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Neumann lines and Rhabdites |
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| Meteorite - Uwet, IIAB Iron |
| Figure 2. Scale bar 60 µm. |
| Neuman lines and rhabdites. Uwet, IIAB Iron. |
| Neumann lines are intricate,
parallel markings visible in the kamacite phase of many hexahedrite iron
meteorites. These patterns arise from shock-induced changes in the kamacite
crystal lattice, usually triggered by high-energy impacts on the meteorite's
parent body, though they can also form upon entering Earth's atmosphere. First
identified in 1848 by Johann G. Neumann, they are regarded as secondary
structures, shaped by events after the meteorite's initial formation.
Rhabdites are iron-nickel phosphide minerals found within the kamacite matrix. They take shape during the primary cooling and crystallization of the meteorite's parent body. As the molten iron-nickel mixture solidifies, these phosphide minerals precipitate out and become encased within the kamacite structure. A notable subtype, prismatic rhabdites, are elongated, prism-like crystals of iron-nickel phosphide. Their distinct, fracture-free form contrasts with plate-like rhabdites, which often exhibit microscopic fractures. Despite their specific morphology, prismatic rhabdites remain secondary structures within the kamacite groundmass. Schreibersite and rhabdites share the same chemical makeup, (Fe,Ni)3P. The distinction between the two lies in their crystal size, shape, and formation dynamics. Schreibersite emerges as larger, well-developed crystals under slow cooling conditions, giving atoms time to organize into substantial, defined structures. In contrast, rapid cooling leads to rhabdites, which appear as smaller, needle-like or prismatic crystals formed through exsolution from the metallic matrix. |
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