|
||
Plessitic octahedrite, Ballinoo |
| Listing of Structures |
| Back...Next |
| Meteorite, Ballinoo, IIC Iron |
| Figure 1. Scale bar 300 µm. |
| Meteorite, Ballinoo, IIC iron |
| Recrystallized kamacite lamellae. |
| Plessitic octahedrites form through a prolonged cooling process spanning millions of years, during which a phase separation in austenite forms kamacite and taenite. Though not a mineral, plessite emerges as a fine-scale intergrowth of kamacite and taenite. These meteorites often contain additional minerals such as schreibersite, cohenite, and troilite. Fully plessitic iron meteorites are uncommon, with Ballinoo (classified as IIC) being a notable example. Its entire matrix consists of a fine-grained plessitic mixture. Plessitic octahedrites typically exhibit nickel contents between 9% and 18%, positioning them as a transitional form between octahedrites and ataxites. In Ballinoo, the plessitic matrix features well-defined alpha-phase (taenite) and gamma-phase (kamacite) particles, each measuring approximately 1-2 µm in width. The distinctive spiky formations of kamacite within Ballinoo's plessite are primarily shaped by cooling and crystallization processes, as noted by Zucolotto, M.E. in Faina, Brazilian Plessitic Octahedrite (2015). Slow cooling enables the high-temperature taenite phase to exsolve into kamacite, forming spindles or spikes. Nucleation sites, such as schreibersite (an iron-nickel phosphide), further facilitate this process by providing surfaces for kamacite to initiate and grow. Nickel and phosphorus concentrations in the meteorite also influence the development and morphology of these kamacite spikes. |
| Figure 1. Scale bar 200 µm. |
| Meteorite, Ballinoo, IIC iron |
| Partial recrystallization effects seen in the kamacite lamellae. |
| Listing of Structures |