Pressure induced elastic softening in framework aluminosilicate- albite (NaAlSi3 O8)

Abstract

Albite (NaAlSi3 O8) is an aluminosilicate mineral. Its crystal structure consists of 3-D framework of Al and Si tetrahedral units. We have used Density Functional Theory to investigate the high-pressure behavior of the crystal structure and how it affects the elasticity of albite. Our results indicate elastic softening between 6-8 GPa. This is observed in all the individual elastic stiffness components. Our analysis indicates that the softening is due to the response of the three-dimensional tetrahedral framework, in particular by the pressure dependent changes in the tetrahedral tilts. At pressure <6 GPa, the PAW-GGA can be described by a Birch-Murnaghan equation of state with V0GGA= 687.4 Å3,K0GGA= 51.7 GPa, and K0'GGA= 4.7. The shear modulus and its pressure derivative are G0GGA= 33.7 GPa, and G0'GGA= 2.9. At 1 bar, the azimuthal compressional and shear wave anisotropy AVPGGA = 42.8%, and AVSGGA= 50.1%. We also investigate the densification of albite to a mixture of jadeite and quartz. The transformation is likely to cause a discontinuity in density, compressional, and shear wave velocity across the crust and mantle. This could partially account for the Mohorovicic discontinuity in thickened continental crustal regions.