Equations of State of Stishovite and CaCl2-type SiO2 to Lower Mantle Conditions

SiO₂ is thought to be present in the Earth's lower mantle in subducting plates, which may be comprised of 15-20 wt% free silica. SiO₂ 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 (P-T) space has not been resolved. There have been many previous studies on the equation of state of stishovite, but they span a limited range of pressures and temperatures (up to 54 GPa or 2400 K), 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 T in Kelvin and P in GPa. The stishovite data imply K₀' = 5.24(9) and a quasi-anharmonic T² dependence of -6.0(4) x 10^-6 GPa*cm³/mol/K² for a fixed q = 1, ɣ₀ = 1.71, and K₀ = 302 GPa, while for the CaCl₂-type phase K₀ = 341(4) GPa, K₀' = 3.20(16), and ɣ₀ = 2.14(4) with other parameters equal to their values for stishovite. The behaviors of the a and c axes of stishovite with pressure and temperature were also fit. Compared to the a axis, the c axis is much less compressible (K_0a = 269(4) GPa vs. K_0c = 435(9) GPa) and has a lower thermal expansion (a_a = 2.11(12) x 10^-5 K^-1 vs a_c = 1.70(11) x 10^-5 K^-1). 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. Because this transition is second order with no discontinuity in density, it is unlikely that it would be observable as a seismological reflection, though it may be detectable based on seismic velocities and anisotropy due to the changes in elastic properties. Silica is denser than the surrounding mantle material up to pressures of 58-68 GPa, or depths of 1420-1640 km, with uncertainty due to temperature effects. At shallower depths, silica can contribute to the gravitational force pulling on a sinking slab. At greater depths, silica is less dense than the mantle, providing a source of buoyancy to resist the downward motion of the slab.