A mineral previously known only from moon rocks and lunar meteorites has now been found on Earth. Researchers discovered the substance—dubbed tranquillityite after the Sea of Tranquility, where Apollo 11 astronauts landed on the Moon in July 1969— at six sites in Western Australia. The mineral occurs only in minuscule amounts and has no economic value, but scientists say it could be used for age-dating the rocks in which it occurs.
Soon after the first Apollo astronauts returned from the moon, scientists analyzed samples of igneous rocks they had collected, known as basalts. The rocks contained three previously unknown minerals, two of which—armalcolite and pyroxferroite—were found on Earth within a decade or so. But for the past 40 years, the third mineral, tranquillityite, hasn't been seen anywhere but in moon rocks and in meteorites blasted from the lunar surface by massive impacts. The reddish-brown mineral is mostly composed of iron, silicon, zirconium, and titanium, but it also includes trace amounts of rare elements such as yttrium. Geologists have long sought tranquillityite in Earth rocks, partly because 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. Now, in this month's issue of Geology, Birger Rasmussen, a geologist at Curtin University in Bentley, Australia, and his colleagues report that they've finally found tranquillityite on our planet.
The researchers looked in igneous rocks of Western Australia, particularly in those that didn't show signs of having undergone large-scale metamorphic changes deep within Earth. That's because when tranquillityite is exposed to excessive heat and pressure, it readily transforms into other minerals. The team confirmed the presence of the mineral by firing 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.
"Tranquillityite is not that unique in its overall chemistry, so it's odd that it hadn't been found in Earth rocks before," Rasmussen says. Nevertheless, he notes, the mineral likely remained elusive for several reasons. First, typical bits of tranquillityite—which are shaped like tiny needles that have been pounded flat—are unusually small, about 150 micrometers long, or slightly less than the diameter of the thickest human hair. Second, lunar rocks are more pristine than those on Earth, which are much more likely to have been altered either chemically, by hot fluids rich in dissolved minerals flowing through them, or physically, by geological processes such as plate tectonics, which can carry rocks deep below Earth's surface and subject them to hellish temperatures and pressures. Finally, Rasmussen says, tranquillityite can easily be mistaken for rutile, a similarly colored mineral commonly found in igneous rocks. Only certain types of analyses, such as the electron diffraction analyses conducted by the team, can discern tranquillityite, and samples of Earth rocks typically don't get such detailed scrutiny, he notes. "The lunar samples were so precious, they were studied in great detail," he says. "Geologists really took those rocks apart."
Robert Hazen, an earth scientist at the Carnegie Institution for Science in Washington, D.C., who was not involved in the study, says it's not surprising that tranquillityite hasn't shown up until now. For one thing, he notes, the mineral appears only in small amounts and develops during the late stages of crystallization of molten rocks in oxygen-poor conditions. "You've got to have the right set of conditions for these unusual minerals to form." Then, once tranquillityite has formed, it is unstable over the long term at Earth's surface, where it's exposed to water, oxygen, and living organisms that can dissolve, consume, or otherwise transform minerals.
Despite these challenges, Rasmussen says 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. So if researchers look hard enough elsewhere, they're bound to turn up more.