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Mosaic aggregates in a hexahedrite |
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| Meteorite - Santo Antônio do Descoberto, IIAB Iron |
| Figure 1. Scale bar 150 µm. |
| Meteorite - Santo Antônio do Descoberto, IIAB Iron |
| Irregular troilite, daubreelite and metal mosaic aggregates. Note the abundance of Neumann lines. |
Hexahedrites are a low nickel classification of iron meteorite. They are composed almost exclusively of the nickel-iron alloy kamacite and are lower in nickel content than octahedrites. The nickel concentration in hexahedrites has consistently been reported to be below 5.8% and only rarely below 5.3%. The name comes from the cubic (i.e., hexahedron) structure of the kamacite crystal. Hexahedrites do not display a Widmanstätten pattern but often display Neumann lines, indicative of impact shock at some time after the primary cooling of taenite-kamacite. Mosaic aggregates in hexahedrites form due to the rapid cooling and reheating processes the meteorite undergoes during its time in space and passage through Earth’s atmosphere. These aggregates consist of irregularly shaped kamacite grains with various orientations, creating a mosaic-like appearance. The overall structure results from the transformation of the alpha-iron phase to gamma-iron and back to alpha during heating above 750° C and subsequent rapid cooling. They are secondary structures. |
| Figure 2. Scale bar 100 µm. |
| Meteorite - Santo Antônio do Descoberto, IIAB iron |
| Mosaic aggregates. |
| Figure 3. Scale bar 30 µ. |
| Meteorite - Santo Antônio do Descoberto, IIAB iron |
| Irregular troilite, daubreelite and metal mosaic aggregates. |
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