1. Equations of state and phase boundary for stishovite and CaCl2-type SiO2.
- Author
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Fischer, Rebecca A., Campbell, Andrew J., Chidester, Bethany A., Reaman, Daniel M., Thompson, Elizabeth C., Pigott, Jeffrey S., Prakapenka, Vitali B., and Smith, Jesse S.
- Subjects
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EARTH'S mantle , *SILICA , *CALCIUM chloride , *HIGH pressure (Science) , *SILICEOUS rocks - Abstract
Silica is thought to be present in the Earth’s lower mantle in subducting plates, in addition to being a prototypical solid whose physical properties are of broad interest. It is known to undergo a phase transition from stishovite to the CaCl2-type structure at ~50–80 GPa, but the exact location and slope of the phase boundary in pressure-temperature space is unresolved. There have been many previous studies on the equation of state of stishovite, but they span a limited range of pressures and temperatures, and there has been no thermal equation of state of CaCl2-type SiO2 measured under static conditions. We have investigated the phase diagram and equations of state of silica at 21–89 GPa and up to ~3300 K using synchrotron X-ray diffraction in a laser-heated diamond-anvil cell. The phase boundary between stishovite and CaCl2-type SiO2 can be approximately described as
T = 64.6(49)·P – 2830(350), with temperatureT in Kelvin and pressureP in GPa. The stishovite data imply K 0 ′ $\begin{array}{} K_0^{'} \end{array} $ = 5.24(9) and a quasi-anharmonicT 2 dependence of −6.0(4) × 10−6 GPa·cm3/mol/K2 for a fixed q = 1, γ0 = 1.71, andK 0 = 302 GPa, while for the CaCl2-type phaseK 0 = 341(4) GPa, K 0 ′ $\begin{array}{} K_0^{'} \end{array} $ = 3.20(16), and γ0 = 2.14(4) with other parameters equal to their values for stishovite. The behaviors of thea andc axes of stishovite with pressure and temperature were also fit, indicating a much more compressiblec axis with a lower thermal expansion as compared to thea axis. The phase transition between stishovite and CaCl2-type silica should occur at pressures of 68–78 GPa in the Earth, depending on the temperature in subducting slabs. Silica is denser than surrounding mantle material up to pressures of 58–68 GPa, with uncertainty due to temperature effects; at higher pressures than this, SiO2 becomes gravitationally buoyant in the lower mantle. [ABSTRACT FROM AUTHOR]- Published
- 2018
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