An extremely rare mineral has been found in a western Wisconsin meteorite crater, offering a new glimpse into an astronomical event that happened long before the age of the dinosaurs.

University of Puerto Rico geologist Aaron Cavosie discovered reidite in a rock sample taken from the Rock Elm meteorite impact structure in Pierce County, Wis., a find that makes the Rock Elm site one of only four places on Earth — all meteorite craters — where reidite has been discovered.

“It’s a significant find scientifically,” Cavosie said, adding that it tells scientists new things about the pressures the meteorite presented at impact. “When you find a mineral that has only been reported from three other localities on the entire planet of any age, it is a very rare occurrence.”

The reidite was not brought to the planet by the meteorite. Rather, it was formed during the impact, and it tells scientists new things about an ancient geological instant in the town of Rock Elm.

The meteorite, estimated to have been two football fields wide, struck the planet about 450 million years ago. The impact released the energy equivalent of 63,000 Hiroshima atomic bombs — an event that would have been felt regionally, said William Cordua, a retired University of Wisconsin-River Falls geology professor.

“If you take something of that size moving at supersonic speeds and it hits the Earth, which is (also) going through space at supersonic speeds, that energy gets released at once at one spot, and it’s pretty spectacular,” Cordua said.

It’s likely that the site was a shallow sea at the time, and the meteorite would have caused a tsunami. It also created pressures so intense that rock at the site was altered at the atomic level.

Much of that altered rock was swept away during hundreds of millions of years of erosion that left the Rock Elm site appearing unlike what most people think of as a meteorite crater.

“That’s what surprises people — they expect to see this huge smoking hole in the ground,” said Cordua. “It was initially, but it filled with sediment.”

The tides of time did not completely wipe away the geology created by the impact, however. The site already had been of interest to geologists when Cordua began mapping it in 1983, and a 2004 paper he co-wrote established the site as a meteorite crater.

Cavosie brought students from the University of Puerto Rico to the site in summer 2013 for NASA-funded astrobiology research. They took samples, and Cavosie was looking at the tiny minerals with a scanning electron microscope at the University of Wisconsin-Madison — where he earned his doctorate — when he found the reidite in December.

He sent the mineral to geologists Timmons Erickson and Nick Timms with Curtin University in Western Australia, and they confirmed that it was in fact reidite. The discovery was announced in October.

When he made the find, Cavosie, who called it “a classic case of an accidental discovery,” was looking for tiny fault-like features in the mineral zircon, a common mineral found in sandstone, which is widely exposed at the center of the Rock Elm site.

Those features form under extremely high pressures, but with high enough pressures the atoms are rearranged, changing zircon into a new, denser mineral — reidite.

“Rocks in Earth’s crust do not experience these kinds of pressures from any known process other than an extraterrestrial impact,” Cavosie said.

With reidite present at the Rock Elm site, scientists now know the pressures at impact were at least three times greater than what had been previously known.

While most of the reidite would have eroded away, Cavosie believes the tiny grains he discovered were pushed deep underground in cracks caused by the impact. Hundreds of millions of years and possibly hundreds of meters of erosion later, they were revealed.

The Rock Elm reidite — the oldest known reidite on Earth — is the first time the mineral has been found in sandstone, and Cavosie said it opens the door to investigating whether other sandstone-dominated impact craters also have the mineral.

“I think we just cracked the lid in being able to identify reidite in a much more common rock that occurs in many more impact craters,” he said.

Cordua said the find also means more recognition for the Rock Elm site and possibly more research there, such as searching for other strange minerals and surveys looking deep below the surface.

“There’s other work to do there,” he said, “and I’m hoping that this spurs it.”

Andy Rathbun can be reached at 651-228-2121. Follow him at twitter.com/andyrathbun.