9 November 2015

Smithsonian scientist seeks cosmic clues in a morsel of metal

Smithsonian Institute scientist Glenn MacPherson examines his very tiny sample. Smithsonian Institute scientist Glenn MacPherson examines his very tiny sample. Stephen Bilenky

This week at the lab, a meteorite expert from the Smithsonian Institution is examining a morsel of a mineral found in far-off Siberia, hoping to find clues about its origin.

Glenn MacPherson (pictured above) brought to the lab a single grain of an alloy that, he and his collaborators believe, fell to Earth on a meteorite some 8,000 years ago. Made of aluminum, copper and iron, the mineral, dubbed icosahedrite, is believed to be the very first example of a quasicrystal found in nature.

A quasicrystal is like a crystal in that its atomic structure has both an orderly arrangement and symmetry. But the arrangement doesn't repeat itself the way it does in crystals, and the symmetry is so different that it had been considered impossible by scientists — until they began making quasicrystals synthetically (and, at first, quite accidentally) in the 1980s.

Finding a quasicrystal "in the wild" has been a long-time goal of Princeton University physicist Paul Steinhardt. The leader of this project, Steinhardt had predicted mathematically that quasicrystals were possible at about the same time the first ones were concocted in a lab. The discovery of a naturally-occurring quasicrystal has been both exciting and puzzling, said MacPherson, because the metallic aluminum requires extreme conditions rarely found in nature.

"Having an alloy of metallic aluminum with copper is like this double puzzle that we're still trying to resolve," said MacPherson, who is visiting the MagLab for the first time. "We still don't know how the metal formed. We're slowly inching our way forward. We are here making one important test."

With Munir Humayun of the lab's Geochemistry Group, MacPherson is using a technique called laser ablation with a mass spectrometer to identify trace elements in the sample. One of their goals is to rule out any possibility that the mineral is manmade; they have already detected traces of gold, silver and thallium, none of which they would expect to find in a manmade alloy. They are also looking for trace elements that could shed light on how and when the metal may have been formed, as they believe it was, on an asteroid that itself dates back 4.5 billion years.

Photo by Stephen Bilenky / Text by Kristen Coyne

Last modified on 10 November 2015