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Acicular plessite |
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| Meteorite, Glorieta Mountain, Pallasite, MG |
| Figure 1. Scale bar 150 µm. |
| Meteorite, Glorieta Mountain, Pallasite, MG |
| Acicular plessite field in Glorieta Mountain. |
| Acicular plessite consists of a
fine-grained intergrowth of kamacite and taenite. "Acicular" refers
to the needle-like shape of the kamacite crystals within this
mixture. This textured mixture forms as the meteorite cools from high temperatures, 1,200° C, with the kamacite and taenite phases developing in the retained taenite of the metal. It is one of the several plessite types, including Granular, Net, Comb, and others. Specifically, acicular plessite forms as "fields" in iron meteorites through taenite's slow cooling and phase transformation. As the temperature drops, kamacite forms within the taenite through exsolution, where the taenite phase separates into kamacite and residual taenite. The result is a fine-grained intergrowth of kamacite and taenite, with the kamacite appearing as needle-like structures within the taenite. Acicular plessite in iron meteorites is considered a secondary structure. This is because it forms during the slow cooling and iron phase transformation of the meteorite after its initial solidification. Acicular plessite is typically found in octahedrites, forming in the gaps between the lamellae of kamacite and taenite. |
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