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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 develop
through protracted cooling over timescales of millions of years, during which
the high-temperature austenite (γ) phase undergoes solid-state phase
separation to produce the low-nickel mineral kamacite (α) and the
high-nickel mineral taenite (γ. Plessite, although not itself a mineral
species, constitutes a fine-scale intergrowth of kamacite and taenite that
occupies the regions between coarser Widmanstätten lamellae. Accessory minerals commonly present in these meteorites include the phosphide schreibersite, the carbide cohenite, and the sulfide troilite. Fully plessitic iron meteorites remain uncommon. Ballinoo, a member of chemical group IIC, provides a well-documented example in which the entire metallic matrix consists of a fine-grained plessitic aggregate. Plessitic octahedrites typically contain between approximately 9 and 18 wt% nickel and occupy a transitional structural position between conventional octahedrites and the nickel-rich ataxites. In Ballinoo the plessitic matrix is composed of intimately intergrown kamacite (α) and taenite (γ) particles whose individual widths are on the order of 1–2 µm. Distinctive spiky or spindle-shaped kamacite morphologies within this matrix arise principally from the kinetics of cooling and crystallization. Slow cooling permits the progressive exsolution of kamacite from the parent taenite, generating elongated spindles or spikes. Heterogeneous nucleation is frequently facilitated by pre-existing schreibersite crystals, which supply favorable interfaces for the initiation and subsequent growth of kamacite. Local concentrations of nickel and phosphorus further modulate both the abundance and the morphological character of these kamacite spikes. |
| Figure 1. Scale bar 200 µm. |
| Meteorite, Ballinoo, IIC iron |
| Partial recrystallization effects seen in the kamacite lamellae. |
| References Buchwald, V. F. (1975). Handbook of Iron Meteorites. University of California Press. Goldstein, J. I., & Michael, J. R. (2006). The formation of plessite in meteoritic metal. Meteoritics & Planetary Science, 41(4), 553–570. Tornabene, H. A., et al. (2020). Genetics, age and crystallization history of group IIC iron meteorites. Geochimica et Cosmochimica Acta, 288, 1–18. Zucolotto, M. E., Carvalho, W. P., Tosi, A. A., & Mendes, J. C. (2015). Faina, a new Brazilian plessitic octahedrite from group IAB. 78th Annual Meeting of the Meteoritical Society, Abstract #5382. |
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