Widmanstätten

Widmanstätten pattern refers to a distinctive microscopic intergrowth of iron-nickel phases observed in some iron meteorites and in certain laboratory-made alloys. When a polished section is etched with acid, it reveals a network of elongated, blade-like structures that contrast with the surrounding metal, producing a characteristic crosshatched or ladder-like appearance.

The pattern arises from the slow cooling of metallic iron-nickel alloys, allowing two phases to segregate: kamacite, a lower-nickel, body-centered structure, and taenite, a higher-nickel phase. As taenite exsolves into kamacite during cooling, lamellae of kamacite grow within taenite along specific crystallographic planes, creating the regular bands seen after etching. The lamellae orientation typically follows octahedral {110} planes, reflecting the underlying crystal geometry.

Widmanstätten patterns are most commonly found in octahedrite iron meteorites, though analogous structures can be produced in the laboratory by controlled heat treatment of iron-nickel alloys. They are not a diagnostic indicator of age but provide a record of very slow cooling within the meteorite’s parent body, such as an asteroid core, on timescales of millions of years. The patterns can be erased by annealing or more intense heating.

History and terminology: the pattern is named after Austrian scientist Alois von Widmanstätten, who described the features in the 19th century; the phenomenon is sometimes referred to as Thomson structures in some literature. Observation requires etching and optical microscopy to reveal the contrast between kamacite and taenite.