Presentation Information
[PPS12-09]Experimental Study on Precursors of Framboidal Magnetite in Asteroids
*Tomohito Kondo1, Masaya Sakakibara1, Tomoya Yamazaki1, Yuki Kimura1 (1.Institute of Low Temperature Science, Hokkaido University)
Keywords:
Asteroid,Crystal Growth,Magnetite,Mesocrystal,Green Rust
[Introduction]
Framboidal magnetite is an aggregate of magnetite particles of hundred nanometers to micrometers in size observed in primitive asteroid-derived samples1, 2. It is considered to have formed by aqueous alteration about 4.6 billion years ago, but it has not been reproduced experimentally, and a unified understanding of the formation conditions and process has not been fully achieved. To clarify this, we performed reproduction experiments by precipitation from aqueous solution.
In this study, we used a co-precipitation method as a possible magnetite formation process in asteroids, which is a common method to synthesize iron-oxide nanoparticles by adding base to an iron-salt solution and precipitating iron species. In co-precipitation method, it has been suggested that magnetite can form from precursor phases such as green rust, a hexagonal platelet-like iron hydroxide rich in Fe2+, and ferrihydrite, a poorly crystalline hydrous Fe3+ oxyhydroxide3. To synthesize particles as large as those of framboidal magnetite, base was supplied using gas–liquid equilibrium of aqueous ammonia to prevent a rapid increase in pH and to synthesize larger particles at low supersaturation. To mimic the temperature environment after ice melting by asteroidal heating, the synthesis was performed under a warming condition from low temperature to room temperature.
[Experimental Methods]
A 10 mL iron-salt solution (FeCl3·6H2O: 30 mmol, FeCl2·4H2O: 30 mmol) in a 45 mm inner-diameter container was placed into a PTS bag together with 42 mL of 1.2 % aqueous ammonia, sealed under N2 gas, and left in a freezer at 1 °C for 4 days (Fig. 1). After that, the suspension containing the synthesized particles was collected into microtubes and left at room temperature for 7 more days to proceed the reaction. After the reaction, the samples were washed with pure water, dispersed in ethanol, dropped onto TEM grids, and observed by TEM. We also observed samples that were dropped onto TEM grids immediately after standing in the freezer for 4 days, without warming to room temperature.
[Results and Discussion]
From the electron diffraction patterns, we confirmed that most particles in the sample were magnetite. In TEM observation, magnetite particles of 10 to 200 nm were observed, and aggregates larger than 100 nm similar to framboidal magnetite were partly observed (Fig. 2A). Also, hexagonal platelet-like particles consisting of many small particles were observed (Fig. 2B). From the electron diffraction patterns, this particle was magnetite, and because the crystal orientations were aligned, it is a magnetite mesocrystal. In the same sample, oriented goethite was also confirmed, which looks like it remained to surround the outer rim after dissolution of a similar hexagonal platelet (Fig. 3).
In the sample dropped onto the TEM grid immediately after collection at low temperature, ferrihydrite considered as a precursor of magnetite, and feroxyhyte, which is known to be obtained by rapid oxidation of iron(II) hydroxide, were observed in addition to magnetite, and hexagonal platelet-like single-crystal hematite particles similar to the magnetite mesocrystal were also observed (Fig. 4).
Because green rust typically has a hexagonal platelet morphology, the hexagonal platelet-like magnetite mesocrystal and goethite suggest that green rust was present as a precursor phase. In addition, the magnetite aggregates and magnetite mesocrystal are considered to have formed gradually with the involvement of Fe2+-rich green rust. The hematite single crystals are also considered to have formed by oxidation of green rust during processes such as exposure to dissolved oxygen and/or drying on the TEM grid. In asteroids, magnetite growth from Fe2+-rich green rust precursor phases under reducing and low-temperature conditions is considered to contribute to growth into submicrometer-sized particles and aggregation into framboidal magnetite. In future work, we will attempt direct observation of green rust, which we could not directly observe in this study.
1) Nakamura, T. et al. Formation and evolution of carbonaceous asteroid Ryugu: direct evidence from returned samples. Science 379, eabn8671 (2022).
2) Nozawa, J. et al. Magnetite 3D colloidal crystals formed in the early solar system 4.6 billion years ago. J. Am. Chem. Soc. 133, 8782–8785 (2011).
