Cloudy taenite-rimmed plessite and kamacite

Cloudy taenite is a specific, high % Ni, secondary structure in iron meteorites. Its appearance and composition characterize it.

It forms due to the slow diffusion of nickel and iron atoms after the initial crystallization of the primary metallic phases forming fine taenite intergrowths.

Cloudy taenite appears as a 'cloudy' submicroscopic two-phase mixture of taenite and another Ni phase, often tetrataenite (49% to 57% nickel, sometimes called 'clear' taenite).

Taenite rims can form through nucleation and growth. During the slow primary cooling, taenite can nucleate and grow around other phases, such as kamacite or plessite fields. It can form rims around existing structures due to the differences in stability of the iron-nickel phases at various temperatures.

Prepared specimens of Grant will often display a bright metallic rim around both plessitic fields and kamacite lamellae. This is due to the high-Ni tetrataenite. Cloudy taenite typically contains 30% to 40% nickel.