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Vermicular schreibersite |
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| Fort Stockton, Iron meteorite. |
Figure 1. Scale bar 400 µm. |
| Fort Stockton, Iron meteorite, classification not refined. |
| Vermicular schreibersite |
| Schreibersite is an iron-nickel phosphide
forming from phosphorous. In many iron meteorites, schreibersite crystallized
from metal-sulfide melts during the cooling and solidification of small bodies
in the early solar system. Going back further, phosphorous likely originated in stellar processes (nucleosynthesis), becoming part of the material that coalesced to form our Solar System. As metal "melts" begin to cool in differentiated asteroids, schreibersite appears late in the formation of iron-nickel alloys, austenite, taenite, and kamacite. Its composition—iron, nickel, and phosphorus—gives it an affinity to other iron-nickel alloys, often forming (crystallizing) alongside taenite and kamacite. Vermicular schreibersite, also called "islands" or "arcs" of schreibersite, refers to its shape. The question is how to get there from a crystallized form.This form of schreibersite often presents as thin, sinuous, filamentary threads or rounded submm-shaped "blebs" alongside kamacite and taenite. Its form, however, is unlikely to be the result of migration or nucleation. The formation likely involves local melting due to impact events that should be evidenced in the kamacite lamellae. Vermicular schreibersite forms through a process involving slow cooling and solidification: 1. Schreibersite begins to nucleate within the metallic matrix as the mass cools. 2. A prolonged cooling rate allows for the development of fine, thread-like structures that are characteristic of vermicular schreibersite. 3. As cooling continues, the schreibersite crystals grow in a worm-like filamentous form, mostly along grain boundaries or within kamacite lamellae. Note: Some of the above are conjectures based on incomplete information. The Fort Stockton, Texas, iron meteorite, found in 1952, was never published in the Meteoritical Bulletin, but it has official status in the Bulletin. The main mass is in the Monnig Collection, TCU, Fort Worth, Texas. |
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