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Schreibersite crystals in an ataxite |
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| Meteorite, Gebil Kamil, Ungrouped iron, ataxitic structure |
| Figure 1. Scale bar 1 cm. |
| Meteorite, Gebel Kamil, Ungrouped, ataxitic structure. |
| Crystals of schreibersite, troilite, and daubreelite enveloped in swathing kamacite. Reflected light image. The black color is a lighting artifact. Note the shear deformation in the lower left corner. |
| The Gebel Kamil iron meteorite fell in Egypt
over 3000 years ago. It is notable for containing large schreibersite crystals.
At a 20% nickel content, it is classified as an ungrouped, Ni-rich iron with an ataxitic structure. In most irons, schreibersite crystals are typically tiny, millimeter-sized. In Gebel Kamil, the ataxitic structure is highlighted by cm-scale crystals of schreibersite that nucleated with kamacite spindles, forming a micro-Widmanstätten pattern. As the meteorite cooled, swathing kamacite formed around the schreibersite crystalline structures that also contain troilite and daubreelite. The formation sequence of most significant minerals depends on the cooling rate and the specific conditions during the meteorite’s solidification. During cooling, kamacite (a low-nickel iron phase) precipitates out of taenite (a high-nickel iron phase). Schreibersite, an iron-nickel phosphide, often forms at lower temperatures and can nucleate on kamacite as secondary crystallization. For a 20% Ni-rich ataxite, this is a likely formation scenario: One of the first minerals to form as the molten metal (austinite) begins to cool is taenite. It is a high-temperature phase of iron-nickel alloy with a face-centered cubic structure (FCC). As the temperature drops, kamacite, a body-centered cubic (BCC) iron-nickel alloy, begins to form. Kamacite and taenite often coexist in iron meteorites. Next would be schreibersite (it is recognized here that troilite forms at a little higher temperature but it is not part of the discussion). Schreibersite is an iron-nickel phosphide mineral. It can form at various cooling stages and is often associated with kamacite and taenite. The general sequence is taenite forms first, followed by kamacite and schreibersite. But, as seen in the following images, the large-sized schreibersite crystallization in Gebel Kamil is surrounded by swathing kamacite that nucleates out of taenite at around 723 degrees C, surrounding the schreibersite crystallization that typically nucleates in kamacite around 600 degrees C, or maybe a little higher depending upon the rate of cooling. So, it would seem that the large schreibersite crystallization preceded the swathing kamacite? No. Schreibersite is an iron-nickel phosphide. Phosphorus can influence the surrounding metal phases and lower the surrounding metal's melting point, allowing swathing kamacite to form around the schreibersite structures as the meteorite continues its primary cooling. |
| Figure 2. Scale bar 1 cm. |
| Meteorite, Gebel Kamil, ungrouped, ataxitic structure. |
| Crystals of schreibersite, troilite and daubreelite enveloped in swathing kamacite. Reflected light image. The black color is a lighting artifact. |
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