HR: 17:35h
AN: MR44A-06    [Abstracts]
TI: The 10 Å phase at high pressure by first principles calculations and implications for the petrology of subduction zones
AU: Fumagalli, P
EM: patrizia.fumagalli@unimi.it
AF: Università degli Studi di Milano, Dipartimento di Scienze della Terra, Università degli Studi di Milano, Milano, Italy
AU: * Stixrude, L
EM: stixrude@umich.edu
AF: University of Michigan, Department of Geological Sciences, University of Michigan, Ann Arbor, MI 48109-1005, United States
AB: The structure and the physical properties of the 10 Å phase, Mg₃Si₄O₁₀(OH)₂ · nH₂O, with n=0, n=1 and n=2 is explored over a wide range of pressure with density functional theory in the local density (LDA) and generalized gradient approximations (GGA). The equation of state depends strongly on the amount of water. All the investigated structures present a layer offset with respect to the ideal phlogopite like stacking along the x direction that influences the beta angle: tetrahedral rings and water molecules are not exactly aligned along the c nor the c* directions in the n=1 and n=2 structures. Water is bound to the tetrahedral sheets and is located off the midplane of the interlayer. The water dipole vector lies almost parallel to the tetrahedral sheets at zero pressure; the angle it forms with the ab plane increases with pressure in both n=1 (up to 34°) and n=2 (up to 28°) structures. Water molecules act both as acceptor and donor oxygen groups for hydrogen bonds. Water oxygens accept hydrogen from the intra-layer hydroxyls, leading to H-bonds at the boundary between the weak and the strong limit, in agreement with the relatively high Raman frequency at 3620 cm^-1 assigned to this vibrational mode. Water molecules donate hydrogen to both upper and lower basal oxygens. Interactions between water molecules and basal oxygens are complicated by the occurrence of polifurcated, mainly asymmetric, bent hydrogen bonds. Nonetheless, within the stability field of the 10 Å phase, LDA results yield distances between basal oxygens and water hydrogen that fall between the weak and strong limit in agreement with Raman signals at 3672 and 3593 cm^-1. GGA simulations suggest even stronger interactions between water molecules and basal oxygens.
DE: 1038 Mantle processes (3621)
DE: 3613 Subduction zone processes (1031, 3060, 8170, 8413)
DE: 3919 Equations of state
DE: 3924 High-pressure behavior
SC: Mineral and Rock Physics [MR]
MN: 2007 Joint Assembly