Reidite is one of the rarest minerals on earth—it's been found in only four places. The most recent is Rock Elm, Wisconsin, where scientists discovered the material in a crater, Phys.org reports. The crater has been around since the geologic period known as the Middle Ordovician, and that means the sample found there is the oldest preserved reidite ever found, at some 450 million years old. What makes it even more unusual is that it was found in sandstone, unlike any of the other reidite finds. That fact could drive experts to take a new look at other sandstone impact structures, Phys.org notes.
Reidite is actually a particularly dense form of another mineral, zircon, LiveScience reports. Zircon becomes reidite via soaring pressures and temperatures following a meteorite impact. "Rocks in Earth's crust do not experience these kinds of pressures from any known process other than an extraterrestrial impact," a researcher tells TwinCities.com. The mineral has previously been found at meteorite impact sites in Virginia, Germany, and China; the first discovery of reidite in nature occurred in 2001. (In other recent geological finds, California yielded a six-pound gold nugget.)