High Pressure‐Temperature Study on the Thermal Equations of State of Seifertite andCaCl₂‐Type SiO₂

Abstract

In this study, we have determined the thermal equations of state (EoS) of CaCl2‐type SiO2 and seifertite (α‐PbO2‐type SiO2) from 55 to 147 GPa up to 3500 K using X‐ray diffraction in laser‐heated diamond anvil cells. The phase transition from stishovite to CaCl2‐type SiO2 is in good agreement with previous experimental results, whereas the transition from CaCl2‐type SiO2 to seifertite was observed to strongly rely on the starting material. For CaCl2‐type SiO2, we obtained a bulk modulus, KT0 = 245(7) GPa with a fixed KT′ = 4, V0 = 48.1(2) Å3, γ0 = 1.3(3), q = 1(fixed), and θD0 = 1100(400) K. Seifertite has a greater KT0 = 290(10) GPa (KT′ = 4 fixed) than CaCl2‐type SiO2. Other thermoelastic parameters of seifertite are V0 = 92.3(5) Å3, γ0 = 1.6(2), q = 1 (fixed), and θD0 = 1600(200) K. Using the obtained results and phase boundary in previous studies, we have found that the phase transition from stishovite to CaCl2‐type SiO2 cannot cause a noticeable change in density but can lower the bulk sound velocity (VФ) by ~10%. This phase transition in the subducted oceanic crust with 20 vol.% SiO2 was estimated to generate a ~2% discontinuity in VФ, which could be related to the observed seismic velocity anomalies at depths of 1,400–1,800 km. At the bottom of the lower mantle, CaCl2‐type SiO2 to seifertite phase transition can lead to a 0.3% jump in ρ and a 0.8% jump in VΦ considering 20 vol.% SiO2 in the subducted mid‐ocean ridge basalt, which may contribute to the formation of the D″ discontinuity above the core‐mantle boundary.

Evidence of shock-compressed stishovite above 300 GPa

“…The stishovite structure from the refined diffraction pattern at 18 ± 2 GPa reveals a relative change in volume of V/V 0 = 0.95 at a density of 4.51 g/cm 3 . The achieved pressure is well within the stishovite stability field found in static compression experiments 2,5,10,[27][28][29][30] . However, a departure from equilibrium behavior is observed at higher pressures: at 123 ± 5 GPa, Bragg reflections of compressed stishovite are still apparent and refined lattice parameters reveal a relative volume change of V/V 0 = 0.80 at a density of 5.35 g/cm 3 .…”

“…However, a departure from equilibrium behavior is observed at higher pressures: at 123 ± 5 GPa, Bragg reflections of compressed stishovite are still apparent and refined lattice parameters reveal a relative volume change of V/V 0 = 0.80 at a density of 5.35 g/cm 3 . These conditions are within the equilibrium phase stability field of α-PbO 2 type silica [10][11][12][13][14][15][16] . At higher pressures, Bragg reflections of compressed and ambient (marked with an asterisk) stishovite are observed.…”

D" Reflection Polarities Inform Lowermost Mantle Mineralogy

“…Ohta et al. (2008) show that the post‐perovskite phase transition in MORB could generate a reflector, but the stishovite to seifertite transition could also be a candidate for this observation (Andrault et al., 2014; Tsuchiya et al., 2004), although the depth of the transition is still debated (Grocholski et al., 2013; Ohta et al., 2008; Sun et al., 2019). Both cases would show polarities of PdP and SdS consistent with a velocity decrease as represented by case 2a in Table 2.…”

In situx-ray diffraction study of dynamically compressedα-cristobalite using a dynamic diamond anvil cell

“…Its high-pressure phase transformations, phase equilibria, and melting process are essential for understanding the interior structure and dynamic nature of planetary formation as well as impact phenomena. The high-pressure modifications of SiO 2 have therefore been extensively studied and a large number of theoretical predicted silica structures have been experimentally confirmed [1][2][3][4][5][6][7][8][9][10][11]. At temperatures exceeding 1470 • C and ambient pressures, SiO 2 crystallizes in the cubic β-cristobalite structure and, upon cooling to temperatures below ∼250 • C, transforms to tetragonal α-cristobalite (space group P4 1 2 1 2).…”