Primary and Secondary Structures - Meteorites
New England Meteoritical Services
 

Recrystallized Taenite Lamellae aggregates in Seneca Falls
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 Meteorite - Seneca Falls, IIAB Iron.
 
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Figure 1.. Scale bar 100 µm.
Meteorite, Seneca Falls, IIAB Iron.
Recrystallized taenite aggregates.
 
 
Figure 1.
Recrystallized taenite aggregates in the Seneca Falls IIAB iron meteorite. Scale bar=100 µm.

Figures 2–4.
Additional views of recrystallized taenite aggregates in the Seneca Falls IIAB iron meteorite. Scale bar=100 µm (each).


In iron meteorites, taenite (γ-FeNi; face-centered cubic) occurs as lamellae within the Widmanstätten pattern intergrown with kamacite (α-FeNi; body-centered cubic). In the Seneca Falls IIAB iron, segments of these taenite lamellae exhibit pronounced recrystallization and fragmentation into aggregates of amoeba-like or spheroidal particles ranging from ~50 to 400 µm. These textures record a secondary, shock-induced thermal event superimposed on the primary slow-cooling history of the parent body.

Formation Mechanism
Such recrystallized aggregates form through intense shock reheating followed by rapid or moderately slow cooling. Hypervelocity impacts on the parent asteroid generate transient high-pressure and high-temperature excursions that can exceed several hundred GPa and hundreds of °C.

This causes:
• Mechanical fragmentation of existing taenite lamellae
• Localized melting or solid-state recrystallization
• Subsequent reorganization of the γ-phase into polycrystalline aggregates that minimize surface energy. Recrystallization initiates in the temperature window of approximately 500–700 °C, where stored strain energy from shock deformation drives the nucleation and growth of new, strain-free grains. Prolonged residence near or just below the γ/(α+γ) eutectoid boundary (~700–800 °C, depending on Ni content) favors spheroidization: the taenite decomposes or breaks into rounded, amoeboid particles as interfacial energies are minimized through diffusion-controlled grain-boundary migration and Ostwald ripening (where larger particles grow at the expense of smaller ones). Upon further cooling, these particles may partially transform to kamacite or plessite (fine intergrowths of α and γ).

Petrographic Context in Seneca Falls
The Seneca Falls IIAB meteorite displays clear evidence of shock modification. The segmented and recrystallized taenite lamellae contrast with the more regular Widmanstätten structures expected from primary, slow subsolidus cooling (typical of IIAB irons at rates of ~1–10 °C/Myr). The amoeba-shaped morphologies are diagnostic of shock-reheated and annealed metal, where plastic deformation and transient heating allowed the taenite to recrystallize in situ without wholesale melting of the bulk meteorite. Similar features are observed in other shocked iron meteorites and provide petrographic markers for impact events on differentiated parent bodies.

Broader Implications
Recrystallized taenite aggregates serve as important indicators of secondary processing in iron meteorites. They complement other shock indicators such as Neumann bands (deformation twins in kamacite), martensite formation, or localized melting. In the context of IIAB irons—magmatic group members derived from a fractionally crystallized metallic core—these features document late-stage collisional evolution in the asteroid belt, potentially associated with the disruption of the parent body. Such impact histories influence cooling rate determinations, as shock reheating can reset or modify diffusion profiles used in metallographic thermometry. Overall, these structures highlight the transition from primary nebular/parent-body differentiation processes to secondary asteroidal impacts, offering insights into the dynamic environment of the early Solar System.

 
 
 
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Figure 2. Scale bar 100 µm.
Meteorite, Seneca Falls, IIAB Iron.
Recrystallized taenite aggregates.
 
 
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Figure 3. Scale bar 100 µm.
Meteorite, Seneca Falls, IIAB Iron.
Recrystallized taenite aggregates.
 
 
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Figure 4. Scale bar 100 µm.
Meteorite, Seneca Falls, IIAB Iron.
Recrystallized taenite aggregates.
 
 
 
 References
• Buchwald, V. F. (1975). Handbook of Iron Meteorites. University of California Press. (Detailed petrography of shock features).
• Clarke, R. S., Jr., & Goldstein, J. I. (1978). Schreibersite growth and its influence on the metallography of coarse-structured iron meteorites. Smithsonian Contributions to the Earth Sciences. (Relevant phase relations and recrystallization).
• Goldstein, J. I., et al. (various metallographic studies on iron meteorites).
• Meteoritical Bulletin Database: Seneca Falls (IIAB iron).
• Scott, E. R. D. (2007). Iron meteorites: Composition, age, and origin. Oxford Research Encyclopedia of Planetary Science. (Overview of magmatic groups and shock history).
• Wasson, J. T. (1985). Meteorites: Their Record of Early Solar-System History. W. H. Freeman.
 
 
 
 
 
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