While digging around an ancient meteorite impact crater in Wisconsin, researchers discovered an extremely rare mineral that’s only previously been found in three other places. The mineral, called reidite, is a dense form (polymorph) of the fairly tough gemstone zircon, which is produced when the latter is subjected to very high pressures. While researchers can form this mineral in the laboratory, naturally occurring reidite is scarce.

The discovery was made in Rock Elm, a 6.5 km diameter impact structure located in western Wisconsin, USA. The crater has been dated to the Middle Ordovician period, which extends from 450 to 470 million years ago, making it the oldest known reidite. It’s been proposed that during this period, a huge meteor shower occurred that was the result of two colossal asteroids, measuring at least 100 km across, slamming into one another between Mars and Jupiter. The event released a huge cloud of smaller rock fragments that took a few million years to reach Earth, and some are still hitting our planet now.

Although reidite has been found in three other impact craters-- Chesapeake Bay Crater in Virginia, Reis Crater in Germany and Xiuyan Crater in China-- Rock Elm is the last place that scientists expected to find the mineral. “No one in their right mind would have looked for reidite in sandstone,” geochemist and study author Aaron Cavosie told Live Science.

Zircon transforms into reidite when meteorites slam into the ground because shock waves from the impact cause a dramatic increase in temperature and pressure at the site. The high pressures cause the building blocks of the mineral to rearrange, becoming tightly repacked. The resulting mineral is similar in composition to zircon, but around 10% more dense. Reidite can also be formed under high-pressure or shock recovery laboratory experiments. In fact, reidite was only known from lab-made samples for around 30 years before it was first discovered in nature in 2001.

The new samples of reidite were found amongst “shocked,” or shock-metamorphosed, zircons. Shock metamorphisms, or simply shock effects, are the modifications in rocks and minerals caused by the passage of shock waves. After first examining the samples by microscopy, Cavosie and his colleagues confirmed the presence of reidite by bombarding them with electrons. Since different minerals scatter, or diffract, electrons in a unique way, the researchers were able to identify reidite in the samples.

Since it takes pressures between 30 to 53 gigapascals to morph zircon into reidite, its presence at Rock Elm means that the meteorite impact resulted in much higher pressures than previously believed. Earlier estimates based on the presence of shocked quartz suggested that the resulting pressure likely didn’t exceed 10 gigapascals. Furthermore, because zircon will be present in any crater carved from sandstone, there is likely much more reidite to be found on Earth than anticipated.

[Via Live Science]