Why do iron meteorites show a crisscross pattern when cut and polished?
— The mark of cooling at 100°C per million years, a slowness no lab on Earth can match
Hidden inside the cut face of an iron meteorite are bands that cross each other at an angle. This pattern, it turns out, can't be made on Earth. What it needs isn't a special material — it's millions of years of waiting.
In the meteorite corner of a science museum sits a flat-cut slab of metal. Its surface is crossed at several angles by thin lines and wide bands — like overlapping lattice windows.
The placard just says: "Cross-section of an iron meteorite. Pattern appears after acid etching." Nobody carved it. The pattern was already inside the metal itself.
Cut open a cast-iron frying pan and you'll find nothing like it. Same element, iron — so why does only the iron from space carry this pattern?
Two kinds of "slow" make the pattern
An iron meteorite is a fragment of the metal core that once sat at the centre of an asteroid. A thick rock shell wrapped around it, letting heat leak into space only bit by bit.
As it cooled, the iron and nickel split into two separate metals. For nickel atoms to migrate through solid metal takes an extraordinarily long time.
In short, the pattern is a record of slow cooling. Cool too fast, and the atoms don't have time to migrate — the pattern never grows. Let's look at each step.
Why is the centre of an asteroid so hard to cool?
About 4.5 billion years ago, countless small bodies were forming around the Sun. In the larger ones, the interior melted, and heavy iron and nickel are thought to have sunk to the centre, forming a metal core.
Look at the left side of Figure 1. A thick rock shell wraps around the metal core. Rock conducts heat poorly. And outside lies the vacuum of space — there's no wind to carry heat away either. Heat can only trickle out as light from the shell's surface.
Estimates put the cooling rate of the core at an almost unbelievable few degrees to a few thousand degrees Celsius per million years. Taking a middle value of 100°C per million years, that's a drop of just 0.0001°C per year. Over a human lifetime, the thermometer's needle wouldn't move at all.
Later, these bodies collided and shattered, scattering core fragments through space. One that happened to fall to Earth is what we call an iron meteorite.
The pattern is the trace of nickel "moving house"
While still very hot, iron and nickel form a single, evenly mixed metal. But once the temperature drops below roughly 700°C, a separate, nickel-poor metal begins to form and grow as plate-shaped crystals.
For a plate to grow, the nickel atoms already there have to move out of the way. The displaced nickel piles up at the plate's edge, forming a thin, nickel-rich layer. That's the origin of the wide bands and thin lines on the right of Figure 1.
This is where the second kind of slowness kicks in. Inside solid metal, atoms can only inch forward, weaving between their neighbours. And the distance they can travel doesn't grow in proportion to time — multiply the time by 100, and the distance only grows tenfold.
So growing a band about a millimetre wide takes millions of years of waiting. However slowly a factory on Earth cools metal, it's a matter of days to months at most. That's why the pattern in iron meteorites is said to be impossible to reproduce artificially.
The bands appear to cross in several directions because the original metal's atomic arrangement has four directions along which plates grow easily. A cut through plates lying at an angle to those directions looks like a crisscrossing net.
The pattern was first reported around 1804 by Thomson, and again in 1808 by Widmanstätten, who is said to have noticed it after heating a meteorite plate over a flame and seeing it discolour unevenly from place to place. Today, the presence of this pattern is one of the clues used to tell whether an iron meteorite is genuine.
By measuring the band width and how the nickel concentration changes across it, you can work backwards to the cooling rate. The slower the cooling, the thicker the rock shell around the core must have been. This is how the sizes of asteroids that no longer exist have been estimated at roughly tens to hundreds of kilometres across.
Summary
The pattern in iron meteorites is the record of cooling over millions of years at the centre of an asteroid, wrapped in a thick rock shell. As it cooled, nickel atoms slowly migrated through the solid metal, splitting into nickel-poor bands and nickel-rich lines. Because the distance atoms travel grows only with the square root of time, the same pattern can never form in the short timescales available on Earth.
It wasn't the material that made the pattern — it was time.
The cut face of an iron meteorite is a fossil of "slowness": 100°C per million years.
How this meteorite came to be accepted as "a rock that falls from the sky" is covered in Why didn't scholars used to believe that rocks fall from the sky?. The story of how slow cooling grows bigger crystals appears in Why do alum crystals grow into such beautiful shapes?, and Earth's own iron core is covered in the story of Earth's centre.
- Dissolve as much alum as you can in hot water and split it between two cups. Handle hot water with an adult's help.
- Cool one cup quickly in ice water. Put the other in a polystyrene box, put the lid on, and let it cool slowly overnight.
- The next day, compare the crystals that formed at the bottom. The one cooled in the box should have grown bigger grains. The polystyrene box is playing the role of the asteroid's "rock shell."
If you spot the cut face of an iron meteorite at a science museum, try counting how many directions the bands run in. The wider the bands, the more slowly that meteorite cooled.
Want to know more? — Terms, formulas, and how this connects to textbooksWe've marked which level each part belongs to, from middle-school science to university specialist courses
- MSCovered in middle-school science
- HSCovered in high-school Chemistry, Physics, or Earth Science
- HS+Advanced high-school content, or textbook sidebar material
- UnivNot covered in high school — university-level specialist content (metallurgy, planetary science)
- ResearchNot yet settled even at university level — an active research question
MSTerms: this phenomenon has a name
- Widmanstätten pattern: the crisscrossing plate-shaped pattern seen in the cut face of iron meteorites, named after its discoverer.
