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[PPS08-05] Crystal growth conditions through 3D shape diversity of magnetite particles in Ryugu samples
Keywords: X-ray nanotomography, TEM, acqueous alteration, crystal morphology
Samples returned from the C-type asteroid Ryugu consist of phyllosilicates, Fe sulfides, magnetite, carbonates, and apatite, corresponding to CI chondrites [1]. 3D structures of Ryugu particles (<~100 μm) were revealed using X-ray nanotomography (XnCT) (spatial resolution: ~200 nm). While Fe sulfides, carbonates, and apatite exhibit euhedral shapes, magnetite displays diverse 3D shapes, including spherulite (Sph), plaquette (Plq), framboid (Frm), rod (Rod), equant (Eqn), whisker (Whs), and cube (Cub) [2]. Based on the textural relationships of the minerals, their precipitation sequence was determined, and a model for aqueous alteration processes in the Ryugu parent body was proposed [2]. In this model, poorly crystalline phyllosilicates and Sph precipitated from highly saturated aqueous solutions formed by the dissolution of reactive amorphous silicates into water. As supersaturation decreased, Plq and Frm, and eventually euhedral crystals of minerals precipitated. An evolution model of magnetite crystal shapes was also proposed based on the shapes and precipitation sequence, but information about crystal orientations was insufficient. In this study, we obtained crystal orientations and qualified 3D shapes to achieve a more detailed understanding of the aqueous alteration processes.
A novel method was developed to determine crystal orientations by acquiring SAED patterns of FIB-milled TEM sections of magnetite with known 3D shapes by XnCT [4]. The results revealed that Plq had two stacking modes with thin {100} and {110} plates. Rod and Whs elongated along the <100> direction. Sph exhibited a complex relationship between elongation direction and crystal orientation, as evidenced by electron diffraction mapping. From the face angles in the 3D images, it was estimated that Eqn consist mainly of {311} and Cub of {100}. These findings, excluding {110} plates in plaquettes, are consistent with previous studies on Sph and Eqn [5-7]. Consequently, magnetite shapes were categorized into {100}, {110}, and {311} forms, and one without specific crystal orientations.
Crystal shapes represent the most stable configurations, minimizing the total surface free energy for a given volume (equilibrium form). By contrast, actual crystal shape is governed by growth environment and conditions (growth form). Under high supersaturation, growing interface instability occurs, leading to growth even with a large surface area (high surface free energy). Here, we adopted a zeroth-order approximation, assuming that (1) crystals are prisms with length, L, and width, W, and (2) crystal faces have the same free energy, γ. The ratio of the total surface free energy to the bulk free energy, ε, was expressed in terms of L, W, γ, and the free energy per unit volume, G, as ε = 2(1/L + 2/W) (γ/G). This allowed for the relative evaluation of ε in the W-L diagram with constant ε/(γ/G) values. L and W for magnetite crystals from CT images (needles for Sph, thin plates for Plq, and separated grains for Frm were used) showed that ε decreased in the following order: Sph > Frm ({311} form) > Whs ({100} form) ~ Plq ({100}/{110} form) > Cub ({100} form) > Rod ({100} form) > Eqn ({311} form). This order is consistent with the precipitation sequence, indicating that magnetite with lower ε precipitated as supersaturation decreased. However, considering the crystal face classification, the order should be {311} → {100} → {110} → {100} → {311} forms. This indicate that environmental factors, such as pH and impurities in solution, may affecting crystal face appearance, have undergone repeated changes, complicating the straightforward explanation of these observations.
[1] Nakamura et al. (2023) Science, 379, eabn8671. [2] Tsuchiyama et al. submitted to GCA. [3] Tsuchiyama et al. (2022) JAMS meeting, abstract. [4] Ohno et al. (2024) LPSC, abstract #1488. [5] Chan et al. (2016) Am. Min.101, 2041. [6] Nozawa et al. (2011) JACS, 133: 8782. [7] Dobrică et al. (2023) GCA 346, 65-75.
A novel method was developed to determine crystal orientations by acquiring SAED patterns of FIB-milled TEM sections of magnetite with known 3D shapes by XnCT [4]. The results revealed that Plq had two stacking modes with thin {100} and {110} plates. Rod and Whs elongated along the <100> direction. Sph exhibited a complex relationship between elongation direction and crystal orientation, as evidenced by electron diffraction mapping. From the face angles in the 3D images, it was estimated that Eqn consist mainly of {311} and Cub of {100}. These findings, excluding {110} plates in plaquettes, are consistent with previous studies on Sph and Eqn [5-7]. Consequently, magnetite shapes were categorized into {100}, {110}, and {311} forms, and one without specific crystal orientations.
Crystal shapes represent the most stable configurations, minimizing the total surface free energy for a given volume (equilibrium form). By contrast, actual crystal shape is governed by growth environment and conditions (growth form). Under high supersaturation, growing interface instability occurs, leading to growth even with a large surface area (high surface free energy). Here, we adopted a zeroth-order approximation, assuming that (1) crystals are prisms with length, L, and width, W, and (2) crystal faces have the same free energy, γ. The ratio of the total surface free energy to the bulk free energy, ε, was expressed in terms of L, W, γ, and the free energy per unit volume, G, as ε = 2(1/L + 2/W) (γ/G). This allowed for the relative evaluation of ε in the W-L diagram with constant ε/(γ/G) values. L and W for magnetite crystals from CT images (needles for Sph, thin plates for Plq, and separated grains for Frm were used) showed that ε decreased in the following order: Sph > Frm ({311} form) > Whs ({100} form) ~ Plq ({100}/{110} form) > Cub ({100} form) > Rod ({100} form) > Eqn ({311} form). This order is consistent with the precipitation sequence, indicating that magnetite with lower ε precipitated as supersaturation decreased. However, considering the crystal face classification, the order should be {311} → {100} → {110} → {100} → {311} forms. This indicate that environmental factors, such as pH and impurities in solution, may affecting crystal face appearance, have undergone repeated changes, complicating the straightforward explanation of these observations.
[1] Nakamura et al. (2023) Science, 379, eabn8671. [2] Tsuchiyama et al. submitted to GCA. [3] Tsuchiyama et al. (2022) JAMS meeting, abstract. [4] Ohno et al. (2024) LPSC, abstract #1488. [5] Chan et al. (2016) Am. Min.101, 2041. [6] Nozawa et al. (2011) JACS, 133: 8782. [7] Dobrică et al. (2023) GCA 346, 65-75.