Crystal structure, equation of state, and elasticity of hydrous aluminosilicate phase, 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 (Al 2 SiO 4 (OH) 2 ) using first principles simulation. Topaz-OH is a hydrous phase in the Al 2 O 3 –SiO 2 –H 2 O (ASH) ternary system, which is relevant for the mineral phase relations in the hydrated sedimentary layer of subducting slabs. Based on recent neutron diffraction 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 in topaz-OH. Upon full geometry optimization we find two distinct space group, an orthorhombic P bnm and a monoclinic P 2 1 /c for topaz-OH. The topaz-OH with the monoclinic P 2 1 /c space group has a lower energy compared to the orthorhombic P bmn space group symmetry. The pressure–volume results for the monoclinic topaz-OH is well represented by a third order Birch–Murnaghan formulation, with V 0 mon  = 348.63 (±0.04) A 3 , K 0 mon  = 164.7 (±0.04) GPa, and K 0 mon  = 4.24 (±0.05). The pressure–volume results for the orthorhombic topaz-OH is well represented by a third order Birch–Murnaghan formulation, with V 0 orth  = 352.47 (±0.04) A 3 , K 0 orth  = 166.4 (±0.06) GPa, and K 0 orth  = 4.03 (±0.04). While the bulk moduli are very similar for both the monoclinic and orthorhombic topaz-OH, the shear elastic constants and the shear moduli are very sensitive to the position of the proton, orientation of the O–H dipole, and the space group symmetry. The S-wave anisotropy for the orthorhombic and monoclinic topaz-OH are also quite distinct. In the hydrated sedimentary layer of subducting slabs, transformation of a mineral assemblage consisting of coesite (SiO 2 ) and diaspore (AlOOH) to topaz-OH (Al 2 SiO 4 (OH) 2 ) is likely to be accompanied by an increase in density, compressional velocity, and shear wave velocity. However, further studies of physical properties and lattice preferred orientation of several key hydrous aluminosilicates needs to be prioritized for gaining better insight into the transport of water via subduction of sediments.