Researchers have recently discovered a rare mineral that used to be found only from moon rocks and lunar meteorites in at least six sites in Western Australia.

Led by Birger Rasmussen, a geologist at Curtin University in Bentley, Australia, the researchers reported that they found as tranquillityite in igneous rocks of Western Australia. Tranquillityite is a reddish brown mineral that is mostly composed of iron, silicon, zirconium, and titanium, and has traces of yttrium which is a rare element.

The mineral was found in the sample of igneous rocks collected by the first Apollo astronauts from the moon. Analysis showed that the rocks contained three previously unknown minerals, the armalcolite and pyroxferroite, and tranquillityite. The first two minerals were found on Earth after about a decade but the third has not been seen anywhere but in moon rocks and meteorites until recently.

Named after the Sea of Tranquility, where Apollo 11 astronauts landed on the Moon in July 1969, the mineral has long been sought by geologists as studies of lunar samples suggest that accurately measuring the proportions of radioactive isotopes in the mineral could be used to ascertain the age of the rocks.

The researchers discovered the rare mineral in the rocks of Western Australia that have not undergone large-scale metamorphic changes deep within Earth. According to Rasmussen, when tranquillityite is exposed to excessive heat and pressure, it readily transforms into other minerals.

To confirm the presence of the mineral, the researchers fired high-speed electrons through tiny rock samples. They noted that the flecks of tranquillityite scattered the electrons in a distinctive pattern matching that produced by lunar samples of the mineral.

According to Rasmussen, tranquillityite may not been discovered earlier for several reasons. He noted that the typical bits of tranquillityite-which are shaped like tiny needles that have been pounded flat-are unusually small, about 150 micrometers long. In addition, lunar rocks are more pristine than those on Earth, which are much more likely to have been altered either chemically or by geological processes such as plate tectonics, he said.

Another reason, according to the researchers, is that tranquillityite can easily be mistaken for rutile, a similarly colored mineral commonly found in igneous rocks. Only specific analyses such as the electron diffraction analyses, can discern tranquillityite.

"The lunar samples were so precious, they were studied in great detail," Rasmussen says. "Geologists really took those rocks apart." However, the researcher noted that the fact that tranquillityite has now been found in six widely scattered sites in Western Australia suggests that it might be more common than thought in igneous rocks.