page_title: Widmanstätten pattern
section_title: Lamellae formation mechanism
hierarchical_section_title: Widmanstätten pattern / Lamellae formation mechanism
caption_attribution_description: English: Original description: Widmanstätten texture in the surface of an etched meteorite from the Gibeon cluster, Namibia. Deutsch: Ursprüngliche Beschreibung (Übersetzung vom Einsteller): Widmanstätten-Struktur in der Oberfläche eines angeätzten Meteoriten aus dem Gibeon-Streufeld, Namibia.
context_page_description: Widmanstätten patterns, also known as Thomson structures, are figures of long nickel–iron crystals, found in the octahedrite iron meteorites and some pallasites. They consist of a fine interleaving of kamacite and taenite bands or ribbons called lamellae. Commonly, in gaps between the lamellae, a fine-grained mixture of kamacite and taenite called plessite can be found. Widmanstätten patterns describe features in modern steels, titanium and zirconium alloys.
context_section_description: Iron and nickel form homogeneous alloys at temperatures below the melting point; these alloys are taenite. At temperatures below 900 to 600 °C (depending on the Ni content), two alloys with different nickel content are stable: kamacite with lower Ni-content (5 to 15% Ni) and taenite with high Ni (up to 50%). Octahedrite meteorites have a nickel content intermediate between the norm for kamacite and taenite; this leads under slow cooling conditions to the precipitation of kamacite and growth of kamacite plates along certain crystallographic planes in the taenite crystal lattice.
  The formation of Ni-poor kamacite proceeds by diffusion of Ni in the solid alloy at temperatures between 700 and 450 °C, and can only take place during very slow cooling, about 100 to 10,000 °C/Myr, with total cooling times of 10 Myr or less. This explains why this structure cannot be reproduced in the laboratory.
  The crystalline patterns become visible when the meteorites are cut, polished, and acid etched, because taenite is more resistant to the acid. 
  The dimension of kamacite lamellae ranges from coarsest to finest (upon their size) as the nickel content increases. This classification is called structural classification.
caption: The fine Widmanstätten pattern (lamellae width 0.3mm) of a Gibeon meteorite.
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