Meteorite strikes can create unexpected forms of silica

As a meteorite travels through the atmosphere and crashes into the earth, how does its violent impact alter the minerals found at the landing site? What can the short-lived chemical phases created by these extreme effects scientists learn about the minerals that exist at the high temperature and pressure conditions found deep in the planet?

New work led by Carnegie’s Sally June Tracy examined the crystal structure of the silica-mineral quartz under shock compression and has been challenging for years assumptions about how this ubiquitous material behaves under such intense conditions. The results are published in Science Advances.

“Quartz is one of the most abundant minerals in the earth’s crust, found in a multitude of different rock types,” Tracy explained. “In the lab, we can track a meteorite impact and see what happens.”

Tracy and her colleagues – Stefan Turneaure of Washington State University (WSU) and Thomas Duffy of Princeton University, a former Carnegie Fellow – used a specialized cannon-like gas pistol to accelerate projectiles in quartz tests at extremely high speeds – several times faster than a bullet fired from a rifle. Special X-ray instruments were used to distinguish the crystal structure from the material which after impact forms less than one millionth of a second. Experiments were performed at the Dynamic Compression Sector (DCS), which is operated by WSU and located in the Advanced Photon Source, Argonne National Laboratory.

Quartz consists of one silicon atom and two oxygen atoms arranged in a tetrahedral lattice structure. Because these elements are also common in the Earth’s silicate-rich mantle, it was possible to detect the changes that quartz undergoes under high pressure and temperature, such as those found in the Earth’s interior, also details on the geological history of the planet.

When a material is subjected to extreme pressures and temperatures, the internal atomic structure can be reshaped, causing the properties to shift. For example, both graphite and diamond are made of carbon. But graphite, which forms at low pressure, is soft and opaque, and diamond, which forms under high pressure, is super hard and transparent. The different arrangements of carbon atoms determine their structures and properties, and this in turn affects how we keep and use them.

Despite decades of research, there has been a lengthy debate in the scientific community about what form silica would take at an impact event, or under dynamic compression conditions such as those employed by Tracy and her collaborators. Under shock loads, silica is often assumed to transform into a dense crystalline form known as stishovite – a structure that is believed to exist in the deep earth. Others have argued that because of the fast time scale of the shock, the material will take over instead of a dense, glassy structure.

Tracy and her team were able to demonstrate that contrary to expectations, when subjected to a dynamic shock greater than 300,000 times normal atmospheric pressure, quartz undergoes a transition to a new turbulent crystalline phase, the structure of which is intermediate between fully crystalline stishovite and a completely unopened glass. However, the new structure may not last once the burst of intense pressure has subsided.

“Dynamic compression experiments allowed us to put this long-running debate to bed,” Tracy concluded. “What’s more, impact events are an important part of understanding planetary formation and evolution and ongoing research can reveal new information about these processes.”