Polymorphism of feldspars above 10 GPa

Area of Science:

• Mineralogy
• Geophysics
• Crystallography

Background:

• Feldspars are primary rock-forming minerals in Earth's crust.
• Previous studies on feldspar deformation under pressure were limited to ~10 GPa.
• Pressure-induced deformation in feldspars typically involves tilting of aluminum-oxygen (AlO4) and silicon-oxygen (SiO4) tetrahedra.

Purpose of the Study:

• To investigate the structural behavior of feldspars at extreme pressures.
• To discover new high-pressure polymorphs of anorthite, albite, and microcline.
• To understand the implications of these high-pressure phases for Earth's upper mantle.

Main Methods:

• In situ single-crystal X-ray diffraction (XRD) studies.
• Experiments conducted up to 27 GPa.
• Analysis of structural changes and phase transitions in feldspar samples.

Main Results:

• Discovery of novel high-pressure polymorphs for anorthite, albite, and microcline.
• Phase transitions triggered by significant tetrahedral distortions.
• Observed increase in aluminum (Al) and/or silicon (Si) coordination numbers at high pressures.

Conclusions:

• High-pressure feldspar polymorphs can exist in the Earth's upper mantle.
• These phases may influence the geodynamics of cold subducting slabs.
• The findings expand the known phase diagram of essential crustal minerals.