Since 1979, scientists have been searching for a piece of the Earth's most abundant mineral. Far out of reach of scientists, the mineral is more than 400 miles deep in the lower mantle.
Now, after five years of work, Oliver Tschauner, a mineral physicist in UNLV's department of geoscience, and Chi Ma, a mineralogist from the California Institute of Technology, have discovered the mineral in a piece of a 4.5-billion-year-old meteorite that fell near the Tenham station in Australia in 1879.
The pair named the mineral in honor of Percy Bridgman, a physicist who won the 1946 Nobel Prize for pioneering research on solids under high pressure. For decades, scientists have simply known the mineral by its chemical components and crystal structure -- silicate perovskite.
In order to officially identify and name a mineral, one must know the chemical composition and crystal structure of the mineral. Though synthetic examples have been studied, no naturally occurring samples of the mineral had ever been found.
Before the Fall
The mineral in the meteorite was created through a that occurred approximately 470 million years ago. "Before its fall, the Tenham meteorite was part of a larger asteroid that collided with another asteroid and broke into many pieces," Tschauner said. "It exhibits signs of strong shock-induced transformations, meaning it endured high temperatures and pressures as the result of this collision. These high-pressure conditions, similar to what we see in the Earth's mantle, is why bridgmanite could form in this meteorite, and why we don't see it at the surface of Earth."
According to Tschauner, people had previously suspected bridgmanite was located in shocked meteorites. Grains of shock-generated high-pressure minerals are known to be tiny (micrometer scale), so scientists used electron diffraction to search for bridgmanite. However, the electron beam destroys this mineral, transforming it into glass.
Tschauner and Ma instead used a micro-focused high energy X-ray beam to collect diffraction signals from the Tenham material. This approach enabled the research team to analyze the structure; the composition was analyzed after collecting diffraction data.
New Analytical Methods
The project required development of new analytical methods and was dependent on a new generation of ultra-fast X-ray detectors, which only became available two years ago. These new detectors permitted a fine-grid mapping for fishing out the tiny grains of bridgmanite, Tschauner said.
"Bridgmanite makes up approximately 70 percent of the volume of the lower mantle and approximately 38 percent volume of solid Earth in total," Tschauner said. "Its physical, chemical, and rheological properties are key in understanding the lower mantle.
"Because the mineral didn't have an official name, there was a problematic terminological vagueness in literature about the lower mantle, 'MgSi-perovskite,' 'silicate perovskite,' or plainly wrong 'perovskite.' This vagueness is now removed," he said.
The mineral and mineral name were approved on June 2 by the International Mineralogical Association's Commission on New Minerals, Nomenclatures and Classification.