- Kamacite and taenite: the nickel-poor metal (wide bands) is kamacite; the nickel-rich metal (thin lines) is taenite.
- Octahedrite: the type of iron meteorite that shows this pattern, named for the octahedron — the plates grow along the faces of a regular octahedron.
MSHSWorking it out with formulas: how many years does the pattern take to grow?
Assuming a constant cooling rate, we can calculate how long it takes to pass through the temperature range where the pattern grows. We'll also look at the relationship between band width and time.
| In symbols | t = ΔT ÷ R (rough band width: x ≒ √( D × t )) |
| In words | Time taken = temperature drop ÷ cooling rate. Distance an atom travels = square root of (mobility × time) |
| Where it comes from | The first is division in the same form as "distance = speed × time." The second comes from the nature of "diffusion," where atoms move by randomly changing direction as they go |
| t | Time spent passing through the temperature range where the pattern grows (unit: million years) |
| ΔT | Width of the temperature range where the pattern grows (unit: °C) |
| R | Cooling rate (unit: °C per million years) |
| x | Rough distance a nickel atom can travel (unit: m) |
| D | Diffusion coefficient — how easily atoms move; drops sharply as temperature falls (unit: square metres per second) |
| Temperature at which the pattern starts growing (depends on nickel content) | roughly 700°C |
| Temperature at which atoms become almost unable to move | roughly 400°C |
| Example cooling rate (middle of the estimated range) | 100°C per million years |
| Temperature drop ΔT | 700 − 400 = 300 °C |
| Time taken t (in millions of years) | 300 ÷ 100 = 3 |
| Temperature drop per year | 100 ÷ 1,000,000 = 0.0001 °C |
| Temperature drop over a human lifetime (80 years) | 0.0001 × 80 = 0.008 °C |
| Time multiplier needed to make the band 10× wider | 10 × 10 = 100 times |
The pattern takes roughly 3 million years to grow. Over that time, a human lifetime would see less than 0.01°C of cooling. And because travel distance grows only with the square root of time, making a band 10 times wider takes 100 times as long. That's why a few days or months on Earth never reaches a millimetre-wide band.
HSHS+Crystal structure and atomic mobility
HSMetal atoms form regularly arranged crystals. Nickel-rich taenite has a face-centred cubic structure; nickel-poor kamacite has a body-centred cubic structure. As temperature falls, the face-centred cubic form becomes less stable, and body-centred cubic plates precipitate out.
HS+The diffusion coefficient, which describes how easily atoms move, is strongly temperature-dependent — it follows an Arrhenius-type relationship and drops sharply as temperature falls. So the pattern grows mainly at higher temperatures, and mostly stops below about 400°C.
UnivThe iron–nickel phase diagram and reading off cooling rates
The pattern's formation is explained by the iron–nickel binary phase diagram together with diffusion-controlled growth based on Fick's law. Nickel concentration inside taenite forms an "M-shaped" profile — high at the plate edges, low in the middle. Comparing this profile against diffusion calculations to derive a cooling rate is a technique called the metallographic cooling rate method, in use since the 1960s.
📖 For the derivation and further reading: Widmanstätten pattern (Japanese Wikipedia) ／ Fick's law (Japanese Wikipedia)
ResearchWhat's still not fully understood
- Why do fragments of the same core cool at different rates? Some types of iron meteorite show widely different cooling rates despite apparently coming from the same core. One proposed explanation: the core was stripped of its rock shell and cooled while exposed.
- Traces of an asteroid's magnetic field, hidden in the gaps between bands Even finer structures at the edges of taenite are thought to record the asteroid's magnetic field at the time. Research is underway into whether the metal core itself generated that field.
- Going to look at an asteroid's core directly A spacecraft is on its way to Psyche, an asteroid thought to be made of metal. Whether it really is an exposed core should become clear once the probe arrives and takes measurements.
In other words, everything in this article is "the best explanation we have so far." Cooling-rate estimates vary considerably depending on the method and the meteorite.
Connections to textbooks (by level)
| Level | Subject / unit | Where in this article |
|---|---|---|
| MS | Science: "States of matter," "The solar system" | How slow cooling grows bigger crystals; asteroids |
| HS | Chemistry: "Metal crystals"; Earth Science: "Formation of the solar system" | Face-centred vs. body-centred cubic; asteroid cores |
| HS+ | Chemistry: "Reaction rate and temperature" | The Arrhenius equation; diffusion halting at low temperature |
| Univ | Metallurgy, planetary science | Phase diagrams, Fick's law, the metallographic cooling rate method |
| Research | Meteoritics | Variation in cooling rates, paleomagnetism, the Psyche mission |
| — | Everyday connections | Alum and rock-candy crystals; quenching and annealing in blacksmithing |
- Wikipedia (Japanese): "Widmanstätten pattern" (ウィドマンシュテッテン構造)
- Wikipedia (Japanese): "Fick's law" (フィックの法則)
- Goldstein, J. I., Scott, E. R. D., Chabot, N. L. (2009) Iron meteorites: Crystallization, thermal history, parent bodies, and origin. Chemie der Erde 69, 293–325.
- Yang, J., Goldstein, J. I., Scott, E. R. D. (2007) Iron meteorite evidence for early formation and catastrophic disruption of protoplanets. Nature 446, 888–891.
※This article is a general-audience science explainer. The figures given are approximations meant to aid understanding of the underlying mechanism. Cooling rates and temperatures vary depending on the study and the meteorite.