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Social Circle |
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Meteorite - Social Circle, IVA Iron. |
Meteorite - Gibeon, IVA Iron. |
All recrystallized structures in Social Circle are secondary. |
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| Figure 1. Fully recrystallized microstructure of the Social Circle (IVA) iron meteorite. All kamacite is recrystallized; taenite and plessite fields are largely decomposed and show incipient spheroidization. Scale bar: 600 µm. |
| Meteorites: Social Circle (IVA iron) and Gibeon (IVA iron) All recrystallized structures observed in the Social Circle meteorite are secondary structures. The Social Circle iron meteorite, a fine octahedrite of the IVA chemical group recovered in Georgia, USA (total known mass ~100 kg), exhibits pervasive recrystallization throughout its mass. This results in a uniform granulated texture in which primary kamacite has been fully recrystallized into equant grains, while taenite and plessite fields show advanced breakdown and the onset of spheroidization. These features indicate significant post-formation thermal processing that overprinted the original Widmanstätten pattern. In his seminal 1975 Handbook of Iron Meteorites, Buchwald documented the absence of evidence for artificial (terrestrial) reheating and concluded that the recrystallization resulted from cosmic processes. He noted prominent “rhythmic growth lines” extending across large portions of the specimen, interpreted as evidence of cyclic thermal excursions. These were attributed to repeated close approaches to the Sun—orbits with small perihelion distances—during which solar radiation provided sufficient heat to drive recrystallization without complete melting. Comparative Analysis with Gibeon For context, the Gibeon IVA iron meteorite (Namibia) displays partial recrystallization of its fine octahedral Widmanstätten pattern, affecting kamacite and taenite to a lesser degree than in Social Circle. Gibeon retains clearer vestiges of the primary intergrowth texture, highlighting the more extreme thermal history experienced by Social Circle within the same chemical group. Implications for Thermal History The pronounced recrystallization in Social Circle, without shock-melted troilite in all interpretations, supports models of solar heating at low perihelion. This phenomenon provides rare insight into the orbital dynamics of meteorite parent-body fragments in the inner Solar System. Subsequent studies have explored both solar radiation and impact-induced shock heating as potential mechanisms, with the rhythmic banding favoring periodic solar influence. Such features underscore the complex post-accretionary histories of IVA irons, which record diverse cooling rates and collisional processing within their parent asteroid’s core. |
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| Figure 2. Recrystallized microstructure of the Gibeon (IVA) iron meteorite. The fine octahedral Widmanstätten pattern of kamacite and taenite shows recrystallization, though less extensively than in Social Circle. Scale bar: 600 µm. |
| Primary / Foundational References
McCallie, S.W. (1927). “Notes on the Social Circle meteorite.” American Journal of Science, 5(76), 360. (Initial description after recognition by the Georgia state geologist.) Henderson, E.P. & Perry, S.H. (1951). “A restudy of the Social Circle, Georgia, meteorite.” American Mineralogist, 36(7-8), 603–608. (Key re-analysis, density, composition, and structural observations on granulation/reheating.) Buchwald, V.F. (1975). Handbook of Iron Meteorites (Vol. 3, p. 1151). University of California Press. (Detailed entry on history, structure, and metallurgy.) Modern / Mineralogical References Isa, J., Ma, C., & Rubin, A.E. (2016). “Joegoldsteinite: A new sulfide mineral (MnCr2S4) from the IVA iron meteorite, Social Circle.” American Mineralogist, 101(5), 1217–1221 (and associated LPSC abstract). (Describes a new thiospinel mineral named after Joe Goldstein; also notes occurrences in other meteorites.) “Meteorites in Georgia” (1966) monograph (Georgia Department of Mines, Mining & Geology) — includes photos, analysis, and context for the specimen. |
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