A model of the crystal structure of cristobalite X-I, which had never before been discovered in other materials. This high-pressure phase of cristobalite is made up of two layers (green and blue), each composed of Si?O. Credit: Leonid Dubrovinsky
A research group at the University of Bayreuth has found a long-sought explanation for the apparent contradictions implicit in the composition of lunar and Martian meteorites. In cooperation with the German Electron Synchrotron (DESY) in Hamburg, the European Synchrotron Radiation Facility (ESRF) in Grenoble and research partners in Lyon and Vienna, the Bayreuth scientists led by Prof. Dubrovinsky were able to demonstrate how meteorites could contain within narrow spaces minerals whose formation conditions are quite different.
When asteroids or comets collide with the moon or Mars, it results in high pressure and high temperatures that abruptly alter the rocks at the point of impact. Chunks of the altered rocks are often catapulted down to Earth. Many of these meteorites have puzzled scientists, mainly for two reasons: 1. they contain Seifertite, a mineral formed from silicon dioxide (SiO2) under extreme pressure. Asteroid and comet impacts that produce such high pressure would had to have been so intense that they would have melted or shattered large areas of rock on the moon and Mars. However, research does not suggest that any such catastrophes ever occurred.
Second, Seifertite is often found right next to the mineral cristobalite, which forms from silicon dioxide at considerably lower pressures.
Scientists at the University of Bayreuth’s Bavarian Research Institute of Experimental Geochemistry & Geophysics (BGI) have now succeeded in explaining this strange meteorite composition. The Xray facilities PETRA III and DESY in Hamburg and the European Synchrotron Radiation Facility (ESRF) in Grenoble enabled researchers to expose cristobalite samples to intensive radiation and high pressures of up to 83 gigapascals, which corresponds to around 820,000 times the pressure of Earth’s atmosphere. The diffraction pattern of the X-ray illustrated how the mineral was altered under various pressures. A crucial difference was found to exist between hydrostatic pressure, where the mineral was compressed from all directions with equal strength, and non-hydrostatic pressure, in which the mineral is compressed unevenly creating strong tensions. The researchers were surprised by the findings:
High non-hydrostatic pressure transforms cristobalite to Seifertite – even if it is weaker than the extremely high pressure that would be required to form Seifertite directly from silicon dioxide. However, when cristobalite is exposed to high pressure that only varies slightly from the uniformity of hydrostatic pressure, it develops a new crystalline structure. This structure, cristobalite X-I, had never been observed in a silicate. As soon as the “quasi-hydrostatic” pressure decreases, the cristobalite reverts to its original structure.
These findings suggest a simple solution to the puzzle of the meteorite: the Seifertite contained in the meteorite is not necessarily a product of extreme impacts that had dramatic effects on the moon and Mars. It can rather be formed, as a result of less severe impacts, from cristobalite under lower – yet still quite high – non-hydrostatic pressures. “The cristobalite that borders the Seifertite can be explained as having been formed under decreasing pressure from cristobalite X-I; the latter only formed temporarily under quasi-hydrostatic pressure,” explains Dr. Ana ÄŒernok. “The assumption that both quasi-hydrostatic and non-hydrostatic pressures can arise within a narrow space when the moon, Mars, or other planets are abruptly struck meshes well with previous findings in the field of meteorite research,” Prof. Dubrovinsky adds.
Dubrovinsky emphasizes that the new findings are of the utmost importance for meteorite research: “Minerals such as cristobalite and Seifertite do not per se allow one to draw any clear conclusions about the formation of the meteorites. Our measurements have shown that identical crystals can have very different origins. It also became clear that there is another factor – in addition to high pressures and high temperatures – that should be considered more seriously in analyses of meteorites: namely, the (sometimes extremely high) mechanical tensions that result from varying pressure zones on the rock structure.”
https://www.uni-bayreuth.de/en/university/press/press-releases/2017/065-meteorites/index.html
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