Elasticity of Hydrous Aluminosilicate Mineral, Topaz-OH (Al2SiO4(OH)2) at High Pressures

Abstract:

We examined the equation of state and high-pressure elasticity of the hydrous aluminosilicate mineral topaz-OH (Al₂SiO₄(OH)₂) using first principles simulation. Topaz-OH is a hydrous phase in the Al₂O₃-SiO₂-H₂O (ASH) ternary system, which is relevant for the mineral phase relations in the hydrated sedimentary layer of subducting slabs. Based on recent experiments, it is known that the protons in the topaz-OH exhibit positional disorder with half occupancy over two distinct crystallographic sites. In order to adequately depict the proton environment in the topaz-OH, we examined five crystal structure models with distinct configuration for the protons. Upon full geometry optimization, we find that there are two distinct crystal structures for the topaz-OH. The first crystal structure has an orthorhombic Pbnm space group symmetry, and the second crystal structure has a monoclinic P2₁/c space group symmetry. At static conditions, the monoclinic (P2₁/c) topaz-OH has lower energy compared to the orthorhombic (Pbnm) topaz-OH. The energy of the monoclinic (P2₁/c) topaz-OH remains stable at least up to 40 GPa, i.e., pressures beyond the thermodynamic stability of the topaz-OH. Based on the results from first principles simulation, the equation of state for the monoclinic topaz-OH is well represented by a third-order Birch-Murnaghan formulation, with V₀ = 348.63 (±0.04) ų, K₀ = 164.7 (±0.04) GPa, and K'₀ = 4.24 (±0.05). The equation of state for the orthorhombic topaz-OH is well represented by a third-order Birch-Murnaghan formulation, with V₀ = 352.47 (±0.04) ų, K₀ = 166.4 (±0.06) GPa, and K'₀ = 4.03 (±0.04). While the bulk modulus is very similar for both the monoclinic and orthorhombic topaz-OH, the shear elastic moduli are very sensitive to the position of the proton and the orientation of the hydroxyl (O-H) groups.

In the hydrated sedimentary layer of a subducting slab, transformation of a mineral assemblage consisting of coesite (SiO₂) and diaspore (AlOOH) to topaz-OH (Al₂SiO₄(OH)₂) is likely to be accompanied by an increase in the density, compressional velocity, and shear wave velocity. To gain better insight into the transport of water via subduction of sediments, further constraints on the elasticity at high pressures and temperatures are required.