Scientific accolades don鈥檛 get much bigger. Percy Bridgman, an American physicist active during the first half of the 20th century, has just had one-third of the planet named after him, although it鈥檚 a chunk of Earth that we will probably never see.
Earth鈥檚 lower mantle is largely composed of magnesium iron silicate, in the form of a mineral with a . Given that the lower mantle is about 2000 kilometres thick, this mineral makes up 38 per cent of Earth鈥檚 entire volume, so it is easily our planet鈥檚 most common mineral.
It is surprisingly rare at Earth鈥檚 surface, though. So rare, in fact, that geologists have struggled to find a natural sample. And the rulebook is strict: a mineral can鈥檛 be formally named with no natural sample to describe it.
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Now, at the University of Nevada in Las Vegas and his colleagues have finally discovered a sample that meets the criteria, meaning the mineral is nameless no longer. It will now be known as bridgmanite.
Crucial sample
鈥淚 would like to give professor Tschauner and his colleagues my congratulations,鈥� says at Hiroshima University in Japan, one of the many geologists who has previously hunted for the crucial sample. Finally, he says, we can complete the naming of the mineral.
The bridgmanite that Tschauner and his colleagues found doesn鈥檛 come from Earth鈥檚 interior, though. It actually arrived on Earth in 1879, buried within a meteorite that slammed into Australia. The experienced temperatures of 2000掳C and pressures of 24 gigapascals during its journey through the solar system 鈥� extreme enough to replicate conditions deep inside the Earth and allow bridgmanite to form.
The mineral should have broken down as the space rock returned to ambient temperature and pressure. The fact that it didn鈥檛 implies that the drop in temperature and pressure must have been extremely rapid, essentially 鈥渇reezing鈥� the bridgmanite in place before it could decay. Today it exists in certain regions of the meteorite as 鈥渃rystallites鈥� between 40 and 200 nanometres long, says Tschauner.
It鈥檚 important to be able to name bridgmanite finally, he says, but having natural samples of the mineral are significant for another reason. They can now be chemically analysed to reveal some of the trace elements that can naturally slot into bridgmanite鈥檚 crystal structure 鈥� which will help refine models of how the deep mantle behaves.
Mantle deep
For too long, says Tschauner, geologists and physicists have relied on a crude reductionist approach to understanding conditions deep inside the Earth. Because we know so little about the deep mantle, theoretical models usually only consider the abundant elements 鈥� magnesium, silicon and oxygen 鈥� that we know must be present. 鈥淵ou can鈥檛 just ignore the rest 鈥� it doesn鈥檛 work,鈥� says Tschauner.
For instance, earlier this year at the University of Alberta in Edmonton, Canada, and his colleagues discovered samples of ringwoodite, another mantle mineral, in diamond from deep within the mantle. The minor elements locked inside the ringwoodite provided good evidence for a massive 鈥渙cean鈥� of water within Earth鈥檚 interior. Without the actual sample, we wouldn鈥檛 have known about the ocean.
Pearson says discovering the first natural samples of bridgmanite in a meteorite is important. 鈥淏ut it鈥檚 not the same as finding the mineral in a terrestrial sample,鈥� he cautions. 鈥淛ust as ringwoodite had been found for the first time in nature in a meteorite, its identification did not foretell the importance of water in the deep Earth.鈥�
In other words, the search for bridgmanite from Earth鈥檚 mantle will continue, and there is no knowing what secrets it might reveal about Earth鈥檚 interior.
And who was Percy Bridgman? A Nobel laureate, Bridgman is sometimes called the father of high-pressure experiments. 鈥淗is advances are what led to the ability to synthesise and study deep Earth materials,鈥� says at Arizona State University in Tempe. 鈥淗e made mineral physics possible.鈥�
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