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Elongated Cohenite |
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| Meteorite: Uruacu, IAB Iron. |
| Figure 1. Scale bar 1.4 mm. |
| Meteorite: Uruacu, IAB Iron |
| Elongated cohenite. |
| Elongated Cohenite in the Uruaçu IAB
Iron Meteorite Figure 1. Elongated cohenite crystals in the Uruaçu IAB iron meteorite. Scale bar=1.4 mm. Figure 2. Additional view of elongated cohenite. Scale bar=1.4 mm. Mineralogy and Petrography Cohenite ((FeNiCo)3C), an orthorhombic iron carbide, is a common accessory mineral in many iron meteorites, particularly those with appropriate carbon contents and relatively low phosphorus. In the Uruaçu meteorite (classified as a coarse octahedrite within the IAB complex, recovered from Goiás, Brazil, in 1992), cohenite forms distinctive elongated crystals that often display a tarnished bronze luster in polished and etched sections. These bodies are typically aligned parallel to or within kamacite lamellae of the Widmanstätten pattern. Mechanisms of Directional (Anisotropic) Growth The elongated habit of cohenite arises during primary crystallization and cooling in the metallic core (or melt pool) of the parent body. Key controlling factors include: • Temperature Gradients and Cooling Kinetics: Spatial variations in cooling rates establish thermal gradients within the metal. Cohenite crystals preferentially elongate in the direction of the maximum temperature gradient, where diffusion and supersaturation drive anisotropic attachment. The relatively slow cooling characteristic of many IAB irons allows sufficient atomic mobility for extended crystal development. • Chemical Gradients and Bulk Composition: Local heterogeneities in carbon activity, nickel, and other elements promote diffusion-controlled growth. Uruaçu is notably cohenite- and schreibersite-rich, consistent with conditions favoring carbide precipitation from carbon-supersaturated solid solutions. • Pressure-Temperature Stability: Cohenite is stable in the approximate range of 650–610 °C for typical iron meteorite compositions. Ambient P-T conditions during cooling influence nucleation density, growth rate, and preferred orientations. • Heterogeneous Nucleation: Micron-sized inclusions (e.g., troilite, graphite, schreibersite, daubréelite, or minor silicates) serve as nucleation substrates. These particles reduce the activation energy for precipitation and steer subsequent growth along favorable crystallographic or diffusion pathways. In octahedrites, alignment with kamacite plates often reflects structural registry established during the taenite-to-kamacite transformation. Primary vs. Secondary Context These elongated cohenite bodies are primary structures that record the slow cooling history of the IAB parent body (or an impact-generated metal pool). The IAB complex is noted for complex, non-magmatic origins involving partial melting and mixing with chondritic material, which can supply carbon and promote carbide formation. Unlike secondary features (e.g., shock-deformed carbides or decomposition products such as graphite + metal), well-preserved elongated cohenite in Uruaçu preserves the primary metallographic record. Broader Implications Cohenite morphology, orientation, and associations provide valuable proxies for cooling rates, carbon abundance, redox conditions, and thermal evolution in iron meteorite parent bodies. In the context of the IAB complex, such features support models of impact-induced melting on chondritic precursors. Complementary microanalytical methods—such as electron microprobe analysis for partitioning behavior, transmission electron microscopy for interfaces, or electron backscatter diffraction for crystallography—can yield quantitative constraints on formation conditions. Uruaçu thus serves as an excellent example of how accessory carbide phases encode detailed aspects of asteroidal differentiation and cooling histories within the broader Widmanstätten framework. |
| Figure 2. Scale bar 1.4 mm. |
| Meteorite: Uruacu, IAB Iron |
| Elongated cohenite. |
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| References • Buchwald, V. F. (1975). Handbook of Iron Meteorites. University of California Press. • Meteoritical Bulletin Database (Uruaçu entry, IAB complex). • Scott, E. R. D. (2020). “Iron Meteorites: Composition, Age, and Origin.” Oxford Research Encyclopedia of Planetary Science. • Additional literature on carbide stability, occurrence, and growth in iron meteorites (e.g., studies referenced in discussions of cohenite formation temperatures and habits). |
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