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

Abstract

We investigated the properties of Al-bearing SiO 2 (with 4 or 6 wt% Al 2 O 3 ) 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 CaCl 2 -structured to α-PbO 2 -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 SiO 2 -Al 2 O 3 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 [ V 0 = 92.73(10) A 3 , K 0 = 304.2(3.0) GPa, K ′ 0 = 4.59 (fixed), Θ D0 = 1130 K (fixed), γ 0 = 1.61(3)]. At 300 K, the volume decrease at the CaCl 2 to α-PbO 2 transition is 0.5(1)%, a value slightly lower than the 0.6% reported previously for Al-free samples. At high temperature, the Gruneisen parameter of seifertite is found to be similar to that of stishovite. Nevertheless, the Δ V / V across the CaCl 2 -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 CaCl 2 -form and seifertite are defined. The analysis of all available data sets suggests different substitution mechanisms for the two SiO 2 polymorphs. Al-substitution could occur via O-vacancies in the CaCl 2 -form and via extra interstitial Al in seifertite. This would result in a density increase of 2.2(3)% at 300 K for SiO 2 in basaltic lithologies. Alternatively, the same Al-substitution mechanism in both of the SiO 2 -dominated phases would yield a density increase of 0.5(1)%.