The last mineral thought to have been unique to the Moon has been discovered in the remote Pilbara region of Western Australia. It was identified by researchers at The University of Western Australia's Centre for Microscopy, Characterisation and Analysis (CMCA).
Tranquillityite, named after the Sea of Tranquillity* where the Apollo 11 moon-walkers landed in July 1969, was tentatively identified by Professor Birger Rasmussen from Curtin University while studying a polished slice of earthly rock in a scanning electron microscope.
When lunar rocks were first analysed in the 1970s after having been brought to Earth by US astronauts Neil Armstrong, Edwin 'Buzz' Aldrin and Michael Collins, scientists identified three minerals -- armalcolite (after Armstrong, Aldrin and Collins), pyroxferroite and tranquillityite -- that they believed were unique to the Moon.
Armalcolite and pyroxferroite were later found on Earth, but when the CMCA's Dr Janet Muhling and Assistant Professor Alexandra Suvorova and their colleagues from Curtin University, published a recent paper in the journal Geology, they showed for the first time that tranquillityite occurred also on Earth.
To confirm the identity of the Pilbara mineral, Dr Muhling analysed its composition by collecting X-rays emitted when the sample was targeted by an electron beam in the electron microscope. This showed that the terrestrial mineral was made up of the same elements as lunar tranquillityite. Electron diffraction showed that the two minerals have the same crystal structure.
Previously, tranquillityite was thought to exist only in returned moon samples and lunar -- and possibly Martian -- meteorites.
The researchers believe tranquilliltyite is the final 'lunar' mineral to be found on Earth because it is rare, small and prone to change. The Moon lacks water and its minerals are pristine, but even a small amount of water in terrestrial magmas will cause minerals to be altered and difficult to identify.
Tranquillityite, both lunar and terrestrial, is an ideal mineral for determining the age of the enclosing rock by radiometric dating. The Pilbara rocks in which tranquillityite occurs were once thought to have been about 820 million years old but new dating by Professor Rasmussen and colleagues at the John de Laeter Centre for Isotope Research has shown that they are about 1040 million years old.
*The so-called sea is actually a giant impact crater that appears dark because it is filled with dark basaltic rocks. It was first named by Italian astronomer Giambattista Riccioli in 1651.
- B. Rasmussen, I. R. Fletcher, C. J. Gregory, J. R. Muhling, A. A. Suvorova. Tranquillityite: The last lunar mineral comes down to Earth. Geology, 2011; 40 (1): 83 DOI: 10.1130/G32525.1
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