Editing Widmanstätten pattern (section)

==Lamellae formation mechanism==
[[File:Meteoric iron phase diagram taenite kamacite.svg|thumbnail|right| Phase diagram explaining how the pattern forms. First [[meteoric iron]] is exclusively composed of taenite. When cooling off it passes a phase boundary where [[kamacite]] is exsolved from taenite. [[Meteoric iron]] with less than about 6% nickel ([[hexahedrite]]) is completely changed to kamacite.]]
[[File:Widmannstaetten.png|thumb|Widmanstätten pattern, metallographic polished section]]
[[Iron]] and [[nickel]] form [[homogeneous]] [[alloys]] at temperatures below the [[melting point]]; these alloys are [[taenite]]. At temperatures below 900 to 600&nbsp;°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]] [[meteorite]]s 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 450 and 700&nbsp;°C, and can only take place during very slow cooling, about 100 to 10,000&nbsp;°C/Myr, with total cooling times of 10 [[Myr]] or less.<ref>{{citation|title=Iron meteorites: Crystallization, thermal history, parent bodies, and origin|journal=Chemie der Erde – Geochemistry|volume=69|issue=4|pages=293–325|doi=10.1016/j.chemer.2009.01.002|year=2009|last1=Goldstein|first1=J.I|last2=Scott|first2=E.R.D|last3=Chabot|first3=N.L|bibcode=2009ChEG...69..293G}}</ref> This explains why this structure cannot be reproduced in the laboratory.

The [[crystal]]line patterns become visible when the meteorites are cut, polished, and acid-etched, because taenite is more resistant to the acid.

[[File:Widmanstätten pattern kevinzim.jpg|thumb|The fine Widmanstätten pattern (lamellae width 0.3mm) of a [[Gibeon (meteorite)|Gibeon meteorite]].]]
The dimension of [[kamacite]] lamellae ranges from ''coarsest'' to ''finest'' (upon their size) as the nickel content increases. This classification is called ''[[Iron meteorite#Structural classification|structural classification]]''.