Elasticity of phase-Pi (Al3Si2O7(OH)3) – A hydrous aluminosilicate phase

Abstract

Phase-Pi (Al3Si2O7(OH)3) is an aluminosilicate hydrous mineral and is likely to be stable in hydrated sedimentary layers of subducting slabs. Phase-Pi is likely to be stable between the depths of 60 and 200km and is likely to transport water into the Earth’s interior. Here, we use first principles simulations based on density functional theory to explore the crystal structure at high-pressure, equation of state, and full elastic stiffness tensor as a function of pressure. We find that the pressure volume results could be described by a finite strain fit with V0, K0, and K0′ being 310.3Å3, 133GPa, and 3.6 respectively. At zero pressure, the full elastic stiffness tensor shows significant anisotropy with the diagonal principal components C11, C22, and C33 being 235, 292, 266GPa respectively, the diagonal shear C44, C55, and C66 being 86, 92, and 87GPa respectively, and the off-diagonal stiffness C12, C13, C14,C15, C16, C23, C24, C25, C26, C34, C35, C36, C45, C46, and C56 being 73, 78, 6, −30, 15, 61, 17, 2, 1, −13, −15, 6, 3, 1, and 3GPa respectively. The zero pressure, shear modulus, G0 and its pressure derivative, G0′ are 90GPa and 1.9 respectively. Upon compression, hydrogen bonding in phase-Pi shows distinct behavior, with some hydrogen bonds weakening and others strengthening. The latter eventually undergo symmetrization, at pressure greater (>40GPa) than the thermodynamic stability of phase-Pi. Full elastic constant tensors indicate that phase-Pi is very anisotropic with AVP ∼22.4% and AVS ∼23.7% at 0GPa. Our results also indicate that the bulk sound velocity of phase-Pi is slower than that of the high-pressure hydrous aluminosilicate phase, topaz-OH.