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Fusion crust |
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| An iron meteorite's primary
cooling ends between 400° C and 350° C; with further cooling, any
significant diffusion is over. From this point forward, anything that happens, i.e., impacts, shocks, or any thermally altering events, will most likely produce secondary structures. It is the same for stoney meteorites. Still to come after its 4-plus billion-year journey is the atmospheric passage that creates the hallmark indicator of any freshly fallen meteorite - fusion crust. The crust of a freshly fallen stone meteorite is usually black and somewhat glassy. It's a thin exterior coating around 1 mm to 2 mm in thickness, although it can be a little thinner or thicker depending on the transit time in the atmosphere. Air molecules pile up in the front of the falling meteoroid creating pressure and friction against the mass. This friction causes heat and exterior melting that burns away (ablates) a significant amount of the stony or iron/nickel mass. Eventually, the meteoroid loses its cosmic velocity, ablation stops as it slows down, and it falls the last few miles in "dark" flight, not burning or glowing but cooling. Witnessed falls are often reported as "frosting over" from their very cold interiors. Striking Earth, they are now referred to as meteorites. The blackened exterior fusion crust is a secondary structure resulting from the thermally altering event of atmospheric passage. The fusion-crusted exteriors of freshly fallen meteorites are primarily black. Weathering will cause this to fade to a black/brownish coloration. After a few hundred or thousand years, the crust will fade to a brownish color. The following images highlight the fusion crust on stone and iron meteorites. They are mostly self-explanatory but will have a descriptive sentence or two. |
| Note: Buchwald's
"Handbook of Iron Meteorites" 1975, Chapter, "The Physics of the
Fall," expertly discusses this atmospheric passage and is highly
recommended reading. Rubin, "Meteorite Mineralogy" (2021), subchapter
12.1, 12.2, presents a comprehensive discussion of atmospheric passage and
mineral alteration from terrestrial weathering. |
Figure 1. Scale bar 8 mm mm. |
| Allende |
| Pueblito de Allende, Chihuahua, Mexico |
| Fell, February 08, 1969 |
| Stone, Carbonaceous chondrite, CV3 |
| Complete specimen, 660 grams. Fusion crusted, exposed chondrules upper left edge, broken "face" from impact seen on lower right surface. Note the contraction cracking in the fusion crust (the thin glassy crust cracks as it cools). |
Figure 2. Scale bar 14 mm. |
| Dimmitt, |
| Castro County, Texas. |
| Found, 1942 |
| Stone, H3.7, chondrite, unequilibrated, regolith breccia |
| Complete specimen, 1.78 kg. The fusion crust of this stone meteorite has weathered to an oxidized brown color. |
Figure 3. Scale bar 2 mm. |
| Peekskill |
| Westchester County, New York |
| Fell, October 09, 1992 |
| Stone, H6 chondrite, brecciated |
| Peekskill, sectioned, showing 2mm thick fusion crust. |
Figure 1. Scale bar 12 mm. |
| Bruderheim |
| Alberta, Canada |
| Fell, March 04, 1960 |
| Stone, L6 chondrite |
| Complete fusion-crusted specimen, 416 grams. Note the regmaglypted depressions. |
| Bruderheim - continued next image. |
Figure 1. Scale bar 2 mm. |
| Bruderheim |
| Fusion-crusted fragment. The crust is 1-1.2mm in thickness presenting as a charred exterior. |
Figure 1. Scale bar 3 mm. |
| Chiang Khan |
| Chiang Khan, Loei, Thailand |
| Fell, November 17, 1981 |
| Stone, H6 chondrite |
| Sectioned end piece, note the 1.5 mm - 2 mm thick fusion crust. |
Figure 1. Scale bar 1.5 mm. |
| Wold Cottage |
| Yorkshire, Wold Newton, UK |
| Fell, December 13, 1795 |
| Stone, L6 chondrite |
| The Wold Cottage meteorite presents with paches of black fusion crust. This meteorite fell in 1795 and was curated to the standards of the day for over 200 years exposed to atmospheric water, misc fungi and bacteria from handling, temperature changes, etc. Over time, contraction cracks in the crust can wick all of this into the interior causing some of the fusion crust to flake or lift off the surface. |
Figure 1. Scale bar 10 mm. |
| Texline |
| Dallam County, Texas |
| Found 1937 |
| Stone, H5 chondrite |
| A weathered chondrite with partial or patches of fusion crust. The terrestrial age is unknown. |
Figure 1. Scale bar 20 mm. |
| NWA 869 |
| Northwest Africa |
| Found 2000 |
| Stone, L3-6 chondrite |
| The terrestrial age of this meteorite is unknown and could be several thousand years or more. This specimen was likely buried in sand and exposed over and over. The top surface has a weathered relic fusion crust with minute contraction cracking still visible. The rest of the specimen is also highly weathered along the broken side surfaces. |
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