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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, composed of iron (Fe), nickel
(Ni), and phosphorus (P), initially crystallizes from metal-sulfide melts
during the cooling of a planetary body. Its distinctive vermicular shape,
however, likely develops through a series of stages influenced by thermal
evolution, phase relationships, and possibly external factors like impacts.
This is a possible scenario: Initial Nucleation: As the metal-sulfide melt cools within a differentiated asteroid, schreibersite begins to nucleate within the metallic matrix, forming small, discrete crystals. This happens late in the sequence, after the formation of iron-nickel alloys like austenite, taenite, and kamacite. Slow Cooling and Crystal Growth: A prolonged cooling period allows these initial crystals to grow. The slow rate prevents rapid solidification, giving schreibersite time to develop elongated, thread-like structures. This growth often occurs along grain boundaries or within kamacite lamellae, where phosphorus diffusion is favored due to structural or chemical gradients. Vermicular Development: As cooling progresses, schreibersite crystals extend into worm-like, filamentous forms. This morphology-described as sinuous threads or rounded "blebs"-may result from phosphorus diffusion through the metallic matrix, constrained by the surrounding kamacite and taenite. The vermicular shape could reflect minimization of surface energy or adaptation to the host matrix's structure. Note: Some, or a lot, 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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