Why do iron meteorites have crystal patterns?
The patterns grew as molten metal in the cores of asteroids cooled extremely slowly over millions of years, giving nickel atoms time to move and separate the metal into two different alloys. That slow cooling cannot be recreated in the laboratory.
To understand the pattern, picture the inside of an asteroid's metal core early in Solar System history. When it was very hot, iron and nickel formed a single alloy called taenite, in which the atoms sit in a particular arrangement. As the core cooled below several hundred degrees Celsius, that arrangement became unstable for metal with modest amounts of nickel. A second alloy, kamacite, which holds less nickel, began to grow inside the taenite.
Kamacite grows as flat plates along particular directions in the taenite crystal, parallel to the faces of an octahedron, an eight-sided shape. As the plates thickened, they pushed nickel out into the remaining taenite, which became richer in nickel. Nickel atoms move through solid metal extremely slowly, so the plates could only grow to millimetres or centimetres wide because the cooling was itself incredibly slow, often estimated at somewhere between a few degrees and a few thousand degrees per million years. Insulated by the rocky layers around them, cores could cool that gradually.
The amount of nickel controls what you see. Irons with little nickel, under about 6 per cent, become almost entirely kamacite and show no Widmanstätten pattern; these hexahedrites often display fine parallel lines called Neumann lines, caused by shock. Irons with moderate nickel become octahedrites with classic bands, which are broader in lower-nickel metal; Sikhote-Alin, which fell in Russia in 1947, is among the coarsest. Irons very rich in nickel, the ataxites, form textures too fine to see with the naked eye.
Because the cooling took millions of years, the pattern cannot be manufactured in a laboratory at this scale, which makes it a powerful identification clue, although some meteorites lose it through reheating, and not every iron shows it.