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Ropey Schreibersite |
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| Meteorite: Oued Bourdim 001, Pallasite of the Eagle Station grouplet. |
| Figure 1. Scale bar 400 µm. |
| Meteorite: Oued Bourdim 001, Pallasite of the Eagle Station grouplet. |
| Ropey schreibersite (arrowed). |
| The thin, ropey appearance of
schreibersite in iron meteorites results from the slow cooling of a meteorite's
parent body, its nucleation at grain boundaries or phase interfaces, and the
controlled diffusion of phosphorus and nickel. Schreibersite doesn't form randomly. Instead, it preferentially nucleates at specific structural sites within the meteorite, such as the boundaries between grains of taenite and kamacite or the interfaces where these two phases meet. These locations act as natural starting points for the mineral's growth because they are areas of higher energy or slight compositional variation. The development of schreibersite is heavily influenced by the diffusion of two critical elements: phosphorus and nickel. Phosphorus, an essential but scarcer component of schreibersite, must migrate through the solidifying metal to reach the nucleation sites. Similarly, nickel, abundant in the iron-nickel matrix, also diffuses toward these growing crystals. The rates at which these elements move depend on the temperature of the cooling parent body and the local availability of each element. At higher temperatures, diffusion is faster, allowing schreibersite to grow more rapidly, while at lower temperatures, the process slows, potentially limiting the mineral's extent. This temperature-dependent diffusion, combined with the structural constraints of the grain boundaries, shape the thin, ropey structures that can twist and weave through the metallic matrix. If phosphorus is abundant and diffusion is unimpeded, schreibersite may form thicker, more pronounced bands. However, in some iron meteorites, the limited availability of phosphorus and the slow cooling conditions result in finer, more delicate formations. |
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