How Widmanstätten patterns grow

Cut, polish and etch most iron meteorites and a lattice of crossing bands appears: the Widmanstätten pattern. It is made of two nickel–iron minerals that slowly separated as the metal core of an asteroid cooled, over millions of years, deep inside its parent body. The pattern cannot be grown at this scale in a laboratory, which makes it one of the surest signs of an iron meteorite.

1. Step 1 of 7

   A metal core freezes

   Many early asteroids grew hot enough to melt, and their dense metal sank to the centre as a core. As a core cooled, its liquid nickel–iron froze into solid metal. At that stage, while still very hot, the metal is all one mineral, taenite, whose atoms are packed in a face-centred cubic arrangement.

   The taenite crystals that formed were enormous, often metres across: some iron meteorites weighing tonnes were cut from a single parent crystal, and their bands run unbroken from one side to the other. Most iron meteorites contain between about 5 and 20 per cent nickel, and that nickel content will decide what happens next.

2. Step 2 of 7

   Cooling very slowly

   A core is wrapped in hundreds of kilometres, or at least tens, of rocky mantle that acts like a thick blanket, so it cools extremely slowly. Measurements on iron meteorites show cooling rates typically from about one degree to a few hundred degrees per million years, and up to thousands for some groups.

   That slowness is the secret of the pattern. The changes that make it happen in solid metal, where atoms can only shuffle slowly from place to place, and they need millions of years at high temperature to build bands that are visible by eye.

3. Step 3 of 7

   Kamacite plates appear

   Below a temperature that depends on the nickel content, roughly 700 °C for metal with about 8 per cent nickel, taenite is no longer stable on its own. A second mineral, kamacite, begins to grow. Kamacite holds very little nickel, under about 7.5 per cent, and its atoms are packed in a body-centred cubic arrangement.

   Kamacite does not grow at random. It forms thin plates aligned with the four sets of planes that would be the faces of an octahedron within the taenite crystal. A slice through the metal cuts these plates at different angles, so the cut face shows bands running in three or four directions. Small grains of phosphide minerals can help the first kamacite to start growing.

4. Step 4 of 7

   Nickel moves out of the way

   Because kamacite takes in so little nickel, every plate that grows pushes nickel out into the taenite next to it. Nickel atoms can only diffuse slowly through solid metal, so they pile up at the edges of the taenite. Measured across a band, the nickel content traces an M shape: low in the kamacite, rising to sharp peaks at the taenite edges, and lower again in the middle.

   As the metal cools, diffusion slows down further, until somewhere around 400 to 500 °C the plates effectively stop growing. The exact shape of the M depends on how fast the metal cooled, so by measuring it with a microprobe and comparing it with models, scientists can work out the cooling rate of a core that broke apart billions of years ago.

5. Step 5 of 7

   Plessite fills the gaps

   The centres of the taenite regions, between the plates, are left with an awkward in-between composition. As cooling continues they break down into a very fine intergrowth of kamacite and taenite called plessite, which looks dark and speckled under a hand lens.

   The thin taenite rims, by contrast, are so rich in nickel that they resist etching and shine as bright lines between the bands. Below about 320 °C, the outermost rims can rearrange into an ordered form of nickel–iron called tetrataenite, a natural magnetic mineral that has interested engineers looking for magnets made without rare elements.

6. Step 6 of 7

   Band width tells nickel and cooling

   The width of the kamacite bands depends mainly on two things: how much nickel the metal holds and how fast it cooled. More nickel means kamacite starts growing at a lower temperature, when atoms move more slowly, so the bands stay narrow. Iron meteorites are classed by band width, from coarsest octahedrites with bands over 3.3 millimetres wide to finest octahedrites under 0.2 millimetres.

   At the extremes, there is no visible pattern. Hexahedrites, with less than about 6 per cent nickel, become entirely kamacite and show only fine, straight Neumann lines made by shock. Ataxites, with more than about 16 per cent nickel, have structures too fine to see without a microscope.

7. Step 7 of 7

   Revealing the pattern

   The pattern is hidden in an uncut meteorite. To see it, a slice is sawn, ground and polished to a mirror finish, then etched with a dilute acid, usually nitric acid in alcohol. Kamacite and taenite are attacked at different rates and reflect light differently, so the bands stand out within minutes.

   The pattern was first described by William Thomson in Naples in 1804, and independently by Alois von Widmanstätten in Vienna in 1808, who revealed it by heating a slice so that the minerals tarnished to different colours. Today it remains one of the most reliable tests for an iron meteorite, because nothing made on Earth shows a centimetre-scale pattern grown over millions of years.

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