Phase diagram and P-V-T equation of state of Al-bearing seifertite at lowermost mantle conditions.

We investigated the properties of Al-bearing SiO₂ (with 4 or 6 wt% Al₂O₃) at pressures and temperatures corresponding to the lowermost mantle, using laser-heated diamond-anvil cell coupled with synchrotron-based in situ X-ray diffraction. The phase transition from CaCl₂-structured to α-PbO₂- structured (seifertite) polymorphs occurs between 113 and 119 GPa at 2500 K. The range of pressure where the two phases coexist is small. There is a slight decrease of the transition pressure with increasing Al-content. We propose a tentative phase diagram reporting the minerals composition as a function of pressure in the SiO₂-Al₂O₃ system. We also refine the P-V-T equation of state of Al-bearing seifertite based on volume measurements up to more than 160 GPa and 4000 K [V₀ = 92.73(10) ų, K₀ = 304.2(3.0) GPa, K'₀ = 4.59 (fixed), ɵ_D0 = 1130 K (fixed), γ₀ = 1.61(3)]. At 300 K, the volume decrease at the CaCl₂ to α-PbO₂ transition is 0.5(1)%, a value slightly lower than the 0.6% reported previously for Al-free samples. At high temperature, the Grüneisen parameter of seifertite is found to be similar to that of stishovite. Nevertheless, the ▵ V/V across the CaCl₂-form to seifertite transition is found to increase slightly with increasing temperature. Across the phase transition, volume changes can be translated into density changes only when the Al substitution mechanisms in both CaCl₂-form and seifertite are defined. The analysis of all available data sets suggests different substitution mechanisms for the two SiO₂ polymorphs. Al-substitution could occur via O-vacancies in the CaCl₂-form and via extra interstitial Al in seifertite. This would result in a density increase of 2.2(3)% at 300 K for SiO₂ in basaltic lithologies. Alternatively, the same Alsubstitution mechanism in both of the SiO₂-dominated phases would yield a density increase of 0.5(1)%. [ABSTRACT FROM AUTHOR]