Pressure induced elastic softening in framework aluminosilicate- albite (NaAlSi(3)O(8))

Albite (NaAlSi(3)O(8)) 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 [Image: see text] = 687.4 Å(3), [Image: see text] = 51.7 GPa, and [Image: see text] = 4.7. The shear modulus and its pressure derivative are [Image: see text] = 33.7 GPa, and [Image: see text] = 2.9. At 1 bar, the azimuthal compressional and shear wave anisotropy [Image: see text] = 42.8%, and [Image: see text] = 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.