“Maybe the fusion crust got worn off”

A fusion crust is the most characteristic feature of a meteorite. People sometimes say to me, “…but maybe the fusion crust got worn off.” Probably not. Here are some considerations.

• Fusion crusts do not easily wear off. They are glass that is tightly bound to the rock. It is like the glaze on a ceramic pot or coffee mug. A meteorite buried in the soil will not lose its fusion crust, although the crust may get duller and change color. The rock may fragment, but the crust will not disappear. For a meteorite exposed at the earth’s surface the exterior of the rock may break off taking fusion crust with it, but patches of fusion crust will remain on some parts of the rock. Here is one of the best examples that I have seen of a fusion crust (1-2 mm thick?) that is flaking off the rock: Ornans. The meteorite is a carbonaceous chondrite, a type of meteorite that is not as “tough” and coherent as ordinary chondrites.
• Some people tell me that they cleaned or ground off the fusion crust with water or a wire brush. (Why would anybody do that!) If the crust comes off from washing or scraping, then it is not a meteorite fusion crust, it is just some kind or terrestrial rind or coating. If you succeed in removing the fusion crust from a real meteorite, then it is not worth anything to anybody.
• A meteorite that found its way into a glacier, rocky stream, rocky river, or shoreline will likely lose its fusion crust by abrasion against other rocks. It would be impossible to identify such a rock as a meteorite by its appearance unless, perhaps, grains of (possibly rusty) metal are observed. If you tell me that you found a rock on a beach or in a stream bed, then I am not going to tell you that it might be a meteorite. Experienced meteorite hunters do not look for meteorites on beaches or stream beds.
• Many meteorites from northern Africa and Oman do not have fusion crusts because they have been exposed at the surface of the earth for thousands, perhaps tens of thousands, of years and the fusion crust has been ablated away by wind-blown sand. Here is one of the most extreme examples that I have seen: NWA 11259. That process is not likely to occur in North America or Europe, for example, because there are no surfaces that old. In most places in temperate climates, meteorites are buried with time, not exposed at the surface.
• There are certainly some meteorites that have been found in the American west that do not have self-evident fusion crusts, but these are all heavily weathered and fractured rocks that do not usually “look like” meteorites. For such a rock, a chemical test would have to be done to prove that the rock is a meteorite.

Think of it this way: If she is wearing a pretty dress and playing a grand piano on a stage with a symphony orchestra behind her, then she is probably a concert pianist. If she is wearing shorts and jogging down the beach with her dog, then she might be a concert pianist, but probably not. How can you or I tell?

Put another way, if you have a rock that does not have an obvious fusion crust, the I am not going to mislead you by saying that it might be a meteorite. Get a chemical test.

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