Polymorphism of feldspars above 10 GPa

Abstract

Feldspars are rock-forming minerals that make up most of the Earth's crust. Along the mantle geotherm, feldspars are stable at pressures up to 3 GPa and may persist metastably at higher pressures under cold conditions. Previous structural studies of feldspars are limited tõ 10 GPa, and have shown that the dominant mechanism of pressure-induced deformation is the tilting of AlO 4 and SiO 4 tetrahedra in a tetrahedral framework. Herein, based on results of in situ single-crystal X-ray diffraction studies up to 27 GPa, we report the discovery of new high-pressure polymorphs of the feldspars anorthite (CaSi 2 Al 2 O 8), albite (NaAlSi 3 O 8) , and microcline (KAlSi 3 O 8). The phase transitions are induced by severe tetrahedral distortions, resulting in an increase in the Al and/or Si coordination number. High-pressure phases derived from feldspars could persist at depths corresponding to the Earth upper mantle and could possibly influence the dynamics and fate of cold subducting slabs.

Polymorphs of the Gadolinite‐Type Borates ZrB2O5 and HfB2O5 Under Extreme Pressure

“…Accordingly, the BO 4 tetrahedra in β ‐ZrB 2 O 5 undergo an insignificant geometrical distortion upon compression as is evident from the evolution of quadratic elongation and bond angle variance parameters (Figure S9) [57] . Interestingly, the pronounced increase of these parameters at ≈120 GPa likely indicates the upcoming phase transition at higher pressures, in line with previous reports on the pressure‐induced evolution of structures based on tetrahedral layers and frameworks [28, 50, 58–60] . The high ratio of charge to cation size for both Zr 4+ and B 3+ results in the increased stiffness of ZrO 8 and BO 4 polyhedra and, thus, of the overall crystal structure.…”

“…[57] Interestingly,t he pronounced increaseo ft hese parameters at % 120 GPa likely indicates the upcoming phase transition at higher pressures, in line with previous reports on the pressureinduced evolution of structures based on tetrahedral layers and frameworks. [28,50,[58][59][60] The high ratio of charget oc ation size for both Zr 4 + andB 3 + results in the increased stiffness of ZrO 8 and BO 4 polyhedra and, thus, of the overall crystal structure. Indeed, the bulk modulus of b-ZrB 2 O 5 is considerably larger than those of datolite (106(4) GPa) and hingganite-(Y) (124(1) GPa).…”

Frictional properties of anorthite (feldspar): Implications for the lower boundary of the seismogenic zone

“…The results of our anorthite experiments (Fig. 2) show very little compression, they behave as a relatively rigid unit (e.g., Pakhomova et al 2020). This rigidity may be one reason that feldspar is associated with brittle sliding even at high temperatures.…”

Frictional properties of anorthite (feldspar): implications for the lower boundary of the seismogenic zone

“…Commonly, feldspars are thermodynamically stable at pressures up to 3 GPa. Because TO 4 tetrahedra show very little compression, they behave as a relatively rigid unit (e.g., Pakhomova et al 2020). This rigidity may be one reason that feldspar is associated with brittle sliding even at high temperatures.…”