High-pressure researchers discover unusually stable forms of feldspar
University of Bayreuth, Press release No. 089/2020, 3 June 2020
Feldspars are minerals that make up about 60 percent of the earth's crust. In high-pressure experiments, a research team coordinated by the German Electron Synchrotron (DESY) and the University of Bayreuth has now discovered new forms of the mineral. In contrast to the previously known forms of feldspar, which are widespread in the Earth's crust, they remain stable at temperatures of up to 600 degrees Celsius when exposed to pressures such as those prevailing in the Earth's upper mantle. These findings could give new impetus to research into cold subducting plates and change the interpretation of seismic signals. The scientists have presented their discovery in "Nature Communications".
The crystal structure of the feldspar anorthite under normal conditions (left) and the newly discovered high-pressure variant (right). Under normal conditions, the silicon and aluminium atoms form tetrahedra (yellow and blue) with four oxygen atoms each (red). Under high pressure polyhedra with five and six oxygen atoms are formed. Calcium atoms (grey) lie in between. The black lines outline the so-called unit cell, the smallest unit of a crystal lattice. Credit: DESY, Anna Pakhomova.
"Feldspar" is a collective term for a group of minerals which, among all the minerals, make up by far the largest component of the earth's crust. The most common members of this group are anorthite, albite, and microcline. Characteristic for their crystal structures are AlO4- and SiO4- tetrahedra. These are triangular pyramids, which consist of four triangular sides. Four oxygen atoms form the corners of the pyramid, while at its centre there is an aluminium or silicon atom.
Scientists have exposed these ordinary feldspars to pressures of up to 27 gigapascals for the first time. At the German Electron Synchrotron (DESY) in Hamburg and the Advanced Photon Source (APS) in Chicago, they have investigated how the crystal structures change as a result. "At pressures of over ten gigapascals, new forms of anorthite, albite, and microclinic are formed. Strong geometric distortions of the AlO4 and SiO4 tetrahedra arise resulting in the aluminium and silicon atoms being provided with additional neighbouring atoms. This leads to the formation of denser frameworks based on polyhedra, in which one aluminium or silicon atom is bound to four, five, or six oxygen atoms," reports Dr. Anna Pakhomova, first author of the study. The high-pressure researcher is a DESY research assistant and habilitation candidate at the Bavarian Research Institute of Experimental Geochemistry & Geophysics (BGI) of the University of Bayreuth.
At BGI, the newly discovered high-pressure variants of feldspar were examined for their stability. It was found, for example, that the high-pressure variant of anorthite is still stable even at temperatures of up to 600 degrees Celsius when exposed to a pressure of 15 gigapascals. “Such pressure-temperature conditions could be found on Earth in the subductions zones – regions where two lithospheric plates collide, one riding over the other,” explains Prof. Dr. Leonid Dubrovinsky from BGI. “In such geological settings, feldspars are delivered into Earth’s interior along with other crustal material by the descending plate. Our results indicate that in cold subduction zones, if the temperature does not rise above 600 degrees, high-pressure phases derived from feldspars could persist at depths corresponding to Earth's upper mantle. This could possibly influence the dynamics and fate of cold subducting lithospheric plates and alter seismological signatures.”
International research cooperation:
Scientists from the University of Bayreuth, Saint Petersburg State University, the University of Chicago, and DESY were all involved in the work.
The research work at the University of Bayreuth was funded by the German Research Foundation (DFG).
Anna Pakhomova, Dariia Simonova, Iuliia Koemets, Egor Koemets, Georgios Aprilis, Maxim Bykov, Liudmila Gorelova, Timofey Fedotenko, Vitali Prakapenka, Leonid Dubrovinsky: Polymorphism of feldspars above 10 GPa. Nature Communications (2020), DOI: 10.1038/s41467-020-16547-4
Prof. Dr. Leonid Dubrovinsky
Bavarian Research Institute of Experimental Geochemistry & Geophysics (BGI)
University of Bayreuth,
Telefon: +49 (0)921 / 55-3736 oder -3707
University of Bayreuth
Phone: +49 (0)921 / 55-5356