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.
Highlights
Plagioclase feldspar is one of the most important mineral solid-solution series between the end-members- albite (NaAlSi3O8) and anorthite (CaAl2Si2O8)
It is fundamental to address how the crystal structure and chemistry influences the physical property across the plagioclase solid
The unit-cell volume and cell axes predicted with local density approximation (LDA) and generalized gradient approximation (GGA) bracket the previous X-ray diffraction results (Fig. 1)
Summary
The compression behavior of the albite could be described with a Birch-Murnaghan equation of state[32]. The unit-cell volume and cell axes predicted with LDA and GGA bracket the previous X-ray diffraction results (Fig. 1). LDA typically overbinds, whereas the GGA under-binds, i.e., unit-cell volume data are slightly larger than the experimental results (Fig. 1). The inter-axial angles from our structural optimizations of albite using GGA are in excellent agreement with the previous X-ray diffraction studies[26] (Fig. 1). The unit-cell volume predicted by GGA (maTtoa0bd GluePliSap1sr,heFodigwic.st1eV)d.0GTuGhsAien >bgVuLl0keDxpmAbosyhd~ou3wli.4sp%Kre,0LdwDicAht e>erdeKua0sesxiupnnbgiyGt-~cG1eA4ll.5va%ot l0u( GTmaPebalpesrhSeod1w,icFstieKgd.0G1bG)yA. Albite (triclinic symmetry) has 21 non-zero independent elastic constants, cij - three diagonal and compressional elastic stiffness components with, i =j, and i = 1–3; three diagonal and shear elastic stiffness components with i =j, and i = 4–6, and 15 off-diagonal elastic stiffness components with i ≠j and i = 1–6 33. The agreement between the predicted elastic constants at room pressures from this study and previous experimental results are
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