|
||
Neumann lines and Rhabdites |
| Listing of Structures |
| Back...Next |
| Meteorite - Uwet, IIAB Iron. |
| Figure 1. Scale bar 60 µm. |
| Neumann lines and rhabdites in the kamacite of the Uwet iron meteorite (IIAB group). Scale bar=60 µm. |
| Neumann Lines and Rhabdites in
Iron Meteorites Iron meteorites offer a dramatic window into the violent collisional history and slow thermal evolution of differentiated asteroids, with two distinctive microscopic features—Neumann lines and rhabdites—revealing key chapters in that story. Neumann Lines Neumann lines (also known as Neumann bands) are fine, parallel striations or lamellae observed within the kamacite (α-FeNi) grains of many iron meteorites, particularly hexahedrites and coarsest octahedrites. These features represent mechanical twins formed by shock deformation along specific crystallographic planes. They are secondary structures, typically resulting from high-velocity impacts on the meteorite’s parent body during its collisional history in the asteroid belt, although development can occasionally occur during atmospheric entry and deceleration. First systematically described in 1848 by Johann Georg Neumann, these lines provide important evidence of post-crystallization shock events. Their presence, orientation, and density can be used to infer the intensity and directionality of shock pressures (typically in the range of 5–100 GPa or higher for prominent development), helping reconstruct the dynamic history of the parent body prior to delivery to Earth. Rhabdites and Related Phosphides Rhabdites are acicular, prismatic, or needle-like crystals of iron-nickel phosphide, (FeNi)3P, that occur as exsolution products within the kamacite matrix of iron meteorites. They form during the slow cooling of the metallic phase in the parent body as phosphorus, which has limited solubility in kamacite at lower temperatures, exsolves from the supersaturated solid solution. This precipitation occurs as a primary to early-secondary feature during the long-term thermal evolution of the metal. Rhabdites are chemically identical to the related mineral schreibersite ((FeNi)3P). The distinction is primarily textural and genetic: schreibersite typically refers to larger, more equant or tabular crystals that nucleate and grow under slower cooling conditions, allowing for well-developed crystal faces, while rhabdites are smaller, elongated (prismatic or needle-like) forms that result from more rapid exsolution or restricted growth within the kamacite host. Prismatic rhabdites are notable for their relatively fracture-free morphology compared to some plate-like variants, which may develop microfractures due to volume changes or later stresses. Both phases are common accessories in iron meteorites (especially groups with moderate phosphorus contents, such as IIAB, which includes Uwet). Their size, distribution, and morphology serve as sensitive indicators of cooling rates and phosphorus partitioning during solidification and subsequent subsolidus exsolution. In IIAB irons like Uwet, rhabdites are often prominently developed within the kamacite of hexahedritic or coarsest octahedral textures. Broader Significance Together, Neumann lines and phosphide minerals illustrate the multi-stage history of iron meteorites: primary crystallization and slow cooling in the parent body core (recorded by phosphide exsolution), followed by impact-induced shock modification (Neumann lines). Detailed petrographic and microchemical studies of these features, often combined with cooling-rate modeling, constrain the sizes, thermal histories, and collisional evolution of differentiated asteroids. References • Buchwald, V. F. (1975). Handbook of Iron Meteorites. University of California Press. — The definitive reference with extensive descriptions, photographs, and discussions of Neumann bands and rhabdites/schreibersite in many irons, including IIAB examples. • Rubin, A. E. (various papers in Meteoritics & Planetary Science) — Shock effects and microstructure in iron meteorites, including Neumann lines as indicators of impact history. • Wasson, J. T. and others — Chemical groups (e.g., IIAB) and phosphide mineralogy/cooling rates. • Metallographic studies — Additional details on rhabdite morphology and Neumann band crystallography appear in journals such as Geochimica et Cosmochimica Acta and Meteoritics & Planetary Science. |
| Listing of Structures |