High-Pressure Experiments Yield Discovery of New Forms of Feldspars

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 aluminum 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

In high-pressure experiments, scientists have discovered new forms of the common mineral feldspar. At moderate temperatures, these hitherto unknown variants are stable at pressures of Earth’s upper mantle, where common feldspar normally cannot exist. The discovery could change the view at cold subducting plates and the interpretation of seismologic signatures, as the team around DESYscientist Anna Pakhomova and Leonid Dubrovinsky from Bayerisches Geoinstitut in Bayreuth report in the journal Nature Communications.

Feldspars represent a group of rock forming minerals that are highly abundant on Earth and make up roughly 60 percent of Earth’s crust. The most common feldspars are anorthite, (CaSi2Al2O8), albite (NaAlSi3O8), and microcline (KAlSi3O8). At ambient conditions, the aluminum and silicon atoms in the crystal are each bonded to four oxygen atoms, forming AlO4 and SiO4 tetrahedra.

“The behavior of feldspars under increasing pressure and temperature has been intensively investigated before, with the respect to their fate in Earth’s interior,” explains Pakhomova. “Feldspars are known to be stable only at pressures of up to 3 Giga-Pascals along the common pressure-temperature profile of the Earth, while they decompose into denser minerals at higher pressures.” 3 Giga-Pascals (GPa) are equivalent to 30,000 times the normal air pressure at sea level. “However, under cold conditions feldspars may persist metastably at pressures higher than 3 GPa,” adds Pakhomova. “Previous high-pressure structural studies of feldspars at room temperature have shown that the tetrahedral framework of feldspars is preserved up to 10 GPa.”

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