterns of the orientations of multiple platelets within the same plaquette (or conduct electron diffraction mapping) to further reveal how magnetite is stacked.
Samples and Methods We analyzed an analysis of plaquette magnetite particles in particles A0063-FC010 and A0067-FIB006. Ono et al. (2024) described the method for combining CT images, TEM images, and SAED information in detail. One plaquette magnetite particle was examined in A0063-FC010 (Mt_1), while two plaquette magnetite particles were present in A0067-FIB006 (Mt_2 and Mt_3). In Mt_3, SAED was acquired for each of the two plates, while in Mt_1 and Mt_2, SAED was obtained at one location each. For each plaquette magnetite particle, we accurately established the relationship with the 3D crystal shape between crystal orientation from CT images and SAED patterns.
Results and Discussion In Mt_1 of A0063-FC010, it was observed that the platelets constituting the plaquette are stacked in nearly the [101] direction. The platelets were also found to be stacked in approximately the [101] direction in Mt_2 of A0067-FIB006. In Mt_3, SAED patterns were obtained for two adjacent platelets, both of which were stacked in mostly the [001] direction. However, the crystal orientations perpendicular to the stacking direction of each platelet did not match; one platelet's [100] and the other's [110] were nearly aligned in the same direction (i.e., the two platelets were rotated relative to the stacking direction by approximately 45°). Similar results have been reported in previous studies (Chen et al., 2016), where SEM/EBSD mapping results showed groups of platelets with nearly identical crystal orientations stacking along the [001] direction while rotating about the [001] axis. Mt_2 and Mt_3 are proximally located, suggesting that the plaquette with [001] and [101] stacking is formed at the same location. Alternatively, it is possible that a single plaquette contains a mixture of [001] and [101] stacking. Furthermore, since there is internal vacancy (or full of material resembling the matrix in the vacancy) within the platelets in plaquette magnetites observed in this study, the previous study was not reported the presence of such vacancy within the plaquette interior.
Summary In this study, we studied the crystal orientations of plaquette magnetite from two Ryugu particles using 3D CT images and SAED patterns. This revealed that plaquette magnetite not only stacks in the [001] direction, as indicated by previous study, but also in the [101] direction. If Ryugu and CI chondrite plaquettes include the same structural features, it is conceivable that plaquettes stack in either the [001] or [101] direction, with groups of plates sharing the same crystal orientation rotating about the stacking direction. Such complex structures are considered to result from the rapid growth of plaquettes under relatively high saturation conditions. Going forward, we try to obtain SAED patterns of the orientations of multiple platelets within the same plaquette (or conduct electron diffraction mapping) to further reveal how magnetite is stacked.