Kamacite spindles typically form in iron
meteorites with a nickel content ranging from 4% to 7.5%. In irons with higher
nickel levels, such as 13%, the development of large kamacite spindles becomes
less common, resulting in a narrower range of spindle structures.
Plessitic octahedrites represent a transitional
group between octahedrites and ataxites in iron meteorites. These contain
kamacite spindles embedded within a matrix of taenite and plessite, often
accompanied by accessory minerals like schreibersite and troilite. Elevated
nickel content (>13%) tends to inhibit the formation of the
Widmanstätten pattern, a characteristic feature of octahedrites.
In plessitic octahedrites, large kamacite spindles
typically measure 2 to 32 mm in length and 0.5 to 1.5 mm in width. These
spindles are often tapered and exhibit three distinct orientations on cut
surfaces, resembling the Widmanstätten pattern. Their formation occurs
during the slow cooling of the meteorite, as kamacite exsolves from taenite,
giving rise to these elongated, spindle-like structures.
In microscopy images of the Cowra meteorite-an
ungrouped iron with 13.38% nickel-large kamacite spindles appear as swathing
kamacite enveloping primary platy schreibersite crystals. Within the plessitic
matrix, numerous smaller spindles are also visible, having nucleated from
kamacite grains. Buchwald (1975) originally classified Cowra as a plessitic
octahedrite, but modern refinements in classification systems have since
designated it as "ungrouped."
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