Gamma-iron is a form of metallic iron that normally exists only when the metal is hot, and the Chinese Academy of Sciences now reports finding it frozen into grains of glass from the Moon’s far side. Prof. Du Haifeng’s group at the High Magnetic Field Laboratory in Hefei says this is the first time the phase has been identified in natural lunar samples.
The glass came from soil that China’s Chang’e-6 mission returned in 2024, the first ever brought back from the lunar far side. Some of the iron grains inside it carry a stable magnetic signature.
A high-temperature iron that stayed put
According to the Hefei Institutes of Physical Science release of September 24, gamma-Fe is stable under ordinary conditions only at high temperatures. Cool iron settles into a different crystal arrangement, which is why a phase that exists only at high temperature is so unexpected in cold lunar material. The release credits three ingredients with preserving it on the Moon: trace carbon, very fast cooling, and the glassy matrix that locked the grains in place.
A meteorite strike produces exactly such conditions. The impact melts local soil, and the molten droplets chill in moments, so any iron that formed in the hot phase is trapped before it can rearrange. The paper, published in PNAS on September 16 as “Magnetic vortex state of natural lunar γ-Fe,” carries the claim into the scientific record.
Electron holography on three particles
The team cut thin slices from individual particles with a focused ion beam, then examined them by transmission electron microscopy and chemical analysis. To read the magnetism inside single grains, it used off-axis electron holography, a technique that maps magnetic fields at the scale of nanometers. The raw microscopy and spectroscopy data sit in a public dataset deposited by Pengfei Liu of Macau University of Science and Technology, which describes the work as the first discovery of pure gamma-phase iron preserved at ambient temperature in Chang’e-6 impact glass from the South Pole-Aitken Basin.
Details of the sample set come from a secondary write-up of the paper. It says three iron-bearing particles from soil collected at the Apollo crater inside the South Pole-Aitken basin were studied, two of them impact glass and one basalt. Gamma-iron made up 64 to 76 percent of the iron particles analyzed in the glass, while the basalt held only ordinary alpha-iron. The grains ranged from under 10 nanometers to about 140 nanometers across. For scale, a human hair is tens of thousands of nanometers wide, so the largest grain is hundreds of times thinner than a strand of hair, and the smallest are far below what any optical microscope could resolve. That scale explains why electron microscopy and holography, rather than ordinary petrographic study of rock slices, were needed to see the phase at all.
A magnetic fossil from the Moon’s lost field
Size decides the magnetism. Grains larger than roughly 100 nanometers displayed a vortex state, in which the magnetic moments curl around inside the particle instead of all pointing one way. The Hefei release says the larger gamma-Fe grains held stable single-vortex states, which is what makes them candidates for recording magnetic information.
Dr. Li Long of the Hefei team called the find a “tiny magnetic fossil” that may help scientists understand the Moon’s ancient magnetic history. The Moon has no global magnetic field today, yet its rocks carry magnetized patches, and the origin of those anomalies, especially around the South Pole-Aitken basin, is still argued over.
Gamma-iron is not the first unexpected iron compound reported from Chang’e-6 soil. A separate Science Advances paper in November 2025 described crystalline hematite and maghemite in the same soil, and its authors suggested that maghemite may be behind the magnetic anomalies seen around the basin. The two results point at the same region from different directions, one oxide and one metal, and neither paper settles what magnetized that ground.
The limits of a three-particle result
The claim is narrow, and the Hefei institute frames it that way. It says the phase is new to natural lunar samples, and the data come from a handful of particles from one landing area, not a survey of the whole far side, so any statement about how common the phase is on the Moon would go beyond what the samples can show. The same limit applies to the magnetic history: three particles can show that vortex states survive in impact glass, but they cannot by themselves reconstruct what the lunar field did over billions of years. Whether gamma-iron shows up in other Chang’e-6 grains, in Apollo or Chang’e-5 material, or in lunar meteorites has not been reported in the release.
The public record consists of one paper, one dataset and one release from Hefei, all tied to the same few particles. Turning a magnetic fossil into a dated measurement would require reading the preserved vortex states against a known impact age, and none of the three documents reports that step.
This article was produced with the assistance of AI and reviewed by Morning Overview editors prior to publication.
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