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Acicular plessite |
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| Meteorite - Glorieta Mountain, Pallasite, MG. |
| Figure 1. Scale bar 150 µm. |
| Meteorite, Glorieta Mountain, Pallasite, MG |
| Acicular plessite field in Glorieta Mountain. |
| Acicular Plessite in a
Pallasite Meteorite Glorieta Mountain (pallasite, PMG-an) Figure 1. Acicular plessite field in the Glorieta Mountain pallasite. Scale bar: 150 µm. Pallasites Pallasites are distinctive stony-iron meteorites consisting of olivine crystals embedded in a metallic Fe–Ni matrix. They are widely interpreted as samples related to the core–mantle boundary region of differentiated asteroids, although impact-mixing models have also been proposed for some groups. Glorieta Mountain, classified as an anomalous main-group pallasite (PMG-an), is noted for its large, gem-quality olivine crystals and complex metal microstructures. Acicular Plessite Acicular plessite is a fine-grained intergrowth of kamacite (α-FeNi) and taenite (γ-FeNi) in which kamacite occurs as thin, needle-like (acicular) crystals within a taenite or retained-taenite matrix. This morphology develops during slow cooling of the metallic phase through the two-phase kamacite–taenite field (roughly 700–400 °C), where kamacite nucleates and grows preferentially along specific crystallographic directions within the parent taenite. It is one of several recognized plessite varieties—including granular, net, comb, and cellular types—each reflecting subtle differences in cooling rate, local composition, and nucleation conditions. In Glorieta Mountain and other pallasites, acicular plessite commonly occupies interstitial regions or “fields” within the metal matrix, between larger kamacite grains or adjacent to olivine crystals. As a secondary structure, it forms through solid-state exsolution and diffusion-controlled growth rather than direct crystallization from melt. Petrogenetic Significance Acicular plessite is an important metallographic indicator in pallasites for several reasons: • It records the low-temperature cooling history of the metal after the olivine–metal mixture had already assembled. Because it develops well below the solidus of both metal and olivine, it constrains the thermal regime that followed the mixing event. • Its needle-like morphology is sensitive to cooling rate. Acicular textures generally form under relatively faster conditions within the range of meteoritic metallographic rates, or during the earlier stages of plessite development, providing finer resolution than bulk cooling-rate estimates alone. • Variations in the abundance, scale, and distribution of acicular plessite relative to other plessite types (and to features such as cloudy zone or tetrataenite) help reconstruct whether cooling was continuous or interrupted and whether localized reheating or mild shock occurred. • In comparative studies, differences in acicular plessite development among main-group and anomalous pallasites supply additional constraints on shared versus distinct thermal environments. In Glorieta Mountain, the presence of well-developed acicular plessite fields is consistent with the slow asteroidal cooling rates typical of main-group and related pallasites, while also preserving a record of solid-state processes that operated after the distinctive olivine–metal texture had formed. |
| References • Buchwald, V. F. (1975). Handbook of Iron Meteorites. University of California Press. • Goldstein, J. I., & Michael, J. R. (2006). The formation of plessite in meteoritic metal. Meteoritics & Planetary Science, 41, 553–570. • Massalski, T. B., Park, F. R., & Vassamillet, L. F. (1966). Speculations about plessite. Geochimica et Cosmochimica Acta, 30, 649–662. • Yang, J., Goldstein, J. I., & Scott, E. R. D. (2010). Main-group pallasites: Thermal history, relationship to IIIAB irons, and origin. Geochimica et Cosmochimica Acta, 74, 4471–4492. • Meteoritical Bulletin Database, entry for Glorieta Mountain (PMG-an). |
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