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Troilite "Spiders" |
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| Meteorite - Willamette, IIIAB Iron. |
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| Figure 1. Scale bar 80 µm. |
| Meteorite, Willamette, IIIAB iron |
Troilite "Spiders" in the Willamette IIIAB Iron Figure 1. Troilite “spiders” (filamentous extensions from troilite inclusions) in the Willamette IIIAB iron meteorite. Scale bar=80 µm. Figure 2. Additional view of troilite "spider" morphology. Scale bar=120 µm. Figure 3. Detailed troilite filaments intersecting kamacite and plessite fields. Scale bar=50 µm. The main mass of the Willamette meteorite is housed at the American Museum of Natural History, New York. Overview and Classification The Willamette meteorite (also known as Tomanowos to the Clackamas Chinook people) is a large IIIAB iron. It weighs approximately 15.5 tons (14,500 kg) and ranks among the largest iron meteorites recovered on Earth. Its bulk composition is dominated by iron (~91 wt% Fe) with ~7.62 wt% Ni, plus minor Co, P, and trace siderophiles (e.g., Ga, Ge, Ir). Willamette exhibits a complex, recrystallized microstructure with only relict traces of a medium Widmanstätten pattern, the result of significant post-formation shock and thermal processing. Troilite (FeS) occurs as large nodules (typically 1–3 cm or larger), lenticular inclusions, and distinctive “spider”-like filamentous structures. These “troilite spiders”—radiating filaments extending from central troilite masses—are a notable (though not entirely unique) feature, also reported in other irons such as Willow Creek (IAB). Primary Structures During initial slow cooling in the parent body core, Willamette developed a medium Widmanstätten pattern through the exsolution of kamacite (α-FeNi) lamellae within taenite (γ-FeNi). Troilite nodules formed from trapped residual S-rich melts during solidification, commonly accompanied by minor phosphides. These represent primary crystallization and cooling features typical of magmatic iron meteorites in the IIIAB group. "Secondary Structures and Shock-Thermal History", Buchwald (1975) and subsequent studies (e.g., Rubin 2015) describe a multi-stage history involving impact events that profoundly modified the primary textures: 1. Major Shock and Reheating: A significant impact caused intense shock waves and reheating. Troilite nodules were partially shock-melted, allowing molten sulfide to penetrate and inject into surrounding kamacite and plessite fields along multiple directions. Concurrent crushing of metal grains occurred, with metal wedges driven into fractured troilite. Post-shock annealing led to recrystallization of kamacite (producing equiaxed grains) and troilite, with incorporation of small phosphide and taenite particles into the sulfide. 2. Lesser Shock Event: A later, milder impact introduced Neumann lines (mechanical twins) into the recrystallized kamacite. Associated annealing polygonized the kamacite grains but did not cause further widespread recrystallization. 3. Atmospheric Entry and Weathering: Willamette’s terrestrial residence (estimated at tens of thousands of years) in the humid Willamette Valley environment led to extensive terrestrial corrosion. No fusion crust or heat-affected zone remains, consistent with prolonged surface exposure and ablation/weathering. The “troilite spiders” are interpreted as secondary features arising from this sequence. Shock-melting and injection of troilite created filamentous extensions that radiate outward, cross-cutting kamacite and intersecting pre-existing plessite fields. These structures reflect plastic flow and injection of low-viscosity sulfide melt or semi-molten material during impact events. Role of "Troilite Spiders" in Terrestrial Corrosion The meteorite’s exterior displays a dramatic network of large cavities and pits (some exceeding 65 cm in length and 45 cm in depth), distinct from typical regmaglypts (aerodynamic ablation features). These cavities result primarily from prolonged terrestrial weathering. Troilite is particularly susceptible to oxidation; rainwater interacting with FeS produces dilute sulfuric acid (via reactions forming H2SO4), which drives electrochemical corrosion of the surrounding Fe-Ni metal. The spider-like filaments created preferential diffusion pathways and sites for the sulfuric acid attack, accelerating localized dissolution and contributing to the formation of deep cavities. This process highlights how primary (sulfide nodules) and secondary (shock-mobilized filaments) troilite distributions can control post-arrival alteration. Broader Implications Willamette exemplifies the utility of iron meteorites in reconstructing asteroidal collisional histories. Its recrystallized structure and troilite injection features record at least two major impact events on the IIIAB parent body, followed by ejection, atmospheric entry, and extended terrestrial weathering. Comparative studies with other shocked irons refine models of shock pressures, post-shock temperatures, and sulfide-metal interactions under extreme conditions. |
| Figure 2. Scale bar 120 µm. |
| Meteorite, Willamette, IIIAB |
| Figure 3. Scale bar 50 µm. |
| Meteorite, Willamette, IIIAB |
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| Figure 4. |
| The main mass of Willamette, American Museum of Natural History, New York. |
| References
• Buchwald, V. F. (1975). Handbook of Iron Meteorites. University of California Press. • Rubin, A. E. (2015). “Shock effects in the Willamette ungrouped iron meteorite.” Meteoritics & Planetary Science, 50(12), 1984–1994. • Wasson, J. T. (various works on IIIAB irons and troilite in iron meteorites). • Additional sources: American Museum of Natural History documentation; Meteoritical Bulletin Database; Wikipedia summary drawing on primary literature (for basic facts). |
| Cultural/Historical: notes |
| (Tomanowos) appear in AMNH documentation and secondary sources, but mineraloy is best described in the referenced papers listed above. |
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