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 200 km 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 V-0,K-0, and K'(0) being 310.3 angstrom(3), 133 GPa, and 3.6 respectively. At zero pressure, the full elastic stiffness tensor shows significant anisotropy with the diagonal principal components C-11, C-22, and C-33 being 235, 292, 266 GPa respectively, the diagonal shear C-44, C-55, and C-66 being 86, 92, and 87 GPa respectively, and the off-diagonal stiffness C-12, C-13, C-14,C-15, C-16, C-23, C-24, C-25, C-26, C-34, C-35, C-36, C-45, C-46, and C-56 being 73, 78, 6, -30, 15, 61,17, 2, 1, -13, -15, 6, 3, 1, and 3 GPa respectively. The zero pressure, shear modulus, Go and its pressure derivative, Got are 90 GPa 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 (>40 GPa) than the thermodynamic stability of phase-Pi. Full elastic constant tensors indicate that phase-Pi is very anisotropic with AV(p) similar to 22.4% and AV(s) similar to 23.7% at 0 GPa. 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. (C) 2017 Elsevier B.V. All rights reserved.