|
||
Comb plessite |
| Listing of Structures |
| Back...Next |
| Meteorite - Carbo, IID Iron. |
 |
| Figure 1. Comb plessite in the Carbo iron meteorite (IID). Scale bar=900 µm. |
| Plessite: Definition and Formation Plessite is a fine-grained, two-phase intergrowth of kamacite (α -FeNi) and taenite (γ-FeNi), commonly accompanied by minor accessory phases such as schreibersite (FeNi)3P) and tetrataenite (ordered FeNi). It is not a discrete mineral species but a microstructural term encompassing a range of complex textures that develop in the residual taenite domains during the later stages of cooling in iron meteorites. Plessite typically forms within the fields bounded by the Widmanstätten pattern (kamacite lamellae exsolved from parent taenite). These fields represent the final products of diffusional transformation as the meteorite cooled through the Fe-Ni phase diagram’s two-phase (α + γ) region. A continuous rim of taenite often borders plessite fields, reflecting localized Ni enrichment and stabilization of the γ phase at the interfaces. Comb Plessite Morphology Under conditions of particularly slow primary cooling, larger plessite fields can develop distinctive “comb” or “comb-like” morphologies. These exhibit parallel, elongated kamacite platelets or rods arranged in a regular, tooth-like array reminiscent of a comb. This texture arises from oriented nucleation and growth of kamacite within the taenite host, influenced by crystallographic orientation relationships (e.g., Kurdjumov-Sachs or Nishiyama-Wassermann) and diffusion kinetics. Comb plessite is regarded as a secondary structure, overprinting or developing within the primary Widmanstätten framework. Its formation requires sufficient time for diffusion-controlled coarsening and alignment parallel to the host kamacite lamellae. The Carbo Meteorite (IID) The Carbo meteorite, classified as a member of the IID chemical group of iron meteorites, exemplifies comb plessite development. IID irons are characterized by moderate Ni contents (typically ~11–13 wt% Ni) and distinct trace-element signatures (e.g., Ga, Ge, Ir). Carbo’s estimated primary cooling rate through the critical temperature range (~700–300°C) is approximately 10–20°C per million years—relatively slow even among magmatic irons. This protracted cooling allowed extensive diffusion and the development of well-defined Widmanstätten patterns alongside large, comb-textured plessite fields. In Carbo, the alignment of comb plessite structures closely follows the orientation of the primary Widmanstätten kamacite bands, consistent with their formation as secondary features during continued slow cooling of retained taenite. Such textures provide insights into the thermal evolution of the IID parent body core, supporting models of fractional crystallization in differentiated asteroidal cores with subsequent slow cooling in an insulated environment. Broader Implications The study of plessite varieties, including comb plessite, contributes to understanding cooling rates, parent-body sizes, and the timing of metal solidification in the early solar system. Quantitative metallographic analysis of kamacite bandwidths and plessite textures, combined with numerical modeling of Fe-Ni diffusion, allows reconstruction of thermal histories. Anomalies or variations in these structures can also record later perturbations, such as shock reheating or impact events. This microstructural diversity underscores the value of detailed petrographic examination in iron meteoritics, bridging macroscopic classification with microscale processes. Further high-resolution imaging (e.g., SEM, EBSD) and microchemical mapping could refine nucleation mechanisms and quantify diffusion parameters in specimens like Carbo. |
| References • Buchwald, V. F. (1975). Handbook of Iron Meteorites. University of California Press. — The primary reference. Buchwald describes comb plessite in detail across multiple groups (e.g., IIIAB, IVA, and ungrouped irons), noting its occurrence as a fine, oriented intergrowth resembling a comb or feather. He illustrates it in etched sections and links it to local cooling rate variations and Ni/P content. • Goldstein, J. I., et al. (2009). Iron meteorites: Crystallization, thermal history, parent bodies. Chemie der Erde – Geochemistry, 69(4), 293–325. — Discusses plessite varieties, including comb plessite, in the context of Widmanstätten pattern breakdown and martensite decomposition during cooling. • Yang, J., & Goldstein, J. I. (2005). The formation of the Widmanstätten structure in meteorites. Meteoritics & Planetary Science. — Covers mechanisms leading to comb and other plessite textures via martensite decomposition. • Scott, E. R. D. (1977). Various works on iron meteorite groups (e.g., in Geochimica et Cosmochimica Acta). — Notes comb plessite in specific chemical groups as an indicator of cooling history. |
| Listing of Structures |