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Degenerate Plessite |
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| Meteorit -Henbury, IIIAB Iron. |
| Figure 1. |
| Meteorite: Henbury, IIIAB, iron. |
| Degenerate plessite in the Henbury iron meteorite (IIIAB group; bulk Ni 7.41 wt%). Scale bar=1.4 mm. |
| Degenerate plessite represents a distinctive microstructural variant observed primarily in the central regions of plessitic fields within certain iron meteorites. It consists of a fine-grained, disordered intergrowth of kamacite (α-FeNi; low-nickel body-centered cubic phase, typically ~5–7 wt% Ni) and taenite (γ-FeNi; higher-nickel face-centered cubic phase, often 20–50 wt% Ni or more in zoned regions). This texture arises during the slow cooling of the parent body’s metallic core or pool, where the eutectoid decomposition of taenite and associated phase transformations proceed in an incomplete or arrested manner, yielding irregular, poorly delineated boundaries and a chaotic, mottled appearance rather than organized geometric patterns. Formation and Microstructural Characteristics In iron meteorites, plessite forms in the interstices between Widmanstätten kamacite lamellae as the high-temperature taenite phase cools below the (γ α+γ) two-phase field and undergoes diffusional decomposition. Multiple plessite morphologies have been documented, including comb, net, cellular, acicular, duplex, black, and pearlitic varieties, each reflecting specific nucleation and growth conditions influenced by local nickel content, cooling rate, and minor element abundances (e.g., P, S, Co). Degenerate plessite is distinguished by its lack of coherent organization. It appears as a secondary or transitional feature, often centrally located within broader plessitic regions, where initial nucleation and growth of kamacite and taenite interfaces begin but fail to develop into well-defined structures. This results in a fine-scale, heterogeneous blend with diffuse phase boundaries and an overall “degenerate” or incomplete morphology. Nickel diffusion and partitioning play important supporting roles—particularly in lower-bulk-nickel irons (~7 wt% Ni, as in many IIIAB members)—but the dominant control is the meteorite’s thermal history. Slower or interrupted cooling can halt the transformation sequence, preventing full development of more ordered plessite types. In Henbury (a medium octahedrite), this is evident in the central zones of plessitic fields, where taenite margins may thin or fade. This texture is not confined to a single plessite classification but can manifest within or transition from various named varieties. Its occurrence underscores the kinetic complexity of Fe-Ni phase transformations under the extreme conditions of asteroidal cooling (typically 1–100 °C/Myr in the relevant temperature range). Broader Context The study of plessite microstructures, including degenerate forms, provides critical insights into the cooling rates, parent-body sizes, and thermal evolution of differentiated asteroids. Detailed metallographic analysis, as pioneered in comprehensive works on iron meteorites, remains essential for classifying specimens and reconstructing their petrogenetic histories. In the case of Henbury (IIIAB), the presence of degenerate plessite aligns with its classification and observed shock/impact features from the well-documented crater field in Australia. References Buchwald, V. F. (1975). Handbook of Iron Meteorites: Their History, Distribution, Composition and Structure (3 volumes). University of California Press. (The foundational reference for the description, classification, and metallographic analysis of plessite varieties, including degenerate plessite in IIIAB irons such as Henbury.) Additional supporting literature: • Massalski, T. B., & others (speculations on plessite formation mechanisms). • Yang, J., Goldstein, J. I., & Scott, E. R. D. (1997 or later works on plessite microstructures). |
| • Recent studies on plessite microstructures via EBSD and other techniques (e.g., as reviewed in works on duplex/black plessite). |