meteorsScanning electron microscopy image of the micro-structure of albite prior to the rapid compression experiments. The image spans about 0.036 millimeterCredit: Stony Brook University, Lars Ehm

A US-German research team has simulated meteorite impacts in the lab and followed the resulting structural changes in two feldspar minerals with X-rays as they happened. The results of the experiments at DESY and at Argonne National Laboratory in the US show that structural changes can occur at very different pressures, depending on the compression rate. The findings, published in the February 1 issue of the scientific journal Earth and Planetary Science Letters, will aid other scientist to reconstruct the conditions leading to impact craters on Earth and other terrestrial planets.

Meteorite impacts play an important role in the formation and evolution of Earth and other planetary bodies in our solar system. But the impact conditions, including the impactor size, velocity, and the peak pressure and temperature, are usually determined long after the impact occurred by studying permanent changes in the rock-forming minerals in the impact crater. To reconstruct the impact conditions from the rock record in an impact crater hundreds to millions of years after the event requires scientists to reconcile observations from the field with the results of laboratory experiments.

In recent decades, scientists have developed a classification scheme that ties impact conditions to pressure- and temperature-induced changes in rock-forming minerals that can be found in typical rocks in impact craters. The feldspar group minerals albite (NaAlSi3O8), anorthite (CaAl2Si2O8) and their mixture plagioclase (NaxCa1-xAl2-xSi2+xO8) are highly abundant in planetary crusts. Therefore, changes in these minerals with respect to pressure and temperature are widely used as indicators for very large impacts. Such changes include structural transformations or amorphisation, the loss of ordered crystal structure.

However, for the feldspar group minerals, the reported values for the pressure conditions of the amorphisation transition differ vastly if static or dynamic compression techniques are used. "These differences point to large gaps in our understanding of compression rate-induced processes in minerals," says Lars Ehm from Stony Brook University and Brookhaven National Laboratory, the principle investigator of the project. This has far-reaching implications for the interpretation of natural impact events based on the rock record with respect to the velocity, size, and other properties of the meteorite.

The inner structure of minerals and other samples can be investigated with X-rays that are diffracted by the crystal lattice of a material. From the characteristic diffraction pattern, the inner structure of a sample can be determined. This technique has been used and refined for more than a century. It can now also be used to track dynamic processes.

"The emergence of new and very powerful X-ray sources such as PETRA III, Advanced Photon Source, and the European X-ray Free Electron Laser in combination with the recent quantum leaps in X-ray detector technology provide us now with the experimental tools to investigate materials' response to measure the atomic structure at rapid compression conditions," says Hanns-Peter Liermann, head of the Extreme Conditions Beamline P02.2 at DESY's X-ray source PETRA III, where some of the experiments were conducted.

"In our experiment we used gas- or actuator-controlled diamond anvil cells to rapidly compress our samples while we continuously collect X-ray diffraction patterns," explains Melissa Sims, lead author of the study. "This allows us to monitor the changes in the atomic structure during the complete compression and decompression cycle, and not only at the start and end of the experiment as in previous so-called recovery experiments."

The research team was able to observe amorphisation of albite and anorthite at different compression rates in the experiment. They compressed the minerals to a pressure of 80 gigapascals, corresponding to 80,000 times the atmospheric pressure. In the experiments, compression rates from 0.1 gigapascals per second (GPa/s) to 81 GPa/s were used. "The results show that depending on the rate of compression, the minerals undergo the amorphisation transition at very different pressures," Ehm says. "The increase in compression rate lead to a lowering of the observed amorphisation pressure."

For example, at the lowest compression rate of 0.1 GPa/s, albite turned completely amorph at a pressure of 31.5 gigapascals, while at the highest rate of 81 GPa/s this occurred already at 16.5 gigapascals.

"For these reasons, amorphisation in plagioclase minerals is not likely to be an unambiguous standard to suggest specific peak pressures and temperature conditions during meteorite impact," says Ehm. Further investigations are needed to fully understand the behaviour of these minerals and to assess if impact conditions can be gauged against the structure of rock minerals.