Structure and compressibility of Fe-bearing Al-phase D

Due to its large thermal stability, Al-phase D, the (Al,Fe 3+ ) 2 SiO 6 H 2 member of the dense hydrous 11 magnesium silicate (DHMS) phase D, may survive along hot subduction geotherms or even at ambient 12 mantle temperatures in the Earth’s transition zone and lower mantle, playing therefore a major role as 13 water reservoir and carrier in the Earth’s interior. We have investigated the crystal structure and high- 14 pressure behavior of Fe-bearing Al-phase D with a composition of Al 1.53(2) Fe 0.22(1) Si 0.86(1) O 6 H 3.33(9) by 15 means of single-crystal X-ray diffraction. While the structure of pure Al-phase D (Al 2 SiO 6 H 2 ) has space 16 group P 6 3 / mcm and consists of equally populated and half-occupied (Al,Si)O 6 octahedra, Fe- incorporation in Al-phase D seems to induce partial ordering of the cations over the octahedral sites, 18 resulting in a change of the space group symmetry from P 6 3 / mcm to P 6 3 22 and in well-resolved diffuse 19 scattering streaks observed in X-ray images. The evolution of the unit-cell volume of Fe-bearing Al- 20 phase D between room pressure and 38 GPa, determined by means of synchrotron X-ray diffraction in a 21 diamond anvil cell, is well described by a 3 rd -order Birch-Murnaghan equation of state having 22 isothermal bulk modulus K T0 = 166.3(15) GPa and first pressure derivative K' T0 = 4.46(12). Above 38 23 GPa, a change in the compression behavior is observed, likely related to the high-to-low spin crossover 24 of octahedrally coordinated Fe 3+ . The evolution of the unit-cell volume across the spin crossover was 25 modelled using a recently proposed formalism based on crystal-field theory, which shows that the spin 26 crossover region extends from about 30 to 65 GPa. Given the absence of abrupt changes in the 27 compression mechanism of Fe-bearing Al-phase D before the spin crossover, we show that the strength of H-bonds and likely their symmetrization do not greatly affect the elastic properties of phase D solid 29 solutions, independently of their compositions. the single-crystal H-bonds. synchrotron providing insights on the relation and elasticity