News National Discovery of reidite, one of the rarest minerals on Earth, may reveal Australia’s biggest crater

Discovery of reidite, one of the rarest minerals on Earth, may reveal Australia’s biggest crater

The world's rarest mineral, reidite, is shown in an "elector diffraction" image highlighted by the colour purple. Photo: Morgan A Cox
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Researchers have discovered one of the rarest minerals on Earth buried deep within an ancient meteorite crater in Western Australia.

The ultra-rare mineral known as reidite was found deep within the long-buried Woodleigh Crater near Shark Bay, approximately 750 kilometres north of Perth.

The reidite is only formed under the extreme pressure created when rocks from outer space slam into the Earth’s crust.

It is only the sixth time the mineral has been discovered on Earth.

Curtin University research superviser Aaron Cavosie said reidite started life as a far more common mineral – zircon – and only transformed into reidite during the pressure of impact.

“Finding reidite at Woodleigh was quite a surprise as it is much rarer than diamonds or gold, though unfortunately not as valuable,” he said.

Finding may reveal Australia’s biggest crater

The discovery has indicated Woodleigh Crater may be much larger than previously estimated and could reveal it to be the largest impact crater in the country.

Woodleigh has long been buried beneath younger sedimentary rocks, so its size is not yet known and remains hotly debated.

Previous research estimated the crater to be between 60 to 120 kilometres in diameter.

If Woodleigh is found to have a diameter of more than 100 kilometres it would be classified as the largest impact crater in Australia.

“There are not many impact craters on Earth that are larger than 100 kilometres in diameter,” Dr Cavosie said.

“The significance is that once they get to be much larger than 100 kilometres in diameter they enter a class of impact event that are large enough to cause mass extinctions and influence biological evolution.

“For instance, the large impact crater in Mexico that is contributed to causing the demise of the dinosaurs 65 million year ago is 180 kilometres in diameter.

“And that one is not even twice as large as what Woodleigh may be.”

Discovery lay dormant for 20 years

The discovery was made after the research team drilled core samples from the centre of the crater, in a region known to geologists as the central uplift.

“Central uplifts are desirable targets for learning about impact conditions,” Dr Cavosie said.

“They bring profoundly damaged rocks closer to the surface and in some instances are associated with exploration targets.”

Dr Cavosie said the discovery was made by chance by one of his honours students, Morgan Cox.

“Morgan’s worked on a couple of different projects for her thesis,” he said.

“In this one we decided to examine the mineral zircon and how it responds to the high pressures created during meteorite impacts.

“The wild thing is that the drill core had been sitting in the core shed for the Geological Survey of Western Australia for almost 20 years before we examined it and identified that reidite was present.”

A drill core sample lay untouched for 20 years before the reidite discovery. Photo: Aaron J Cavosie

A priceless discovery

Dr Cavosie said the amount of reidite that has been discovered worldwide was microscopic.

“I’m going to guess that the whole of the reported reidite in the history of geology would sit under your fingernail,” he said.

“It’s like this – things have values other than in dollars there’s no commercial value of reidite.

“But from a scientific point of view it’s a priceless mineral, in terms of what it allows you to understand about the bigger picture.”

Monash University Associate Professor Andy Tomkins said reidite was only to be found on the Earth’s surface.

“You wouldn’t really get it anywhere else,” he said.

“Even meteorites wouldn’t have it despite the fact that they are full of impact signs because you don’t get big enough impacts between different asteroids.

“You need to have quite high pressures to form reidite, so it can only form in impact craters about a certain size.”