Plessite
A fine-grained intergrowth of kamacite and taenite filling the spaces between Widmanstätten bands.
Plessite is not a single mineral but a texture: a fine intergrowth of kamacite and taenite. In octahedrites it fills the triangular and polygonal fields left between the kamacite plates, and it is a major component of nickel-rich ataxites. It forms from the central portions of taenite fields that did not transform during the main growth of Widmanstätten kamacite.
As cooling continued, these taenite cores, with their intermediate nickel contents, became unstable at lower temperatures. Many first transformed to martensite, a distorted, supersaturated structure, which then decomposed into kamacite and taenite on very fine scales. Depending on composition and cooling history, metallographers describe several morphologies, such as comb, net, cellular and martensitic plessite, and the variety in one meteorite can record a whole range of local nickel contents. Plessite fields are well displayed in fine octahedrites like Gibeon and in medium octahedrites like Henbury.
Under a low-power microscope or hand lens on an etched slice, plessite appears as dark, finely textured or speckled patches bounded by bright taenite rims. High magnification or electron microscopy is needed to resolve the individual phases.
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