3) LaGrow, A. P. et al. Unravelling the growth mechanism of the co-precipitation of iron oxide nanoparticles with the aid of synchrotron X-Ray diffraction in solution. Nanoscale 11, 6620–6628 (2019).
Framboidal magnetite is an aggregate of magnetite particles of hundred nanometers to micrometers in size observed in primitive asteroid-derived samples1, 2. It is considered to have formed by aqueous alteration about 4.6 billion years ago, but it has not been reproduced experimentally, and a unified understanding of the formation conditions and process has not been fully achieved. To clarify this, we performed reproduction experiments by precipitation from aqueous solution.
In this study, we used a co-precipitation method as a possible magnetite formation process in asteroids, which is a common method to synthesize iron-oxide nanoparticles by adding base to an iron-salt solution and precipitating iron species. In co-precipitation method, it has been suggested that magnetite can form from precursor phases such as green rust, a hexagonal platelet-like iron hydroxide rich in Fe2+, and ferrihydrite, a poorly crystalline hydrous Fe3+ oxyhydroxide3. To synthesize particles as large as those of framboidal magnetite, base was supplied using gas–liquid equilibrium of aqueous ammonia to prevent a rapid increase in pH and to synthesize larger particles at low supersaturation. To mimic the temperature environment after ice melting by asteroidal heating, the synthesis was performed under a warming condition from low temperature to room temperature.
[Experimental Methods]
A 10 mL iron-salt solution (FeCl3·6H2O: 30 mmol, FeCl2·4H2O: 30 mmol) in a 45 mm inner-diameter container was placed into a PTS bag together with 42 mL of 1.2 % aqueous ammonia, sealed under N2 gas, and left in a freezer at 1 °C for 4 days (Fig. 1). After that, the suspension containing the synthesized particles was collected into microtubes and left at room temperature for 7 more days to proceed the reaction. After the reaction, the samples were washed with pure water, dispersed in ethanol, dropped onto TEM grids, and observed by TEM. We also observed samples that were dropped onto TEM grids immediately after standing in the freezer for 4 days, without warming to room temperature.
[Results and Discussion]
From the electron diffraction patterns, we confirmed that most particles in the sample were magnetite. In TEM observation, magnetite particles of 10 to 200 nm were observed, and aggregates larger than 100 nm similar to framboidal magnetite were partly observed (Fig. 2A). Also, hexagonal platelet-like particles consisting of many small particles were observed (Fig. 2B). From the electron diffraction patterns, this particle was magnetite, and because the crystal orientations were aligned, it is a magnetite mesocrystal. In the same sample, oriented goethite was also confirmed, which looks like it remained to surround the outer rim after dissolution of a similar hexagonal platelet (Fig. 3).
In the sample dropped onto the TEM grid immediately after collection at low temperature, ferrihydrite considered as a precursor of magnetite, and feroxyhyte, which is known to be obtained by rapid oxidation of iron(II) hydroxide, were observed in addition to magnetite, and hexagonal platelet-like single-crystal hematite particles similar to the magnetite mesocrystal were also observed (Fig. 4).
Because green rust typically has a hexagonal platelet morphology, the hexagonal platelet-like magnetite mesocrystal and goethite suggest that green rust was present as a precursor phase. In addition, the magnetite aggregates and magnetite mesocrystal are considered to have formed gradually with the involvement of Fe2+-rich green rust. The hematite single crystals are also considered to have formed by oxidation of green rust during processes such as exposure to dissolved oxygen and/or drying on the TEM grid. In asteroids, magnetite growth from Fe2+-rich green rust precursor phases under reducing and low-temperature conditions is considered to contribute to growth into submicrometer-sized particles and aggregation into framboidal magnetite. In future work, we will attempt direct observation of green rust, which we could not directly observe in this study.
1) Nakamura, T. et al. Formation and evolution of carbonaceous asteroid Ryugu: direct evidence from returned samples. Science 379, eabn8671 (2022).
2) Nozawa, J. et al. Magnetite 3D colloidal crystals formed in the early solar system 4.6 billion years ago. J. Am. Chem. Soc. 133, 8782–8785 (2011).
3) LaGrow, A. P. et al. Unravelling the growth mechanism of the co-precipitation of iron oxide nanoparticles with the aid of synchrotron X-Ray diffraction in solution. Nanoscale 11, 6620–6628 (2019).