High-pressure phase transition and equation of state of hydrous Al-bearing silica

Abstract

Stishovite, a rutile-structured polymorph of SiO 2 , is a main component of subducted basaltic lithologies in the lower mantle. At mid lower-mantle depths, a second-order ferroelastic transition to orthorhombic CaCl 2 -type (post-stishovite) structure occurs, causing extensive elastic shear softening. Previous studies showed that Al incorporation can decrease the transition pressure, while it is still debated whether H has a similar effect. Here we report the equations of state, structural evolution, and phase transformation of Si 0.948 Al 0.052 O 1.983 H 0.018 (Al5) stishovite and Si 0.886 Al 0.114 O 1.980 H 0.074 (Al11) post-stishovite samples using diamond anvil cells in combination with synchrotron X-ray diffraction and Raman spectroscopy. The Al5 sample transformed to the orthorhombic polymorph upon compression to 16 GPa, displaying a drop of ~12% in its bulk modulus across the transformation. The Al11 sample did not undergo any phase transition in the pressure range investigated. Single-crystal structural refinements and Raman spectroscopy measurements on the Al5 sample show that the soft optic mode B 1g is decoupled from the tetragonal-to-orthorhombic structural transformation and shows a plateau in the stability field of post-stishovite, between 20 and 30 GPa. This observation indicates that the transformation is not pseudo-proper ferroelastic as in SiO 2 stishovite and that existing Landau expansions are likely not applicable to H-rich Al-bearing silica samples. Using the equation of state parameters of orthorhombic Al5 and Al11 and literature data on SiO 2 post-stishovite we then discuss the possibility of non-ideal mixing along the SiO 2 -AlOOH join.

A model of elastic softening and second order phase transitions in anisotropic phases, with application to stishovite and post-stishovite

“…In the case of stishovite, one structural parameter Q appears to be sufficient to describe the cell and elastic properties over large regions of P -T space. This parameter Q can be associated with tilting of SiO 6 octahedra (Zhang et al, 2023, Figure 2) and directly related values like oxygen positions (Criniti et al, 2023). The stishovite-post-stishovite transition is associated with a sharp minimum in the frequency of rotational vibration of O atoms about the c-axis.…”

Elasticity of Hydrous SiO2 Across the Post‐Stishovite Transition in the Lower Mantle

“…Previous ab initio calculations on stishovite with Al 2 O 3 and δ‐AlOOH defects have shown that H incorporation changes the transition to first order by inducing volume and entropy discontinuities (Umemoto et al., 2016). A recent single‐crystal XRD and Raman study on Al,H‐bearing stishovite indicate a second‐order transition, but not necessarily pseudo‐proper, as Raman shifts of the A g mode plateau between 20 and 30 GPa (Criniti et al., 2023).…”