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https://www.llnl.gov/news/shock-compression-research-shows-hexagonal-diamond-could-serve-meteor-impact-marker

Shock compression research shows hexagonal diamond could serve as meteor impact
marker
Breanna Bishop
Lawrence Livermore National Laboratory
March 14, 2016

In 1967, a hexagonal form of diamond, later named lonsdaleite, was identified for the first time inside fragments of the Canyon Diablo meteorite, the asteroid that created the Barringer Crater in Arizona.

Since then, occurrences of lonsdaleite and nanometer-sized diamonds have
been speculated to serve as a marker for meteorite impacts, having also
been connected to the Tunguska explosion in Russia, the Ries crater in Germany, the Younger Dryas event in sites across North America and more.

It has been hypothesized that lonsdaleite forms when graphite-bearing
meteors strike the Earth. The violent impact generates incredible heat
and pressure, transforming the graphite into diamond while retaining the graphite's original hexagonal structure. However, despite numerous theoretical and limited experimental studies, crucial questions have remained unresolved for short-time high-pressure environments relevant to meteor impacts, particularly the structural state immediately after the shock transit,
the timescales involved and the influence of crystalline orientation.

In a new paper published today by Nature Communications,
a team of researchers, including scientists from Lawrence Livermore National Laboratory (LLNL), provides new insight into the process of the shock-induced transition from graphite to diamond and uniquely resolves the dynamics
of the phase change.

The experiments show unprecedented in situ X-ray diffraction measurements
of dynamic diamond formation on nanosecond timescales by shock compression
of graphite starting at pressures above 0.5 Mbar (1 Mbar = 1 million atmospheres).
The team observed the direct formation of lonsdaleite above 1.7 Mbar,
for the first time resolving the process that has been proposed to explain
the main natural occurrence of this crystal structure being close to meteor impact sites.

"Due to difficulties in creating lonsdaleite under static conditions,
the overall existence of this crystal structure in nature has been questioned recently," said lead author Dominik Kraus. Kraus conducted this research
while working as a University of California, Berkeley Physics Department postdoc sited within LLNL's NIF & Photon Science directorate.
He now serves as the Helmholtz Young Investigator group leader at Helmholtz-Zentrum
Dresden-Rossendorf in Germany.

"However, static experiments cannot mimic fast dynamics such as those
in violent meteor impact events," he said. "Here we show that we can
indeed create a lonsdaleite structure during dynamic high-pressure events. This is interesting for modeling dynamic phase transitions in general,
but also shows that the lonsdaleite found in nature could indeed serve
as a marker for violent meteor impacts."

The experiments were conducted at the Matter at Extreme Conditions (MEC) experimental area at the Linac Coherent Light Source
(LCLS) at the SLAC National Accelerator Laboratory at Stanford. Graphite samples were shock-compressed to pressures of up to 2 million atmospheres
(2 Mbar) to trigger the structural transitions from graphite to diamond
and lonsdaleite. The phase changes in the high-pressure samples were probed with ultrafast (femtosecond) X-ray pulses created by LCLS.


The experiments show dynamic diamond formation starting at pressures above
0.5 Mbar (1 Mbar = 1 million atmospheres) and the direct formation of lonsdaleite above 1.7 Mbar.
According to Kraus, this was the very first in situ structure measurement
of the shock-induced graphite to diamond transition. Before these experiments, all conclusions regarding this structural transition were based on the material that was recovered after applying the shock drive or dynamic measurements of macroscopic quantities, such as density and pressure.

"You won't get rich from our experiments, but the shock-induced transition from graphite to diamond already has important industry applications,"
he said. "For example, nanometer-sized diamonds for fine polishing of materials are created by detonation of carbon-bearing explosives. These explosions typically generate pressures up to ~0.5 Mbar, just above the threshold of diamond formation. Here we show that above 2 Mbar, the lonsdaleite

structure can be generated in a very pure form. Since pure lonsdaleite
is supposedly even harder than diamond, this is highly interesting and
other groups now try to recover these samples after an experiment."

Kraus was joined by LLNL co-authors Tilo Doeppner and Benjamin Bachmann,
and scientists from the University of California, Berkeley, SLAC, the University of Warwick, the Max Planck Institute, Technical University Darmstadt, Helmholtz-Zentrum Dresden-Rossendorf, the University of Oxford
and GSI.

The U.S. Department of Energy's Office of Science and the German Federal Ministry for Education and Research funded the work.

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