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Open mesh Plessite and Kamacite necking |
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| Meteorite, Bristol, IVA Iron |
Figure 1. Scale bar 1.0 mm. |
| Meteorite, Bristol, IVA Iron |
| Kamacite lamellae necking, two plessite fields--open mesh and decomposing comb. |
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| The term "necking" in
iron meteorites refers to a specific type of deformation within their metallic
structure. This phenomenon, often triggered by stress or impact, results in a
narrowing of the meteorite's features, resembling the way metals stretch and
thin under tensile stress on Earth. Iron meteorites consist mainly of nickel-iron alloys, namely kamacite and taenite. Following their initial cooling in space, these meteorites-to-be endured collisions with other bodies and, later, intense shock waves during their fiery descent through Earth's atmosphere. These events caused the metal to twist, tear, or deform, producing distinctive structural patterns within the meteorite. When a meteorite experiences a sudden, powerful impact, shock waves ripple through it, creating extreme pressures and temperatures. These forces drive plastic deformation in the iron alloys, permanently reshaping the material without necessarily breaking it apart. Such shock-induced changes also alter the meteorite's microstructure, sometimes forming features like Neumann lines. The nature of kamacite deformation in necked regions near the meteorite's surface is a telltale sign of atmospheric breakup. In contrast, widespread deformations in larger or deeper sections suggest a more catastrophic event, such as the breakup of a larger body or the impact itself. Necking, along with other signs of deformation or tearing, is considered a secondary feature, reflecting the meteorite's violent history after its initial formation. |
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