Primary and Secondary Structures - Meteorites
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Taenite, Martensite and dense Plessite fields
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Meteorite - Bella Roca, IIIAB Iron.
 
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Figure 1. Scale bar 250 µm.
Meteorite: Bella Roca, IIIAB, Iron
Taenite, martensite, plessite fields.
 
 
 
This is #50, the last topic (for now) of this visual presentation of Primary and Secondary structures - Meteorites (primarily focusing on iron meteorites).



Taenite, Martensite, and Dense Plessite Fields in the Bella Roca IIIAB Iron Meteorite

Figure 1.
Taenite, martensite, and dense plessite fields in the Bella Roca IIIAB iron. Scale bar=250 µm.

Figure 2.
Additional view of taenite, martensite, and dense plessite fields in the Bella Roca IIIAB iron. Scale bar=250 µm.


The Bella Roca IIIAB iron meteorite encapsulates both primary nebular/parent-body differentiation processes and secondary shock modification. As a member of the largest magmatic iron group, it originated through fractional crystallization of a metallic core in a differentiated asteroid. Its microstructures—taenite, martensite, and plessite—record a complex thermal and impact history spanning millions of years.

Primary Structures:
Widmanstätten Pattern and Plessite Slow cooling (~1–10 °C/Myr) of the Ni-rich metal produced the classic Widmanstätten intergrowth of kamacite α-FeNi, (low-Ni) lamellae within a taenite (γ-FeNi, (high-Ni) host. In regions of higher local Ni content or faster final cooling, the residual taenite decomposes into plessite—fine-scale intergrowths of kamacite and taenite. Dense plessite fields in Bella Roca appear as intricate, granular to lamellar zones filling interstices between coarser kamacite plates. Phosphorus, rejected during kamacite growth, concentrated in residual melts and exsolved as schreibersite ((FeNi)3P), a common accessory phase.

Secondary Shock-Induced Features
Subsequent hypervelocity collisions on the parent body introduced shock waves that profoundly modified these primary textures.

Key secondary features include:
• Martensite: Formed by rapid cooling of taenite across the martensite start temperature (Ms, typically ~200–400 °C depending on Ni content). Shock heating followed by rapid quenching transforms portions of taenite into a supersaturated, tetragonal α2-martensite phase, often preserved as acicular or lath-like structures within plessite fields.
• Dense plessite modification: Shock deformation and reheating refine or recrystallize plessite, producing denser, more complex intergrowths.
• Epsilon iron (e-Fe) and other high-pressure phases: Transient high pressures can stabilize the hexagonal close-packed e-phase (hatched appearance in etched sections), sometimes preserving schreibersite inclusions from pre-shock cooling.
• Elemental redistribution: Shock-induced melting or enhanced diffusion creates phosphorus-enriched pockets and secondary precipitates. These features indicate moderate to high shock pressures (e.g., 10–100+ GPa) and variable post-shock cooling rates, common in the collisional environment of the asteroid belt.

Overall History and Significance
Bella Roca exemplifies the multi-stage evolution (chronology) of iron meteorites: primary slow core crystallization - Widmanstätten and plessite development - impact-driven shock reheating - phase transformations, and recrystallization. Metallographic studies of such specimens constrain parent-body size, cooling rates, and impact gardening history. They also provide natural analogs for high-pressure/temperature behavior of Fe-Ni alloys relevant to planetary cores and industrial applications.


 
 
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Figure 2. Scale bar 250 µm.
Meteorite: Bella Roca, IIIAB, Iron
Taenite, martensite, plessite fields.
 
 
 



References
• Buchwald, V. F. (1975). Handbook of Iron Meteorites. University of California Press. (Comprehensive petrography).
• Goldstein, J. I., et al. (various works on Fe-Ni phase transformations, plessite, and martensite).
• Meteoritical Bulletin Database: Bella Roca (IIIAB iron).
• Scott, E. R. D. (2007). Iron meteorites: Composition, age, and origin. Oxford Research Encyclopedia of Planetary Science.
• Yang, J., et al. (shock metamorphism studies in IIIAB irons).
Page #50. This concludes the current visual series on primary and secondary structures in meteorites, primarily focusing on iron meteorites.
 
 
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