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I remember reading about a substance - I think it was a metal - left behind by meteorite impacts and nuclear explosions, but I can't remember the name. What is the name of the substance left behind by nuclear explosions and meteorite impacts?

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Shocked quartz has been found at underground nuclear test sites and major meteor impact sites. It is formed when quartz is subject to intense pressure causing to deform internally along crystal planes. When viewed under a microscopes the planes appear as lines.

Shocked quartz is found worldwide, and occurs in the thin Cretaceous–Paleogene boundary layer, which occurs at the contact between Cretaceous and Paleogene rocks. This is further evidence (in addition to iridium enrichment) that the transition between the two geologic periods was caused by a large impact.

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Was it impactite? It is the name of rocks (not metal) that have been transformed by a meteorite impact. They contain evidence for high pressures (quartz transformed into coesite, its high pressure polymorph) and high temperatures (impact melts, tektites).

Alternatively, you could have heard about the iridium anomaly, an iridium-rich layer associated with the Chicxulub impact. Iridium is indeed a metal. But I don't think that iridium has ever been found in craters left by nuclear explosions.

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The metallic components of meteorites are made of nickel-iron, usually taenite that has a nickel content between 20% and 65% and kamecite with between 5% and 10% nickel.

There are other elements mixed in at low concentrations - notably cobalt at around 0.6% and a mixture of Platinum Group Metals at a few 10's of parts per million. (10ppm = 0.001%). Platinum Group Metals include Palladium, Iridium, Osmium, Ruthenium, Rhodium as well as Platinum.

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