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Dendritic Troilite |
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| Dendritic Troilite in Impact-Melt Chondrites |
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| Figure 1. |
| The original troilite inclusion was a primary feature; the dendritic morphology represents a secondary structure formed by shock-induced vaporization and recondensation. Scale bar=1.5 mm. |
| Dendritic troilite in the Chico L6 impact-melt chondrite. Impact Processes and Dendritic Textures in Chondrites Large dendritic sulfide inclusions exceeding 2 cm in diameter are uncommon in ordinary chondrites. Such morphologies typically result from extreme shock events that cause localized vaporization of primary sulfides, followed by rapid recondensation during cooling. Hypervelocity impacts generate intense shock waves that convert kinetic energy into heat, producing localized melting, vaporization, and a variety of impact-melt textures. These processes can also induce mineralogical transformations and chemical redistribution. Dendritic growth occurs when a vapor or melt phase cools rapidly under conditions favoring skeletal or branched crystal formation, often due to high undercooling and limited diffusion time. In the case of troilite (FeS), the primary (pre-shock) inclusion is vaporized, and the resulting vapor recondenses within the original nodule boundaries upon cooling, yielding the distinctive dendritic residuum classified as a secondary structure. The Chico Meteorite (L6, Impact-Melt Breccia) The Chico meteorite is a significant example of an L-group ordinary chondrite (L6) that experienced substantial impact melting. It was recovered in 1954 as a single 104.8 kg stone in New Mexico. Chico is classified as an impact-melt chondrite/breccia, reflecting heterogeneous shock processing that produced distinct lithologies and melt features visible on the scale of hand specimens (Figures 2 and 3). The prominent dendritic troilite nodule (Figure 1) exemplifies shock-driven textural modification. The primary troilite inclusion was subjected to extreme temperatures during the impact event, leading to vaporization. Upon subsequent cooling, the sulfide vapor recondensed as a dendritic intergrowth rather than reforming the original equant or nodular morphology. This feature did not fully homogenize with the surrounding silicate matrix, preserving sharp boundaries with the host material. Broader Significance Impact-melt chondrites like Chico provide valuable insights into the collisional evolution of ordinary chondrite parent bodies (likely S-type asteroids in the main belt). Key aspects include: • Shock Metamorphism Stages: Chico records high shock pressures (often classified as S6 or equivalent) and post-shock annealing, with melt veins, pockets, and heterogeneous lithologies. • Thermal History: The melt wave and dual lithologies (Figures 2 and 3) illustrate localized energy deposition and variable cooling rates. • Sulfide Behavior: Dendritic troilite demonstrates the mobility of sulfur under impact conditions, with implications for volatile redistribution and the formation of unusual textures in other meteorite types (e.g., irons and stony-irons). • Parent-Body Processes: Such specimens bridge unshocked chondrites and fully differentiated meteorites, recording the transition from primitive accretion to impact-driven differentiation and brecciation. Comparative studies of dendritic sulfides across meteorite classes help constrain impact velocities, peak temperatures, and cooling timescales. Advanced techniques such as X-ray computed tomography (µXCT), electron microscopy, and isotopic analysis further elucidate the timing and conditions of these events. Chico remains an important reference specimen for understanding the role of impacts in shaping the mineralogy and texture of meteoritic materials throughout solar system history. |
| Figure 2. Scale bar 12 mm. |
| Chico, L6 impact-melt chondrite, full slice 1,13kg grams. Note the delination of the thermal melt wave across the slice. |
| Figure 3. Scale bar 6 mm. |
| Chico, two lithologies. |
| References
• Bogard, D. D., et al. (1995). 40Ar-39Ar age and petrology of Chico: Large-scale impact melting on the L chondrite parent body. Geochimica et Cosmochimica Acta. (Key study on Chico’s impact history). • Meteorite-Times / Meteorites-for-Sale. Descriptions and images of Chico L6 impact-melt breccia. • Rubin, A. E. (2002). Smyer H-chondrite impact-melt breccia… (context on dendritic troilite in impact melts). Geochimica et Cosmochimica Acta. • Lunar and Planetary Institute. Meteoritical Bulletin Database: Chico. |
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