Australian scientists have discovered a rare mineral previously known only to be found in lunar rock samples and used it to date an Earth rock which formed over a billion years ago.
Named tranquillityite after the Sea of Tranquility, where astronauts landed on the Moon in 1969, researchers discovered the substance in rocks collected from six sites in Western Australia.
Tranquillityite was first discovered in rocks brought back from the moon soon after the first Apollo mission, along with two other substances - armalcolite and pyroxferroite. Both substances were found in Earth rocks within a decade or so of the 1969 Apollo mission but the third, tranquillityite, wasn't found on Earth until now.
Lead author of the recent Australian discovery, Curtin University professor and palaeontologist Birger Rasmussen, with others, wrote about the find in the journal Geology.
He told Fairfax Media today that he got a "bit of a thrill" out of the discovery.
"In terms of its significance perhaps it's not a 10, perhaps it's a 7 or something, but I got a bit of a thrill," he said. "This was essentially the last mineral which was sort of uniquely lunar that had been found in the 70s from these samples returned from the Apollo mission."
According to Rasmussen, tranquillityite is "very useful" for age-dating rocks in which it is found in.
He and fellow scientists "looked at hundreds of samples" before finding the mineral, he said.
"We had been studying lunar rocks previously, so we'd come across tranquillityite in rocks from the moon, so we knew roughly what to look for and then we hapenned to be looking at similar types of rocks on Earth and we thought 'this mineral should be present, we haven't seen it - I wonder why not? We'll keep an eye out' and eventually we found it."'
He said it was "quite surprising" it hadn't been found in nearly 40 years until now. "I suspect that people weren't really looking that hard I suppose."
The discovery of tranquillityite in certain rocks also helped him and fellow scientists give an age to the rocks underneath rocks with tranquillityite in them, which Rasmussen said were previously thought to be much younger.
Although there are alternative ways to date rocks, the minerals required to date them aren't always present, Rasmussen said. "So having another mineral such as tranquillityite, which has very useful properties for dating - it [adds] another string to your bow."
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