Phase transition and equation of state of dense hydrous silica up to 63 GPa

Abstract

Although it has previously been considered to be essentially anhydrous, Al‐free stishovite can contain up to ∼1.3 wt % of H2O, perhaps through the direct substitution ( Si4+→4normalH+), according to recent studies. Yet the stability of such substitution and its impact on the properties of silica and rutile‐structured hydrous phases (such as δ‐AlOOH and phase H) are unknown at the conditions of the deeper mantle. We have synthesized hydrous and anhydrous Al‐free stishovite samples at 723 K and 9 GPa, and 1473 K and 10 GPa, respectively. Synchrotron X‐ray diffraction patterns show that the unit cell volume of hydrous stishovite is 1.3% greater than that of anhydrous stishovite at 1 bar, suggesting significant incorporation of OH in the crystal structure (3.2 ± 0.5 wt % H2O). At 300 K, we found a lower and broader transition pressure from rutile type to CaCl2 type (28–42 GPa) in hydrous dense silica. We also found that hydrous silica polymorphs are more compressible than their anhydrous counterparts. After the phase transition, the unit cell volume of hydrous silica becomes the same as that of anhydrous silica, showing that the proton incorporation through a direct substitution can be further stabilized at high pressure. The lower pressure transition and the pressure stabilization of the proton incorporation in silica would provide ways to transport and store water in the lower mantle in silica‐rich heterogeneities, such as subducted oceanic crust.

Equation of State of Polycrystalline Stishovite Across the Tetragonal‐Orthorhombic Phase Transition

“…From the inspection of diffraction patterns alone, we locate the phase transition between 40 and 50 GPa. As we would expect different grains of the sintered mate- rial to be subjected to different stress conditions, we cannot exclude that both tetragonal and orthorhombic structures coexist for a range of pressures as observed by Nisr et al (2017).…”

“…Laser annealing further stimulates the relaxation of nonhydrostatic stresses (Andrault et al, ; Singh et al, ). The compression experiments on stishovite powder by Andrault et al () and in helium by Nisr et al () therefore capture the compression behavior of a mechanically relaxed material. In contrast, grain boundaries are locked to each other in sintered polycrystalline stishovite.…”

“…The revised EOS from Fei et al () gave slightly higher pressures for B2‐structured NaCl than those reported by Andrault et al () with a maximum difference of 1.4 GPa and lower pressures for platinum with a maximum difference of 9.3 GPa at the highest pressure. To better assess the effect of the stress environment imposed by the pressure‐transmitting medium, we also compare our results to compression of stishovite powder in helium (Nisr et al, ).…”

“…Consequently, the presence of stishovite‐bearing rocks such as subducted oceanic crust in Earth's lower mantle has been inferred from seismic observations of low shear wave velocities (Helffrich et al, ; Kaneshima & Helffrich, ; Niu et al, ). Previous experimental work on the elastic properties of stishovite and CaCl 2 ‐type SiO 2 was either limited to low pressures (Jiang et al, ) or focused on the equation of state (EOS) of single crystals (Hemley et al, ) and powders (Andrault et al, , ; Grocholski et al, ; Nisr et al, ). The effect of nonhydrostatic compression on the phase transition from stishovite to CaCl 2 ‐type SiO 2 has been investigated on powders (Shieh et al, ; Singh et al, ) and on sintered polycrystalline material (Asahara et al, ).…”

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

“…Polycrystalline Stv synthesized from the large-volume press exhibits a less compressible a axis and a larger volume above 10 GPa (Buchen et al, 2018). On the other hand, the phase transition from CaCl 2 -type SiO 2 to Andrault et al (2003); upper triangles: Wang et al (2012); yellow lower triangles: Nisr et al (2017); red lower triangles: Grocholski et al (2013); blue lower triangles: Grocholski et al (2013); hexagons: Murakami et al (2003); gray short lines: error bars. Error bars are smaller than symbols when not shown.…”