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

Abstract

Albite (NaAlSi3O8) 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 = 687.4 Å3, = 51.7 GPa, and = 4.7. The shear modulus and its pressure derivative are = 33.7 GPa, and = 2.9. At 1 bar, the azimuthal compressional and shear wave anisotropy = 42.8%, and = 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.

Seismic properties and anisotropy of the continental crust: Predictions based on mineral texture and rock microstructure

“…Mookherjee et al . [] theoretically determined values provide the only pressure derivatives available for albite (An0).…”

“…As an alternative, Mookherjee et al . [] showed that albite conversion to jadeite and quartz can lead to >1 km/s increase in velocity, from ~6.5 km/s to ~8.1 km/s. Such conversion may be occurring during the formation of the continental Moho, which would ultimately provide a very strong seismic impedance contrast between the feldspar‐rich crust and the jadeite‐rich mantle [i.e., Jagoutz and Behn , ].…”

“…The slower velocities of plagioclase, compared to clinopyroxene, would increase the difference in acoustic impedance between the granulites in the lower crust (plagioclase bearing) and ultramafics of the upper mantle, and this potentially creates strong seismic reflectors. As an alternative, Mookherjee et al [2016] showed that albite conversion to jadeite and quartz can lead to >1 km/s increase in velocity, from~6.5 km/s to~8.1 km/s. Such conversion may be occurring during the formation of the continental Moho, which would ultimately provide a very strong seismic impedance contrast between the feldspar-rich crust and the jadeite-rich mantle [i.e., Jagoutz and Behn, 2013].…”

“…The body-diagonal constants c 11 and c 33 also showed a minimum around 6 GPa, although not as pronounced as for c 66 ; c 22 showed increasing elasticity as a function of pressure. The elastic softening has been attributed to restructuring of tetrahedral coordinated units, TO 4 (T = Al, Si), in the framework silicate structure of albite Mookherjee et al [2016]. theoretically determined values provide the only pressure derivatives available for albite (An0).3.3.…”

“…Mookherjee et al [2016] have investigated the pressure dependence of elastic constants for albite up tõ 12 GPa using density functional theory. Interestingly, they found that the c 66 elastic constant in albite decreases as a function of pressure with a minimum around 6 GPa.…”