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Plessitic octahedrite, Ballinoo |
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| Meteorite, Ballinoo, IIC Iron |
| Figure 1. Scale bar 300 µm. |
| Meteorite, Ballinoo, IIC iron |
| Recrystallized kamacite lamellae. |
| Plessitic octahedrites form
through slow cooling over millions of years, during which they undergo a phase
separation that forms different iron-nickel minerals - kamacite, taenite,
plessite (plessite is not a mineral, it’s a combination of kamacite and
taenite that has intergrown into a fine mixture). Additionally, the minerals schreibersite, cohenite, and troilite are common. Iron meteorites that are entirely plessitic are relatively rare, Ballinoo, IIC is one of them. The entire matrix of Ballinoo is a fine-grained plessitic mixture. Plessitic octahedrites typically have a nickel content ranging from 9% to 18% and often appear as transitional between octahedrites and ataxites. For Ballinoo, the plessitic matrix consists of well-defined alpha (taenite) and gamma (kamacite) particles of about 1 – 2 µm width. The spike formation of kamacite in the plessite of Ballinoo is primarily influenced by cooling and crystallization, Zucolotto, M.E., Faina, Brazilian plessitic octahedrite, 2015. Slow cooling allows for the formation of kamacite spindles or spikes as the high-temperature taenite phase exsolves into lower-temperature kamacite.The presence of nucleation sites, such as schreibersite (iron-nickel phosphide) crystals, can promote the growth of kamacite spikes. These sites provide a surface for kamacite to nucleate and grow. Nickel content and phosphorus also influence the formation and growth of the spiky kamacite seen in Ballinoo. |
| Figure 1. Scale bar 200 µm. |
| Meteorite, Ballinoo, IIC iron |
| Recrystallization effects seen in the kamacite lamellae. |
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