Deep within the Earth's interior, silicate-perovskite is thought to be the planet's most abundant mineral. However, it exists at such depths that we only know about it through modeling, and now from an asteroid that has given geologists their first look at the material.
Silicate-perovskite is thought to dominate the lower mantle, below the mantle transition zone at 700km and above the iron core that starts 2,900km deep. We have no rocks from such depths; it is only recently that terrestrial ringwoodite, thought to dominate at the transition zone, was found, allowing us to expand our knowledge of that part of the planet.
However, we still have a good idea of the make up of deeper layers of the planet by studying the way earthquakes move through these zones. Combined with modeling of the effects of enormous pressures and high temperatures on the Earth's most common elements this has given geologists a fair idea what to expect exists down there.
Minerals that are hypothetical don't get proper names, but when a previously unseen crystal was found in a meteorite collected from Tenham, western Queensland, it was named bridgmanite after Nobel Prize-winning physicist Percy Bridgman.
However, bridgmanite turns out not to just be a visitor from outer space. “This fills a vexing gap in the taxonomy of minerals,” says Dr Oliver Tschauner of the University of Nevada-Las Vegas. The chemical composition, MgSiO3, matches what is thought to lie between the transition zone and the core. High energy impacts in space have left the Tenham meteorite highly shocked, and Tschauner suggests these impacts created effects equivalent to the pressures deep beneath the Earth's surface. “Shocked meteorites are the only accessible source of natural specimens of minerals that we know to be rock-forming in the transition zone of the Earth,” Tschaunder says.
The Tenham meteorite fell to Earth in 1879, but attempts to study it with scanning electron microscopes destroyed the minerals. “This material is very sensitive to electron beams,” says Dr Chi Ma of Caltech. Ma recognized the presence of bridgmanite, when he turned the electron microscope on, but like a thought experiment in particle physics the action of observing the mineral altered it. Ma, responded by sending samples to Tschauner, whose tool of choice is X-Ray diffraction from synchrotron beams.
It took five years, but Ma and Tsachuaner have now collected enough information about the bridgmanite in the meteorite to satisfy the International Mineralogical Association Commission on New Minerals, Nomenclature and Classification. The announcement is yet to be published on the IMA's website, which appears to be running well behind decisions, perhaps a reflection of the timescales geologists are used to working with.
However, Ma is very pleased. “We are glad no one used [Bridgman] for other minerals,” he said, “This one is so important.”