Iron meteorites are fragments of the metallic cores of shattered asteroids. Their core alloy is Fe–Ni. Above ~900 °C it is a single face-centred-cubic phase called taenite (γ). As the core cools — at a glacial 0.5–300 °C per million years, because rock and metal are terrible insulators only when there's a lot of it and no convection — the alloy enters the two-phase γ+α field of the Fe–Ni phase diagram. A low-nickel, body-centred-cubic phase called kamacite (α) nucleates and grows as parallel plates along the four {111} octahedral planes of the parent taenite crystal, pulling nickel out ahead of the advancing interface by solid-state diffusion.
Nucleation onset: T_n(Ni%) ≈ 720 − 14·(Ni% − 6) °C (higher Ni delays α formation)
Kamacite fraction: f(t) = f_max · √(active fraction) (diffusion-limited growth, ∝ √(D·t))
Lamella spacing: d(mm) ≈ 1.4 · R(°C/Myr)^(−1/2) (empirical, after Wood 1964)
- Nickel content — sets where on the phase diagram cooling starts, the nucleation temperature, and how much kamacite the lever rule allows to form.
- Cooling rate — the real dial meteoriticists read backwards: measuring a meteorite's actual lamella spacing under a microscope and inverting this relation tells you how fast, and therefore how deep inside its parent asteroid, that piece of iron cooled — cooling rates of a few °C/Myr imply core radii of tens of kilometres.
- Progress / Play — scrubs through cooling from a molten 900 °C core down to ~400 °C, where diffusion becomes too slow to move nickel any further and the pattern freezes in for good.
The pattern is normally invisible — it only appears when a meteorite is cut, polished and etched with acid, which attacks the two phases at different rates. That etched cross-hatch of bright kamacite bands on darker taenite is the actual Widmanstätten pattern, named for Alois von Beckh Widmanstätten, who first documented it in 1808.