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

Abstract

High‐pressure silica polymorphs may contribute to lithologies in Earth's lower mantle. Stishovite, the tetragonal rutile‐type polymorph of SiO2, distorts to an orthorhombic CaCl2‐type structure at pressures and temperatures of the lower mantle. We compressed sintered polycrystalline stishovite and determined the unit cell parameters as a function of pressure using synchrotron X‐ray diffraction. The compression behavior of sintered polycrystalline stishovite deviates systematically from previous results on stishovite powder and single crystals. Following an initial stiffening, the bulk modulus of sintered polycrystalline stishovite drops at the ferroelastic phase transition. We analyzed the observed spontaneous strains using Landau theory to predict the complete elastic behavior of sintered polycrystalline silica across the phase transition. The reduction in bulk modulus at the phase transition as derived here from the compression curve of sintered polycrystalline silica may have implications for the seismic detection of silica‐rich materials in Earth's lower mantle.

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

“…Our obtained pressure-volume data for CaCl 2 -type SiO 2 at 300 K were in good agreement with literature results (Figure 4) (Grocholski et al, 2013;Murakami et al, 2003). Based on our results and previous studies, transition from Stv to CaCl 2 -type SiO 2 does not cause any discontinuity in the compression curve because of the second-order phase transition but can affect the volume gradient (Andrault et al, 1998;Andrault et al, 2003;Buchen et al, 2018;Fischer et al, 2018;Hemley et al, 2000;Tsuchiya et al, 2004). A recent experimental study reported a greater volume for both Stv and CaCl 2 -type SiO 2 than literature results and our measurements (Buchen et al, 2018).…”

“…Due to the second-order phase transition, transition from Stv to CaCl 2 -type SiO 2 does not cause a notable change in density along the slab geotherm based on our result and Wang et al (2012) (Buchen et al, 2018;Nisr et al, 2017). Yet the phase transition can induce a 10-11% reduction in V Φ because CaCl 2 -type SiO 2 has a lower bulk modulus than Stv but a similar density, which is consistent with previous work (Buchen et al, 2018). Here our calculated ρ of seifertite using the obtained thermoelastic parameters is~1.5% greater than the previous theoretical prediction, whereas V Φ is~4.5% lower (Driver et al, 2010).…”

“…Pressure-volume (per formula unit) relationship of SiO 2 at high pressures and 300 K. Yellow: Stv; red: CaCl 2type SiO 2 ; blue: seifertite; circles: this study; diamonds:Buchen et al (2018); squares:…”

“…A recent experimental study reported a greater volume for both Stv and CaCl 2 ‐type SiO 2 than literature results and our measurements (Buchen et al, ). Polycrystalline Stv synthesized from the large‐volume press exhibits a less compressible a axis and a larger volume above 10 GPa (Buchen et al, ). On the other hand, the phase transition from CaCl 2 ‐type SiO 2 to seifertite was observed to cause a 2% reduction in the volume with Z normalized to 1 at 300 K (volume per formula unit) (Figure ).…”

“…A recent experimental study reported a greater volume for both Stv and CaCl 2 -type SiO 2 than literature results and our measurements (Buchen et al, 2018). 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.…”