Pressure induced elastic softening in framework aluminosilicate- albite (NaAlSi₃O₈)

Area of Science:

• Mineral Physics
• Geophysics
• Computational Materials Science

Background:

• Albite (NaAlSi3O8) is a key aluminosilicate mineral forming the backbone of Earth's crust.
• Understanding its high-pressure behavior is crucial for interpreting seismic data and crustal dynamics.

Purpose of the Study:

• To investigate the high-pressure structural and elastic properties of albite using Density Functional Theory.
• To elucidate the mechanisms behind pressure-induced elastic changes in albite.
• To explore the implications of albite's high-pressure transformation on crustal discontinuities.

Main Methods:

• Utilized Density Functional Theory (DFT) with the PAW-GGA approach.
• Applied the Birch-Murnaghan equation of state to model pressure-volume data.
• Calculated elastic stiffness constants, bulk modulus, shear modulus, and wave velocities.

Main Results:

• Identified elastic softening in albite between 6-8 GPa, affecting all elastic stiffness components.
• Attributed softening to pressure-induced changes in tetrahedral tilts within the 3-D framework.
• Determined elastic parameters (e.g., bulk modulus = 51.7 GPa, shear modulus = 33.7 GPa) and significant azimuthal anisotropy at 1 bar.
• Modeled densification to a jadeite-quartz mixture, predicting density and velocity discontinuities.

Conclusions:

• Albite's elastic behavior is significantly influenced by its framework's response to pressure.
• The predicted transformation and associated discontinuities offer a potential explanation for the Mohorovicic discontinuity in specific tectonic settings.
• DFT provides a powerful tool for understanding mineral behavior under extreme conditions relevant to geophysics.