|Cheryl Colan, 'Meteor Crater' via Flickr. CC BY-NC 2.0|
Reidite is a rare mineral that has been found only in four crater impacts: the Chesapeake Bay Crater in Virginia, Ries Crater in Germany, Xiuyan Crater in China, and Rock Elm Crater in Wisconsin in the United States.
Researchers discovered the mineral, called reidite, at the Rock Elm impact structure in western Wisconsin. Reidite is a dense form of zircon, one of the hardiest minerals on Earth.
This is the oldest reidite ever found,, said Aaron Cavosie, a geochemist at the University of Puerto Rico in Mayagüez. The Rock Elm meteorite crater is 450 million to 470 million years old, he said.
The Rock Elm crater was gouged out of carbonate rocks and sandstone that contains tiny fragments of quartz and zircon. The earlier reidite discoveries were all in impact melt breccias — a mix of rock that melted and cooled into glass during the impact and unmelted rock fragments.
Zircon morphs into reidite when shock waves from meteorite impacts hike up pressures and temperatures to extreme levels, equal to those deep inside the Earth where diamonds form.
The pressure makes minerals tightly repack their molecules into denser crystal structures. Reidite has the same composition as regular zircon but is about 10 percent denser.