Meteorite strikes may create unexpected form of silica

Date:
August 26, 2020
Source:
Carnegie Institution for Science
Summary:
New research examining the crystal structure of the silica
mineral quartz under shock compression is challenging longstanding
assumptions about this ubiquitous material.



FULL STORY ========================================================================== [Quartz (stock | Credit: (c) ala / stock.adobe.com] Quartz (stock image).

Credit: (c) ala / stock.adobe.com [Quartz (stock | Credit: (c) ala / stock.adobe.com] Quartz (stock image).

Credit: (c) ala / stock.adobe.com Close When a meteorite hurtles
through the atmosphere and crashes to 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 impacts teach scientists about
the minerals existing at the high-temperature and pressure conditions
found deep inside the planet?

==========================================================================
New work led by Carnegie's Sally June Tracy examined the crystal structure
of the silica mineral quartz under shock compression and is challenging longstanding 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 Earth's crust, found
in a multitude of different rock types," Tracy explained. "In the lab,
we can mimic a meteorite impact and see what happens." Tracy and her colleagues -- Washington State University's (WSU) Stefan Turneaure and Princeton University's Thomas Duffy, a former Carnegie Fellow - - used
a specialized cannon-like gas gun to accelerate projectiles into quartz
samples at extremely high speeds -- several times faster than a bullet
fired from a rifle. Special x-ray instruments were used to discern the
crystal structure of the material that forms less than one-millionth
of a second after impact. Experiments were carried out at the Dynamic Compression Sector (DCS), which is operated by WSU and located at the
Advanced Photon Source, Argonne National Laboratory.

Quartz is made up of one silicon atom and two oxygen atoms arranged in
a tetrahedral lattice structure. Because these elements are also common
in the silicate-rich mantle of the Earth, discovering the changes quartz undergoes at high-pressure and -temperature conditions, like those found
in the Earth's interior, could also reveal details about the planet's
geologic history.

When a material is subjected to extreme pressures and temperatures,
its internal atomic structure can be re-shaped, causing its properties
to shift.

For example, both graphite and diamond are made from carbon. But graphite, which forms at low pressure, is soft and opaque, and diamond, which
forms at high pressure, is super-hard and transparent. The different arrangements of carbon atoms determine their structures and their
properties, and that in turn affects how we engage with and use them.

Despite decades of research, there has been a long-standing debate in the scientific community about what form silica would take during an impact
event, or under dynamic compression conditions such as those deployed
by Tracy and her collaborators. Under shock loading, silica is often
assumed to transform to a dense crystalline form known as stishovite --
a structure believed to exist in the deep Earth. Others have argued that because of the fast timescale of the shock the material will instead
adopt a dense, glassy structure.

Tracy and her team were able to demonstrate that counter to expectations,
when subjected to a dynamic shock of greater than 300,000 times
normal atmospheric pressure, quartz undergoes a transition to a novel disordered crystalline phase, whose structure is intermediate between
fully crystalline stishovite and a fully disordered glass. However, the
new structure cannot last once the burst of intense pressure has subsided.

"Dynamic compression experiments allowed us to put this longstanding
debate to bed," Tracy concluded. "What's more, impact events are an
important part of understanding planetary formation and evolution
and continued investigations can reveal new information about these
processes." This research was supported by the Defense Threat Reduction
Agency and the NSF.

Washington State University (WSU) provided experimental support through
awards from the U.S. Department of Energy (DOE)/National Nuclear Security Agency (NNSA).

This work is based on experiments performed at the Dynamic Compression
Sector, operated by WSU under a DOE/ NNSA award. This research used the resources of the Advanced Photon Source, a Department of Energy Office
of Science User Facility operated for the DOE Office of Science by the
Argonne National .


========================================================================== Story Source: Materials provided by
Carnegie_Institution_for_Science. Note: Content may be edited for style
and length.


========================================================================== Journal Reference:
1. Sally June Tracy, Stefan J. Turneaure, Thomas S. Duffy. Structural
response of a-quartz under plate-impact shock compression. Science
Advances, 2020; 6 (35): eabb3913 DOI: 10.1126/sciadv.abb3913 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2020/08/200826151306.htm

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